bis 2b lab manual

Introduction to

BiologyLab ManualBiological Sciences 2B

University of California, Davis

Cover photo: Chrysomelid beetles, Zygogramma tortuosa, feeding and mating on a plant in the sunflower family in Madera Canyon, AZ, USA.

Photo by M. Frances Keller.

Copyright © 2009 by the Regents of the University of California, Davis

ISBN 978-0-7575-5982-2

Kendall Hunt Publishing Company has the exclusive rights to reproduce this work,to prepare derivative works from this work, to publicly distribute this work,to publicly perform this work and to publicly display this work.

All rights reserved. No part of this publication may be reproduced,stored in a retrieval system, or transmitted, in any form or by anymeans, electronic, mechanical, photocopying, recording, or otherwise,without the prior written permission of the copyright owner.

Printed in the United States of America10 9 8 7 6 5 4 3 2 1

iii

Table of Contents Pages

Preface v

Acknowledgements vii

Laboratory:

Discovering Diversity Pre-lab 1–6 Lab Exercise 7–20

Resource Acquisition in Eukaryotic Organisms Pre-lab 21–30 Lab Exercise 31–70

Population Growth Pre-lab 71–78 Lab Exercise 79–92

Competition and Natural Selection Pre-lab 93–100 Lab Exercise 101–112

Mendelian and Population Genetics Pre-lab 113–124 Lab Exercise 125–148

Succession in a Marine Fouling Community Pre-lab 149–156 Lab Exercise 157–166

Trophic Interactions Pre-lab 167–174 Lab Exercise 175–186

Evidence for Incipient Speciation Pre-lab 187–194 Lab Exercise 195–200

v

Lab grades contribute 30% of the final course grade.

Labs are worth 15 points each: 5 points for the pre-lab and 10 points for the post-lab exercise. Pre-Labs must be completed and turned in by 9 a.m. on Monday of the lab week. If you do not turn in the pre-lab, you will not be allowed to attend lab and will forfeit all points for the week. You are encouraged to discuss the pre-lab in a study group, but written answers should be entirely your own.

Pre-lab submission will be done electronically via the Smartsite, but in extreme situations a hard copy will be accepted. Place in the B.L.C., or under the door by 9 a.m. Monday of the lab week.

Because of the nature of the labs and because all lab sections are full, if you miss your lab for any reason you may not attend another lab that week. There are no makeups; however, we will drop your lowest score. So if you miss a lab, that will be the one dropped score.

Course Material FeeBIS 2B has a Course Material Fee of $22. This fee is automatically waived for students who are Pell Grant-eligible. Students who are unable to afford the fee but who are not Pell Grant-eligible financial aid recipients may seek a waiver from the office of Evolution and Ecology (EVE). You may pick up a waiver form from the receptionist in Room 2320 of Storer Hall. You are also encouraged to seek financial aid from the Financial Aid Office. Complete and return the waiver to the EVE Office before the 20th day of instruction. Only in extreme cases of financial hardship unaddressed through financial aid can the fee be waived. Further instructions are listed on the waiver form.

Preface

Grading

Lecture 70% 350 pts Lab 30% 150 pts Total 100% 500 pts

Lab Points Breakdown

8 labs @ 15 pts 120 pts Dropped score –15 pts Lab practical 45 pts Total 150 pts

vii

Acknowledgments

This lab manual would not have been possible without the contributions of numerous individuals. Listed alphabetically, they are Jarrett Brynes, Scott Carroll, Yui Chu, Malia Collins, Kymron deCesare, Lisa Dorn, Wes Dowd, Lynn Epstein, Mara Evans, Anne Genissel, Tom Goliber, Rick Grosberg, Lesley Hamamoto, Annaliese Hettinger, Marcel Holyoak, Susan Keen, M. Fran Keller, Annie Kim, Jan Lim, Thien Mai, Trieu Mai, Maja Makagon, Julin Maloof, Robert McKee, Tim Metcalf, Sergey Nuzhdin, Noa Pinter-Wollman, Pat Randolph, Dave Rizzo, Erik Runquist, Ernesto Sandoval, Cipriano Santos, Jay Stachowicz, Maureen Stanton, Julia Svoboda, Edward Thomsen, Douglas Walker, and the staff of the Sacramento-Yolo Mosquito Vector Control District. For technical assistance with the electronic portions of this lab manual, we thank Kirk Alexander, Brian Miller, Matt Lee, and their staff from Academic Technology Services at UCD. We also thank the students of a UC Davis Honors Challenge course who reviewed early drafts of this lab manual. They include Isaac Chang, Johnny Chau, Dani Cooper, Dattesh Dave, Rachel Dettorre, Larissa Kozisek, Daniel Lev, Chelsea Marshall, Monisha Paripatyadar, Christina Rae, Preeti Upadhyaya, and Tiffany Young. Finally, we thank the Department of Evolution and Ecology, the Department of Plant Biology, and the College of Biological Sciences at the University of California, Davis for their support in the development of BIS 2B.

Discovering DiversityGoals and ObjectivesAt the end of this laboratory you should be able to:

1. Describe the different ways diversity can be measured and how estimates of diversity are calculated. Be prepared to outline calculation methods.

2. Describe two methods of sampling used to estimate diversity.

3. Explain why sampling is needed.

4. Explain why and how a rarefaction plot is made, what data are used, and how these plots are interpreted.

5. Explain what is meant by diversity.

Pre-lab Introduction for Lab 1Read and complete the pre-lab exercise before Monday 9 a.m. of lab week. Read the rest of the lab to get a general idea of what you will be doing.

Biodiversity encompasses all the organisms (animals, plants, fungi, bacteria, algae) on the earth. About 1.75 million species have been identified so far, but global species estimates range from 5 to 30 million species. This means there are still several million species we have not yet identified. Some types of organisms are very diverse and others are much less so. For example, there are 300,000 known beetle species but only 4,500 species of mammals. One of the most diverse forests in the world lies in Lambir Park, Sarawak (Malaysia)—here more than 1,100 tree species are found in a forest of 500,000 trees, all in an area the size of four square city blocks. Estimating biodiversity is difficult; imagine how many places on earth have to be studied. Many habitats have never been explored or are disappearing as human populations expand. If we can only conserve some habitats, which should we choose? Field biologists make inventories of the species composition and relative abundances of organisms in unique habitats through out the world using some of the techniques you will learn this week. Some information is stored in a biological inventory database maintained by UCD [ice.ucdavis.edu] and at other sites around the world.

Lab 1

1

© Keen 2008. Used with permission.

Figure 1-1

2 Lab 1: Discovering Diversity

Imagine yourself arriving on an unstudied island, charged with the simple task of finding out which organisms are present. You notice immediately that the island is too large to search it completely. You will have to sample the island by studying only a manageable part of it. How will you take a sample that accurately represents the whole island? This type of sample is called a representative sample. The problem of sampling comes up repeatedly in biology, in statistics, in quality control studies, and in many other areas. Shortly, you will have to decide on a sampling scheme for a small biodiversity study to be completed in lab this week.

Two commonly used sampling schemes are transect and quadrat sampling. In transect sampling, a biologist runs a line, called a transect, through the habitat. Every organism found within a fixed distance from the line, perhaps half of a meter on either side of the line, is recorded. A transect sample produces a list of species present and the number of occurrences of each species within the area of the transect.

For quadrat sampling, the habitat is divided into rectangles of known size, called quadrats. A certain number of quadrats are randomly selected for sampling. The numbers and types of each species found in each quadrat are recorded, along with the total area sampled.

Figure 1-2 shows two transects through the island. To use transect sampling, you would place a tape measure across the island (see note in lab on transects) and walk along it, recording every specimen within 0.5 m of the tape measure. The tape measure would serve as the transect line.

Figure 1-3 shows an imaginary grid projected over the island. The stippled areas represent five randomly selected quadrats (see note in lab on using random number tables). To use quadrat sampling, you would record every organism present in each quadrat.

After sampling was completed, you would have an estimate of the number of species present, the number of times each species occurred, and the total area sampled.

Figure 1-2 Figure 1-3© Keen 2008. Used with permission.

© Keen 2008. Used with permission.

Lab 1: Discovering Diversity 3

There are many ways to think about diversity: it could be just the number of species in an area. The sample above has five species. Another way to think about diversity is to look at how many of each species are present. This is referred to as relative abundance. Perhaps there are three different plant species in a typical grassland habitat, but in one grassland area 99% of all plants belong to one species, whereas in another grassland area each species is represented by one third of the plants present. Should a measure of diversity consider these kinds of differences? There is a diversity index (the Shannon-Wiener index) that includes an estimate of relative abundance. We will show you two ways to analyze diversity in the sample just presented.

To analyze the diversity of a biological sample, you could use:

(1) the total number of species, or S ( S = 5 species for our example).

(2) the Shannon-Wiener index, H', a value called “H prime” that takes into account the relative abundance of the different species (i).

To calculate the Shannon-Wiener index*, H', you will use proportions:pi, is the proportion (p) of all individuals collected that belong to species i. For the first species i = 1. To calculate p1, notice that there were 4 specimens collected for species 1 from a total of 16 specimens. Here the proportion p1 is 4/16 or 0.25.

Pre-Lab Questions—Record Your Answers on the Online Version 1. Why would you use the above sampling methods to

collect data on the diversity of an area? (Hint: How would the information collected using transect or quadrat sampling differ from the information collected if you had just divided the habitat into quarters and sampled one of the quarters?)

2. Look at the imaginary island in Figure 1-4. Five species are present. From what you can see, would you encounter them all using the two transects drawn in Figure 1-2?

3. From what you can see, would you encounter them all using the five quadrats shown in Figure 1-3?

Suppose you completed a study of this habitat using quadrat sampling. Your data table, showing the number of specimens of each type in each quadrat, might look something like the table below:

Species: 1 3 4 TotalQuad- Lady 2 Dotted Plain 5 number ofrat beetle Tree bush bush Ant specimens

1 0 1 0 0 3 42 1 0 0 1 0 23 1 1 2 0 1 54 1 0 1 0 1 35 1 0 0 0 1 2Total number 4 2 3 1 6 16

© Lim 2008. Used with permission.

Figure 1-4

* This index was developed from information theory; read more about it in an ecology text.


Next, multiply pi by log2 pi. Read “log2 pi” as “the log base two of pi.” Most calculators include logs to the base 10 but not to the base 2. However, to convert from one log to another, we multiply by a constant that we will call the conversion factor. The conversion factor to convert from log10 pi to log2 pi is 3.322. Multiply your value for log10 pi by 3.322, to get a new value for log2 pi.

In this example, pi = 0.25. Use your own calculator to confirm that the log10 of 0.25 is –0.602. Now, multiply –0.602 by the conversion factor, 3.322. Did you find a value of –2.00? You will find this number in the chart below in the space for log2pi for species 1.

After conversion, multiply pi by log2 pi for each species. For the first species you will multiply 0.25 by –2.0, giving you a value of –0.50. The values for all five species are shown in the last row of the table below.

Species: 1 3 4 TotalQuad- Lady 2 Dotted Plain 5 number ofrat beetle Tree bush bush Ant specimens

1 0 1 0 0 3 42 1 0 0 1 0 23 1 1 2 0 1 54 1 0 1 0 1 35 1 0 0 0 1 2Total number 4 2 3 1 6 16

Pi 4/16 2/16 3/16 1/16 6/16 0.25 0.125 0.188 0.0625 0.375 log10 Pi –0.602 –0.903 –0.726 –1.204 –0.426 log2 Pi –2.00 –3.00 –2.415 –4.00 –1.41 pi (log2 pi) –0.50 –0.375 –0.454 –0.25 –0.531

The final step is to sum (�) the values. The term � represents addition of all the values of pi (log2 pi). We sum all the species (i) from 1 to the maximum number of species found (S). Because S = 5 in our example, we add five values: p1 (log2 p1) + p2 (log2 p2) + p3 (log2 p3) + p4 (log2 p4) + p5 (log2 p5).

The formula for the Shannon-Wiener index (H') is:

Notice that the final number is multiplied by negative one to get a positive value for H'. For our imaginary habitat, the value of H' = 2.11. Calculate H' from the data in the table for yourself.

4. Did you get the correct value for H'?

5. Make a diagram, flow chart, or ordered list to outline the steps you will take to calculate H'.

6. On the following page is a data table containing species counts from two sites. Fill in parts of the table as directed, and then use data from the table to discover what H' tells us about the diversity of these two sites.

For each site, fill in the blank areas of the table and calculate the Shannon-Wiener index, following the methods in your flow chart. Remember that the conversion factor for log10 to log2 is 3.322.

s

H' = –� pi (log2 pi)

i =1


7. Compare the two areas:

Site 1 Site 2

Number of species: ____________________ Number of species:______________________

Number of Individuals: ________________ Number of Individuals: __________________

Value of H' = _________________________ Value of H' = ___________________________

8. Based on your study, what information can you get from the Shannon-Wiener index that you cannot get from the number of species?

9. Is there a difference in diversity between the two sites according to number of species present?

______________________________________________Explain your answer.

10. Does a high value of the Shannon-Wiener index mean the site is more diverse or less diverse?

Site 1 Site 2

Total Total num- num-Species: 1 2 3 ber of Species: 1 2 3 ber of /Quad- lady Dotted speci- /Quad- Lady Dotted speci- rat beetle Ant Bush mens rat beetle Ant Bush mens

1 1 2 1 1 0 5 0

2 1 2 0 2 0 5 0

3 5 0 1 3 0 2 0

4 1 1 1 4 0 5 0

5 0 4 0 5 1 1 1

Total Total number number

pi pi

log 10 pi log 10 pi

log 2 pi log 2 pi

pi(log2pi) pi(log2pi)


11. Compare the two values of the Shannon-Wiener index for the sites. Which site is more diverse according to the index?

12. Biologists think about conservation as a way to preserve something that exists now so that future generations will be able to see it. If your goal was to conserve the three species from our diversity study and you had to select one site that must be preserved, which site would you choose? ___________________________________________________

13. Why did you choose this site?

14. If ants, in particular, were extremely rare in the region and therefore worthy of special preservation efforts, how would that affect your choice of site to conserve?

The laboratory exercise begins on the next page.

7

Lab 1 Discovering DiversityToday’s goal is to estimate two aspects of diversity (number of species and H') for one habitat. You will work in teams of four students. Each group will work together on one of the habitats available [a greenhouse plant community (GPC), soil community (SC), or local plant community (LPC)]. Two groups of four students will collect data from the same type of habitat and compare their data at the end of the lab.

Within the First 30 MinutesYour TA will organize you into groups of four. Each group should then decide what tasks each member will perform.

Organization PhaseYour TA will provide a bag of labeled tablets. Select a tablet, and find the three other students with the same tablet.

My habitat is ___________________________________________

The first task for each group is to assign one of four tasks to each student. The tasks are as follows:

Sampling coordinator—defines a transect or quadrat and makes sure that the sampling is done in the right place

Sampler—brings each organism in the sample to the group’s attention

OTU cataloguer—stores, draws, photographs, or makes notes on each specimen as necessary once the group decides on the Operational Taxonomic Units (OTU)

Data recorder—writes names and numbers for each specimen in each sampling unit

Task Name Email

Sampling Coordinator:

Sampler:

OTU Cataloguer:

Data Recorder:

Your TA:


Once each group member has selected a task, discuss how you will begin to work together to accomplish the project.

(a) Look at the study site.

(b) Examine your study system and the equipment available (tape measures, string, premade quadrats in several different sizes, random number tables, soil cores, cameras, bowls, and vials).

(c) Imagine the group beginning work and talk out the process.

(d) Discuss potential problems and things about which you are unclear with your TA.

Within the Next 60 MinutesSample your study habitat and collect data. Spend no more than 60 minutes collecting data. If you are not finished by this time, you must stop. If you finish early, you may begin analysis.

Sampling PhaseFirst, choose a sampling method (transects or quadrats). Discuss the possibilities and decide which method seems most appropriate for your study.

The sampling method selected was _____________________________________________

We selected this method because:

If you selected quadrat sampling, you will record organisms in each quadrat. If you selected transect sampling, you will divide your transect into quarters and record the organisms in each quarter of the transect separately. There must be eight samples (quadrats or quarter-transects) taken. Your TA will give you sampling equipment.

To sample a habitat, you will have to decide how to identify an organism and how to remember what you called it. You do not have to know the species name. You can label it species 1, or you could call it “blue bug” or “spiky plant.” You must be consistent with whatever groups (OTUs) that you define.

Scientists exploring new environments use quick descriptive names all the time; later, after careful study in the laboratory, they give scientific names to the organisms. The goal of this exercise is to learn to estimate diversity, so it matters that you keep track of the different kinds of organisms you find.

To help you remember what names you have chosen for each specimen, we have digital cameras for the LPC habitats and small vials for voucher specimens for the SC habitat. It is also a good idea to draw each specimen and place a marker next to each sample OTU as you find them. You can decide what is appropriate as you see the OTUs. All students in a group must be able to explain the entire project, as well as recognize and label all OTUs.

Data collection grids are in this manual beginning on p. 13. Your data must be collected and recorded in an orderly manner. Be clear on collection methods and subsequent calculations. Your group will present data and calculations for your TA’s initials at the end of lab.

Within the next 45 minutes of lab, complete your analysis and prepare for the presentation and discussion.


Analysis PhaseCount the total number of species (OTUs) you found: _________________________________________

Calculate the Shannon-Wiener Index for diversity of your system. H' = __________________________

Find the other group of students working on the same habitat you studied. Compare data. Did you find and identify similar OTUs?

Sampling method used: My group: Other group:

____________________ ____________________

Estimated number of OTUs: My group: Other group:

____________________ ____________________

Estimated value of H': My group: Other group:

____________________ ____________________

Rarefaction PlotsDo you think that you found all the OTUs or species present in the habitat you studied? Suppose you sampled 10 individuals and found two different OTUs. Suppose you increased your sample to 100 individuals and found 20 OTUs; does it follow that sampling 1,000 individuals would lead you to 200 OTUs?

Do you suspect there is a relationship between the number of organisms you study and the number of species or OTUs you would find? Rarefaction plots are used to discover this relationship.

Below is an example of a rarefaction plot for a study of small mammals in a forest in French Guiana. The x-axis shows combined numbers of individual small mammals trapped as more samples are taken. The y-axis shows the combined numbers of small mammal species as more and more individuals are collected.

Rarefaction plot of the number of small-mammal species in relation to the number of individuals trapped on 5 transects with 20 trap stations per transect. Mauffrey, J.-F., C. Steiner, and P. Catzeflis. 2007. Small-mammal diversity and abundance in a French Guianan rain forest: test of sampling procedures using species rarefaction curves. J. Trop. Ecol. 23:419–25.

Figure 1-5


Notice that as more individuals are collected, more species are found. Does it look like this will continue indefinitely?

(yes or no) ______________

Examine the shape of the curve. Using your best estimate of the shape of the curve, approximately how many mammal individuals should be collected to include all the mammal species in the study area?

Type your data from each quadrat or quarter-transect into the Excel spreadsheet as directed by your TA. When all students have entered their data, you will have access to everything. In the meantime, go over the data and ideas presented so far and identify any problem areas. If you have time, start making the rarefaction plots mentioned in the homework (your TA will provide graph paper). Once data are entered, your TA will lead a short discussion of the issues covered today, using plots of class data or a plot from the literature, depending upon class progress.

Discussion During the Final 30 Minutes of LabGet together with the other seven students who studied your habitat, and select one representative from the group of eight students who studied each habitat to give a 2-minute presentation to the class. Prepare the presentation as a group. Your presentation should cover:

(a) a brief description of the habitat studied (30 seconds)

(b) sampling method(s) used (10 seconds)

( c) the major problems encountered during the study (20 seconds)

(d) the number of species found (10 seconds)

(e) the two Shannon-Wiener index values (10 seconds)

(f) a comparison of validity of the two estimates (40 seconds)

As you listen to the talks, record the Shannon-Wiener Index values:

Habitat type Estimate 1 for H' Estimate 2 for H'

1.

2.

3.

Your TA will guide a group discussion of rarefaction plots and their shapes. On the next page are descriptions of plots you will make at home, followed by questions to answer and turn in for next week. Be sure you understand the concepts in this lab and know how to do the homework. Use the discussion time to ask questions and clarify problem areas.

11

HomeworkThese questions are listed on the 2B Web site, so you may download them and type your answers below the question instead of handwriting them, if you wish. Do your own work!

1. Each student will make rarefaction plots using the data from each study group for the three habitats studied today. There will be three plots, one for each habitat, so use different symbols on the plot for each of the two samples from the same habitat. Be sure to label your axes. (3 points)

Details: To make a rarefaction plot you will need to know how many individuals were collected in each sampling unit (quadrat or quarter transect), as well as how many species or OTUs were found among these individuals. Arrange your data in order of the number of new species or OTUs found in each sample. Begin your plot with the sample with the most species. The first data point to plot is the total number of individuals found in the sampling unit with the most new OTUs (x axis), plotted against the number of OTUs present in that sample (y axis). To get the next point, count the combined number of individuals in the first two samples (x axis) and plot it against the new total number of OTUs for the two samples combined (y axis). Continue in the same fashion with all your remaining samples. If the curve does not rise smoothly, you have made a mistake in ordering your samples, and you will need to redo the plot.

2. Spend a few minutes examining the shape of the curves for each habitat. Did the two study groups find similar curves for each habitat? Describe your observations. (1 point)

3. Do you think most of the species present in your habitat were found in the sample? What evidence supports your conclusion? (1 point)

4. In lab, the class examined three habitats. Place the habitats in order from most diverse to least diverse. What characteristics of the habitats do you think might be related to the differences in diversity? (1 point)

Name: _______________________________ TA: __________________________ Date: ___________


Below are four broad conceptual questions. Your TA will assign two of these questions to answer as part of the homework. (2 points per question)

5. Imagine doing a study like the one you did in lab but in an area the size of the UCD campus. Would you make any changes to your sampling design? Describe these changes and explain why you made them. If you didn’t make any changes, explain why not.

6. Today you examined species diversity in one fairly simple environment. Suppose you were estimating the number of beetles in Yolo County. Explain why it might matter if the county had dry areas, swampy areas, forests, and meadows all within its boundaries.

7. Each species of organism contains a certain amount of genetic variation. That genetic variation results in differences among organisms in everything from size, shape, and color to chemical compounds. Because humans never know what aspects of this naturally occurring variation will be useful, a wise course of action is to preserve as much variation as possible. Summarize, in a paragraph or two, the major factors that should be considered when preserving biodiversity for future generations.

(If you are interested, read about how an anti-cancer drug, paclitaxel (Taxol), was discovered as part of a study of naturally occurring diversity in forest trees: www.rinr.fsu.edu/fall2002/taxol.html.)

8. Briefly describe a type of variation you observed within your most abundant OTU (here we are asking about differences that you noticed among individual specimens from the same “species”). How could you determine whether that variation is because of genetic differences, differences in the environment each organism experiences, or some combination of the two?


Voucher specimen OTU Name Description # or photo # (if used)

Date _________________ Time __________________ TA Initials ___________________

Community _____________________________________ Group A or Group B? _________


Voucher specimen OTU Name Description # or photo # (if used)




Number Sample #: OTUs present of individuals per OTU

1.

2.

3.





4.

5.

6.



7.

8.


For each site, fill in the blank areas of the table and calculate the Shannon-Wiener index, following the methods in your flow chart. Remember that the conversion factor for log10 to log2 is 3.322.

H' = ___________________________________

Site:

Species: 1 2 3 4 5 6 7 8 9 10 11 Total numberSampling ofUnit specimens

1

2

3

4

5

6

7

8

9

Totalnumber

pi

log10 p1

log2 pi

pi(log2pi)


Group Work Summary—Complete this Sheet at Lab’s End and Turn it in. Circle one:

Did your group work well together? Yes No

Was your project completed successfully? Yes No

Did you do your fair share of work? Yes No

Did others do their fair share of work? Yes No

Who, if any one, did not make an adequate contribution to the group?

Who, if anyone, made an excellent contribution to the group?

Signature



Name ___________________________________________________________________________

Resource Acquisition in Eukaryotic Organisms

Goals and ObjectivesAt the end of this laboratory you should be able to:

1. Describethebasicmetabolicrequirementsofeukaryoticorganisms.

2. Explainhoweukaryoticautotrophs,inparticularplants,acquireresources(includingoxygen,carbon,nutrients,andwater)fromthelivingandnonlivingworld.

3. Explain how eukaryotic heterotrophs acquire resources (including oxygen, carbon, nutrients, andwater)fromthelivingandnonlivingworld.

4. Describe particular structures used for resource acquisition in plants (including roots, shoots,vascularsystems,andleaves)andanimals(includingteeth,jaws,guts,andvascularsystems).

5. Describethestructuresassociatedwithresourceallocationandstorageinplants.

6. Compareandcontrastthebodyplansofsingle-celledandmulticellularautotrophsandheterotrophsintermsoftheirabilitiestoacquireresources.

7. Explaintheroleofautotrophsandheterotrophswithinacommunity.

8. Calculateasurface-area-to-volumeratioandexplainwhatthisratiomeans.

9. Recognizeplantsforthenaturalselectionexperimentweek4(StationI).

Pre-lab Introduction for Lab 2 Read and complete the pre-lab exercise before Monday 9 a.m. of lab week. Read the rest of the lab to get a general idea of what you will be doing; there will not be time to read along during lab.

Inmuchoftoday’slabyouwillbeexamininglivinganimals,plants,andfungi(alleukaryoticorganisms.)Thismaybethefirsttimeyouhavereallylookedattheseorganismswithaneyetofiguringouthowtheyliveandwhytheyhavecertainstructures.Youmaywonderhowtheteethofarabbitdifferfromthoseofafox,andwhy.Youmaywonderwhysunflowersaresotallwhencomparedwithstubbystrawberries,orwhythepotatoplantmakesthestorageorgansthatweservebaked,mashed,orfried.Thesequestionshaveanswersatseveraldifferentlevels—theteethofarabbitdifferfromtheteethofafoxbecausearab-biteatsplantsandafoxeatsmeat.Whatdoyougetfromeatingaplantthatyoudonotgetfrommeat,andviceversa?Doesagrowingplantneedthesamethingsasagrowingrabbitorafox?Theansweris“itdependsonwhatyoumean.”

Lab 2

21

22 Lab2: ResourceAcquisitioninEukaryoticOrganisms

Allthreeneedoxygen,energy,andnutrients.Manyoftheorganisms’needsstemfromtheneedsofthecellsthatmakeuptheirbodies.Sothepre-labtodaystartswitharefreshercourseonsimplecellularmetabolismthatyoulearnedinBIS2A.Onceyouarecomfortablewithcellsandtheirrequirements,youcanexplorethemultitudeofcellarrangementsinlargeorganisms.

Thelivingthingsaroundussurvive,grow,andreproduce.Whatisneededforsurvivalandgrowth?PerhapsyourecallfromBIS2Athatorganismsneedasource of energyandasource of carbon,aswellasasetofchemicalbuildingblocks(hydrogen,nitrogen,calcium,etc.)toconstructandreplenishtheirbodies.ThecurrencyforbiologicalenergyisthenumberofATPmolecules,butthesourcesforenergyandcarbonvaryamongorganisms.Fordifferentkindsoforganisms,wecanasktwoquestions:“howisATPmade?”and“fromwhereiscarboncollected?”

Biologists divide cells and multicellular organisms into two very broad categories: autotrophs andheterotrophs.Ourfirsthintofthemeaningofthesetwowordscomesfromtherootwordsintheirnames:“troph”isfromtheGreek“trophe”meaning“food,”whereas“auto”means“self”(asinautomobile).Didyoudeducethatautotrophsmaketheirownfood?“Hetero”means“different,”suggestingthatheterotrophsfeedonsomethingdifferent—otherorganisms—insteadofmakingtheirownfood.

Thinkingfirstaboutautotrophs,considerthesourcesforenergyandcarbon.ThesunistheultimatesourceofEarth’senergy.Thesun’sraystransmitenergyasheatandlight,socapturinglightenergyandusingitforyourselfwouldbeonewaytobegin“self-feeding.”Inaprocesscalledphotosynthesis,lightenergy (in photons) is captured by pigments, such as chlorophyll, and is used to make ATP. ATP isessentially a storage molecule for energy. The energy stored in ATP provides the energy to combinecarbonfromcarbondioxidewithotherbuildingblockstomakecarbohydrates.Carbohydratesincludestarches and sugars and have the general chemical formula CxHyOz. Sugars are a form of short-termenergystorage,whilelargercarbohydratesandstarchesareusedforlonger-termstorage.Thecommonsugar glucose (C6H12O6) is the final product of photosynthesis. The process is called photosynthesisbecausethepigmentsusethelightenergyinphotons(photo-)tosynthesize(make)glucose.Thefigurebelowshowsenergycollectionandcarbonfixationinaphotosyntheticeukaryoticcell(a“plant”cell);theinsetshowsreactionswithinachloroplast,whichhousesthechlorophyll.ThefixationofcarbontakesplaceasthreemoleculesofCO2entertheCalvincycle.Thethreecarbonatomsarecombinedintoasmallsugar molecule through a series of reactions that uses nine molecules of ATP and six molecules ofanotherenergysource(NADPH).Thetwosmallsugarsthenjointoformglucose.

©ThienMai2008.Usedwithpermission.

Figure 2-1

Lab2: ResourceAcquisitioninEukaryoticOrganisms 23

Photosynthesis is one method of autotrophy. It occurs in plants, algae, and some single-celledeukaryoticforms,aswellasincyanobacteriaandafewotherprokaryotes.

Anothermethodofautotrophy,calledchemoautotrophy,alsousescarbondioxide(CO2)asacarbonsource,butenergyforATPproductiondoesnotcomefromharvestinglight.Instead,energyisderivedfrom breaking apart the chemical bonds in substances (oxidizing the substances) such as hydrogensulfide(H2S),ammonia(NH3),ormethane(CH4).Thefigureaboveshowsanexampleofchemoautotrophyusing hydrogen sulfide. Recall the reduction/oxidation (redox) reactions from BIS 2A where ATP isproducedandusedtodrivefixationofcarbonintheCalvincycle.Noticethesimilaritiesanddifferencesbetweenthisprocessandphotosynthesis.

Certain prokaryotic organisms are chemoautotrophs. There are no chemoautotrophic eukaryotes,but some large eukaryotes, like the thermal vent tubeworms in the deep ocean, have developed anintimateassociation(calledsymbiosis)inwhichchemoautotrophicprokaryoteslivewithintheirtissuesand provide them with food. [oceanexplorer.noaa.gov/explorations/03windows/logs/jul28/media/riftiaeast_wall1.html]

In contrast to autotrophs, heterotrophs feed on other organisms, living or dead, as a source ofenergy,carbon,andotherchemicalbuildingblocks(hydrogen,calcium,potassium,andnitrogentonamejustafew).Heterotrophyisusedbyanimals,fungi,andsomesingle-celledeukaryotes,aswellasbymostprokaryotes.Howdoes itwork?Pictureananimal eatingaplant—theplantproduced the sugarsandothercompounds(proteins,starches)thatmakeuptheplantbodybyusinglightandotherresources.What happened to the energy that the plant collected? It was stored in high-energy chemical bondswithincompounds.Aheterotrophusesasmallamountofenergytobreakthesebondsduringdigestion.Lowerenergy(morestable)productsareformedresultinginanetgaininenergyfortheheterotroph.Someofthechemicalbuildingblocksfromthesereactionswillbereconstructedintheconsumer,whileothersarereleasedaswaste.Wastesareprimarilyawaytoexcreteexcessnitrogenandothersubstancesthatcannotbedigested.

TherelationshipbetweeningestionandcellularmetabolismisshowninFigure2-3,onthenextpage.Ingestion simply refers to bringing food into the body. Cellular metabolism includes the process ofbreaking food down into energy and nutrients. This breakdown, or digestion, occurs in two stages:extracellularandintracellulardigestion.Noticethatorganismsfirsttakefoodintothedigestivecavity.Thisisaninternalsacwhereenzymesbreakdownfood,aformofextracellular digestion.Digestioninthegutcavityisanextracellularprocessbecauseittakesplaceoutsideofcells(inthecavityofthecell-linedsac).Thedigestiveproductsarethenabsorbedintothecellsthatlinethegut,sofurtherbreakdownoffoodproductsisintracellular.

Foodprovidescarbonchainsthatareusedtomakenewcompounds.Thisprocessofbuildingnewcompounds is called biosynthesis and is the second form of cellular metabolism. Food also fuels the

Figure 2-2



productionofATPviatheprocessofcellularrespiration.Thesimplesugarglucoseisbrokendowninaprocesscalledglycolysistoreleasetheenergyneededforcellularrespiration.Morecomplexfood,suchascarbohydrates, fats, and proteins are used as fuel by being broken down further first. The ultimatedestination for the breakdown products is a mitochondrion, where cellular respiration occurs. In amitochondrion, about 38 ATPs are produced per glucose molecule via reactions of the Krebs cycle,electrontransportchain,andoxidativephosphorylation.Oxygenisusedasthefinalelectronacceptorinthisprocess,somitochondriacannotfunctionwithoutoxygen.

Thereareafewhighlyspecializedheterotrophs,suchasfungi,thatdonotconsumeanddigestfoods.Rather, they directly absorb nutrients across their cell walls. When fungi absorb nutrients from deadorganismsorbiologicalwasteproducts,werefertothemasdecomposersorsaprobes.Whenanimalsorplantscollect nutrients from living hostswithoutkillingthehosts(asinamosquitofeedingonyou),werefertotheseorganismsasparasites.Fungi are generally called biotrophs and are not called parasitesunless they live inside the host cell. If the fungi also cause disease,they’recalledpathogens.

Likeanimals,fungirequireanorganiccarbonsourceandmayusethesamesourcetomakeATPandtosupplycarbonmolecules.Whatisunusual about fungi is their absorptive capacity. Chemically simplecompounds (sugars or amino acids) can be absorbed directly acrossfungalcellwalls,butcomplexfooditems(likepolysaccharides)aretoolargetoabsorbdirectly.So,fungihavefoundanotherwaytoeatthem.Afungusexcretes enzymes directly onto more complex food sourcestodigestthem,latertakinginthebroken-downproducts.Figure2-4showsthetipofafungalhypha(abranchofthefungalbody);noticehowitadheres

Figure 2-3


Figure 2-4©Epstein2008.Usedwithpermission.


tightlytothesurfacebeforebeginningdigestion.Itmaysurpriseyoutothinkthatfungidigesttheirfoodsourcesbeforetheyconsumethem,butafewotheranimals,likespiders,octopuses,andseastars,dothesame thing. Digestive enzymes may be secreted onto, or injected into, the prey organisms so that onlyliquefiednutrientsaretakenin.

Sofar,wehaveconsidereddifferentsourcesforenergyandcarboninautotrophsandheterotrophs.There are some differences between these two ways of feeding in eukaryotes, but do the cells of anautotrophdiffergreatlyfromthoseofaheterotroph?Examinethesimplecartoonsofplant,animal,andfungalcells,andcompletetheexercisesonthenextpagetofindout.

Pre-Lab Questions—Record Your Answers on the Online Version 1. SortthecellstructuresshowninFigure2-5intofourcategories:

Unique to plants Unique to animals Unique to fungi Shared


©Goliber2008.Usedwithpermission.

Figure 2-5



2. Foreachfunctionlistedbelow,nametheassociatedstructure:

Function Structure:

Rigidcellularsupport

Flexiblecellularboundary

Siteofcellularrespiration*

Siteofphotosynthesis

*Lookupcellularrespirationinyourtext,ifyoudonotrememberitfromBIS2A.TakeasecondlookatFigure2-3onpage24andnoticewhereoxygenfunctionsincellularrespiration.

3. Whatcomponentofcellsrequiresoxygen?__________________________________________________

4. Whatkindsofcellshavethiscomponent?___________________________________________________

5. Thesimplediagrambelowhasthreeelements:thesun,aplant,andagrasshopperthateatsplants.Totherightofthediagramare13terms,alreadydiscussed.Reviewwhatyouknowbyplacingthenumberforeachtermintheappropriateboxesonthediagram;useallthetermsatleastonce.

Figure 2-6©HamamotoandLim2008.Usedwithpermission.

Terms:1.Photonsupply

2.Autotroph

3.Heterotroph

4.Carbonfixation

5.Digestion

6.Photosynthesis

7.Calvincycle

8.Cellularrespiration

9.Chloroplast

10. O2needed

11. CO2needed

12. O2released

13. CO2released


How does oxygen get to mitochondria in cells?

Ifoxygenispresentintheenvironmentoutsideacell,itwilldiffuseinacrossawetcellmembrane.Diffusioncanmovemoleculesoververyshortdistances,lessthan0.5mm;soifthecenterofacelliswithin0.5mmofanoxygensource,diffusionwillprovideoxygentotheenergy-producingmachinery.Diffusion operates across the surface area of the cell membrane, and it supplies oxygen tomitochondriainthevolume ofthecytoplasm.Surfaceareaismeasuredin2dimensions(forexample,4mm2ofmembranesurfacearea),butvolumeismeasuredin3dimensions(forexample,2mm3ofcytoplasm). So if we want to know how much membrane area is available to supply oxygen to acertaincellvolume,wecancalculateasurface-area-to-volume ratio.Inthisimaginaryexample,theratiois4/2,whichcanbesimplifiedto2/1orsimply2.Thismeansthatforeveryunitofvolumeinthecell,therearetwounitsofmembranetotakeinoxygenortoallowwastestodiffuseout.Thesurface-area-to-volumeratiovarieswiththeparticularshapeinquestion(perhapsasphereversusacube),butforeachshape,sayasphere,theratio declines as the size of the sphere increases.Thismeansthatlarger sphereshave lower surfacearea tovolume ratios thando smaller spheres.Youwilldo somecalculations on this in lab, but can you see how the surface-area-to-volume ratio would limit themaximumsizeofasinglesphericalcellwherediffusionsuppliesoxygen?Ifcellswerenotspherical,whatshapewouldgivethemagreatersurfaceareatovolumeratio?Youwillknowtheanswertothisquestionbytheendoflabtoday.

Diffusionbringsoxygenintorelativelysmallisolatedcellsaslongastheyhaveawetcellmembrane,buthowdoothercellsinorganismsgetoxygen?Mostorganismsaremulticellular—thatis,theyareclustersofcellsthathaveremainedtogetherafterdivisionbymitosis.Thearrangementofclusteredcellsaffectstheeffectivenessofdiffusion.Forexample,cellsarrangedinaflatsheetonthewatersurfacewouldeachhaveanexposedwetmembrane,aswouldcellsinahollowball,ortwolayersofcellsplacedbacktoback.You will see some organisms with these body designs today, but most organisms have much morecomplexarrangementsof cells.Thinkaboutyourownbody.Howareoxygenandother substancesmovedfromcelltocell?Isitclearthatsomesortoftransport systemisneeded?Todayinlab,youwillseeavarietyofsupplyandtransportsystemspresentinmulticellularorganisms,butlet’sworkthroughoneexamplenowbylookingatoxygentransportinthehumanbody.

How does the human body supply oxygen to mitochondria in every cell? In most multicellularorganisms,oxygensupply(respiration)hastwoparts:oxygenpickupandoxygendistribution.Youmayhavetorecallalittlehighschoolphysiologyhere,butitshouldcomebacktoyou.Writebriefanswerstothequestionsbelow.

6. Airentersthehumanbodythroughthenoseormouth,butwhereisoxygenpickedupfromtheair?______________________________________________

7. Thinkingaboutthestructureofthis“pickuporgan,”isthereanythingaboutitsdesignthatwouldmaximizethesurface-area-to-volumeratio?__________________________________________

8. The“pickuporgan”ismoistontheinside(why?___________________________________________)andsurroundedbyameshoftransporttubesontheotherside.Whatdowenormallycallthe“transporttubes?”____________________________________________________

9. Whatflowsinthe“transporttubes”andcarriesoxygen?____________________________________

10.Howdo“transporttubes”bringoxygentoevery cell?________________________________________

11.Howdo“transporttubes”takewastegasesawayfromevery cell?_____________________________

The multicellular body:

Manyofthestructurespresentinmulticellularorganismscanbeseenassolutionstoproblemsofpickupandtransportforvarioussubstances(oxygen,nutrients,minerals,orwastes)aroundthebody.“Transporttubes”typicallyformcirculatoryorvascularsystemsinplantsandanimals.Theseareoftenaccompaniedbyspecializedorgansthatmakehormonesandserveotherfunctions.Allofthesestructuresaddtobody


Partialstructureofcellulose:Noticetherepeatingsugarunits—theycouldprovideanenergysourceifcellulosecouldbebrokendown.Theplantbodyisoftenabout40%cellulose,butsomeparts,likethefibersoncottonseeds,are90%cellulose.Humansusethehighcellulosecontentofcottonfibersforclothproduction.

size,butweknowthatbodiesarenotjustmoundsofcells.Sosomethingisneededtosupportthesetubesandorgans.Supportsystemsorstructuralelements,likebones,areusuallypresent.Boneisoneexampleofastructuralelement,butthereareothers,includingwood,shell,andexoskeleton,tonamejustafew.Theseelementsaremadeofproteinsandminerals,amongotherthings.Inthesectionfollowing,refreshyourmemoryonthechemicalconstituentsofafewstructuralelementsandthenfindoutwheretheseconstituentsoccurinthenonlivingworld.

12. Proteinsaremadeofaminoacids.Whatelementsmakeupaminoacids?Lookupthestructuresofthe20aminoacidsinyourtextandlistthe5elementspresent:

1.________________2.________________3.________________4.________________5.________________

13.Bonecontainslivingcellsandorganicmattersuchascollagen,protein,andpolysaccharides.However,muchofthevolumeofboneismadeupofminerals,whichmaycompriseasmuchas65%ofbonemass. Calcium and phosphate are the most common minerals, with calcium in the form ofhydroxyapatite[Ca10(PO4)6(OH)2]andcalciumcarbonate[CaCO3].Magnesiumhydroxide,fluoride,andsulfatemayalsobepresent.

Wheredoyouthinkthebodygetstheneededminerals?______________________

14.Aplantcellwallismadeofcellulose.Celluloseismadeofmanyrepeatingtwo-sugarunits.Cellulose,boundwith lignin,becomeswood,a largechainpolymerthat isveryhard,stable,anddifficult tobreak apart. From the partial structure shown in Figure 2-7, what are the elements contained incellulose?

1. ________________________ 2.________________________ 3.________________________

Figure 2-7©Santos2008.Usedwithpermission.


Figure 2-8

15.Clearlycelluloseisveryabundantonearth,anditisalong-lastingstablesubstance.Manyanimalscannotdigestcellulose.Giventhis,whatpreventsthebodiesofdeadplantsfromfillingtheearth?Somethingmustdecomposecellulose.This iswherefungaldecompositioncomes in—fungidigestcellulose,asdomanyprokaryotes.Nametwoorganismsthatconsumecelluloseandmakeaneducatedguessastowhethereachbreaksdowncelluloseorsimplyexcretesitasfiber:

1. ___________________________________ 2.____________________________________

Fateofcellulose?_____________________ _____________________________________

Chemical building blocks from the nonliving world:

Wheredoesanorganismgetthechemicalbuildingblocksthatareincorporatedintothebody?Some,aswehaveseen,areeatenasfood.Someexistintheatmosphere,soil,andwaterthatsurroundtheorganisms and are collected during growth. The following chart and diagrams demonstrate whatbuildingblocksareavailabletoorganismsindifferentenvironments.

16. Labelsectionsofthefollowingpiediagramstoshowtherelativeproportionsofatomsandmoleculesinair,water,andearth’scrust.Dataareprovidedintablesonthenextpage.

17. Iftheeaseofoxygenpickupdependsonoxygenconcentration,wouldrespirationbeeasierinairorinwater?_____________.


Atmosphere Seawater Earth’scrust

Sym- % Sym- % Sym- %Element bol Element bol weight Element bol

Chloride Cl 55 Silicadioxide* SiO2 76 Sodium Na 31 Sodium Na 3 Sulphate SO4 8 Magnesium Mg 2 Magnesium Mg 4 Calcium Ca 4 Calcium Ca 1 Potassium K 3 Potassium K 1 Aluminum Al 8 Iron Fe 5

Dissolved Dissolvedgases Gases %vol.

Nitrogen N2 78 Nitrogen N2 11 *Oxygen O2 48 complexedas already SiO2 counted

Oxygen O2 21 Oxygen 02 1 *Silicon Si 28 complexedas already SiO2 countedArgon Ar 1

Carbon CarbonDioxide CO2 0.033 dioxide CO2 83

31

Lab 2: Resource Acquisition in Plants, Animals, & FungiIntoday’slabyouwillexplorehoworganismscollect(acquire)theresourcestheyneedtosurvive,grow,andeventuallyreproduce.Organismscanaltertheirsize,shape,growthplan,andbehaviortoacquirenewresources.Whenresourcesareinshortsupply,organismsmustcompeteforthem.Todaywewillalsolookathowavarietyoforganismscompeteforresources.

Lastweektheclasscollecteddataondiversityinsmallteams,buttodayyouandalabpartnerwillcollectanduseideasfromlabstationssetuparoundtheroom.Therearetwosetsofstations—onesetonautotrophs(stationsAtoF)andanothersetonheterotrophs(stationsGtoL).Halftheclasswillbeginwitheachset—complete a whole set of stations before you move to the other set.AtstationIyouwillbeintroducedtoanexperimentyouwilldoinweekfour.

Tohelpyoumakeconnectionsbetweenallthematerialspresented,theclasswillconstructaconceptmapattheendoflabthisweek.AverysimpleconceptmapwasshowninQuestion5inthepre-lab,butthe class will work on something larger. You will use linking terms such as “contains,” “produces,”“releases,”“takesup,”“excretes,”“eats,”“digests,”or“decomposes”toconnectthetermscoveredinclass.Afewsampletermsandconnectionsareshownbelow.Whatotherconnectionscanyoumake?Youmayaskaboutanyproblemareasintheend-of-classdiscussion.

Figure 2-9


Station A. How Do Photoautotrophs Acquire Energy? Things to DiscoverWhatdoesaphotosyntheticpigmentdo?

Whatphotosyntheticpigmentsexist,otherthanchlorophyll?

Whyshouldoneorganismhavearangeofpigments?

Whatkindsofphotosyntheticpigmentswouldyouexpecttoseeinacoastalalgalivingat20meters?

Wouldyouexpecttoseegreateramountsofeachphotosyntheticpigmentinatropicalunderstoryplantlivinginshadedconditionsorinatropicalplantlivinginbrightlight?

BackgroundSome organisms, called photoautotrophs, convert the energy in sunlight to chemical energy viaphotosynthesis.Thatenergyisused,alongwithotherelements,tobuildbodies.Lightenergyvariesinwavelength.Wavelengthismeasuredinnanometers(nm)frompeaktopeakofawave,asshownbelow.Wavelengthdeterminesthecolorofthelightyousee.

©Chu2008.Usedwithpermission.

Visiblelightismadeupofseveralcolorsorwavelengths.Thechartaboveshowswhichwavelengthsproduceeachcolor.Whatcolorwouldyouseeatawavelengthof600nm?Becauselightoccursatmanywavelengths,autotrophspossesspigmentsthatabsorbdifferentwavelengths.Werefertotheseaslight-harvestingpigments;themostcommononeischlorophyll,whichisfoundinthreeforms:a,b,andc.Chlorophyll a absorbs light at wavelengths of 430 nm and 670 nm (violet and red light). Becausechlorophyllaisabsorbinglightfrombothendsofthevisiblespectrum,thelightitreflectsappearsgreen.This iswhymostplants lookgreentous.Thereareotherpigments, suchascarotenes,anthocyanins,phycocyanins,fucoxanthins,andphycoerythrinsthatspecializeondifferentlightwavelengths.


Figure 2-10


A technique called Thin Layer Chromatography (TLC) can be used to separate pigments. We willshowyouaTLCplatewithseparatedpigmentsfromafloweringplantandfromthreemajorgroupsofalgae.OntheTLCplateyouwillseeseparateverticallanesforfourkindsofautotrophs:

A=Anthophyta-floweringplantsC=Chlorophyta-greenalgae

P=Phaeophyta-brownalgae

R=Rhodophyta-redalgae

Theplateiscoatedwithanadsorbentmaterial.Aconcentratedpigmentextractfromeachplantisplacedatthebaseoftheplate,andtheplateissetinasolventbath.Pigmentsdifferinsolubilityandinthedegreetowhichtheyareadsorbedbythecoatingontheplate.AstheTLCplateadsorbsthesolvent,pigmentsarecarrieduptheplatetodifferentpoints;abandofcolormarkstheplaceeachpigmentstops.Noticethatthebandsdifferincolor;eachcolorrepresentsadifferentlight-harvesting pigment.TwobandswiththesamecoloratthesameplaceontheTLCplaterepresentthesamepigment(seeredovalsonTLCplate).Noticethatnotallbandsarepresentineachorganism.

Whyshoulddifferentorganismshavedifferentpigments?Notallhabitatsreceivethesameamountortypeofsunlight—imaginethelightenvironmentsat30metersbelowthesurfaceoftheoceanorundertheshadeofalargecanopy-formingtree.Autotrophsmustbeabletousethewavelengthsavailableintheirenvironment.Intheocean,waterfiltersoutcertaincomponentsofthevisiblelightspectrum;youcanseewhichpartsofthespectrumpenetratetheoceaninadiagrampostedatthestation.Whatdidyounotice?

At the StationIs there enough energy in light to power a fan? Shine the light on the photocell to find out.

Visiblelightismadeupofseveralcolorsorwavelengthsthatcanbeabsorbedbydifferentpigments.Examine the TLC plate withpigmentsfromafloweringplantandfromthreemajorgroupsofalgae.

Be sure you can explain what the plate shows.Trythe questions onthecardsatthestation.


Station B. Observing PhotosynthesisThings to DiscoverHowwouldyouknowifphotosynthesiswasoccurring?

From what you know about the chemistry of photosynthesis, which dissolved gas(es) are used inphotosynthesisandwhichareproduced?

Howdoestheamountoflightaffectphotosynthesis?

BackgroundAt this station you will demonstrate that photosynthesis occurs in Euglena, a single-celled organism.ManyEuglenaweremixedwithathickeningagentcalledalginate(derivedfromkelp)toformalgaeballs.EachalgaeballcontainsasimilarnumberoflivingEuglenaimmobilizedwithinthealginate.Thealgaeballsare suspended in freshwater.Freshwatercontainsdissolvedoxygenandcarbondioxide fromthesurroundingatmosphere.

Fromlectureandthepre-lab(p.21),recallthemaineventsthatoccurduringphotosynthesis,andaskyourselfwherethecarbonthatismadeintocarbohydratescomesfrom?Whatproductsarereleasedwhenenergyiscapturedandcarbonisfixed?

Tovisualizechangingconcentrationsofgasesinthesolutionaroundthealgaeballs,wehaveaddedabicarbonateindicator.AnindicatorchangescolordependingonthepHofthesolution.Asdissolvedgasesareaddedto,orremovedfrom,thesolutionsurroundingthealgaeballs,thepHofthesolutionwillchange, and the color of the solution will change accordingly. A pH of 1 results from a strong acid,whereasapHof14resultsfromastrongbase.ApHof7isneutral.TheparticularindicatoryouwilluseisverysensitivetochangesinpHbetween7.6and9.2.NearesttheneutralpH(7),theindicatorisyellow,butitchangestoblueasitapproachesapHof9.2(whenthesolutionismorebasic).Usingacolorchart,youcaneasilydeterminethepHofthesolution.

The addition of CO2 to an aqueous solution lowers pH (solution becomes more acidic), whereasremovalofCO2fromanaqueoussolutionraisespH(solutionbecomesmorebasic).Forthosewithaninterestinchemistry,thestatesare:

CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO –3

When Euglena photosynthesize, what happens to the amount of carbon in the solution? Does itincreaseordecrease?

Intheexperimentbelow,youwillplacevialsoflivingalgaeballsinwateratdifferentdistances(5,10,and30cm)fromalightsource.Youwillalsoplaceavialofalgaeballscoveredinfoilandavialcontainingalginateballswithout algaeat5cmfromalightsource.(Whatisthefunctionofthelasttwovials?)

How and why do you expect pH to change as the vials are exposed to light?


At the Station

1. Usingthestrainer,scoopalgaeballs(green)fromthecontainer.Fillfour1-dramvialstoalevelof1.5cm(thetickmark);besuretherearesimilaramountsofalgaeineachvial.Fillonemorevialtothesamelevelastheothers,thistimeusingthealginate balls without algae(white).

2. Labeleachvial:5cm,10cm,30cm,Dfordark,andNfornoalgae.

3. Add4mLoftheindicatorsolutiontoeachvialusingapipette.Makesureyouputexactly4mLineachvial.Capeachvialtightlytopreventgasexchangewiththeairoutsidethevials.

4. DetermineinitialpHforeachvialbyholdingitnexttothecolorguide;recordinitialpHinthechartbelow.Donotmovethecolorguides;theyareplacedtogiveyouconsistentilluminationforcolorcomparisons.

5. Positionthevialsattherequireddistancefromthelightsource:5,10,and30cm.Completelycoverthe“dark”vialusingthealuminumfoilprovided.Makesurenolightcangetin.Placeboththe“dark”vialandthe“noalgae”vialat5cmfromthelightsource.

6. Turnonyourlightsourceandsetthetimerfor20minutes.

ResultsAfter 20 minutes:Is the color change uniform throughout each vial, or is there a different color close to the algae balls? (Comparethecolorofthesolutionthroughoutthevialtothecolornearthealgaeballs.)

Is there a color difference between the algae balls and the no-algae balls?Mixthevialsbyinvertingthemgently.RecordthepHofeachvialbyplacingthevialnexttothepHcolorguide.PromptlyreturnthevialstotheirrespectivepositionsafteryouhavedeterminedtheirpH.Setthetimerforanother20minutes.

After 40 minutes: Mixthevialsbyinvertingthemgently.RecordthepHofeachvialbyplacingthevialnexttothepHcolorguide.

Vialtype InitialpH pHat20mins pHat40mins

Lightat5cm

Lightat10cm

Lightat30cm

Dark

Noalgae

37

Homework (to be turned in at the beginning of lab 3):

1. Beforeyoumixedthesolutionsat10minutes,wasthereadifferentcolorclosetothealgaeballsthanintherestofthesolutioninthevials?______________________________

a. Whymighttherehavebeenacolorchangenext toeachalgaeball,andwhatchemicalchangeswouldresultinsuchacolorchange?(½ point)

b. WhatistherelationshipbetweencolorchangeandpH?(½ point)

c. WhatistherelationshipbetweenpHandphotosyntheticactivity?(½ point)

2. ComparethepHvaluesforvialsat5cm,10cm,and30cmfromthelight.

a. WhichvialhadthehighestconcentrationofCO2after40minutes?(½ point)

b. WhydidCO2varyamongthevials?(½ point)

c. Howdiddistancefromthelightsourceaffectphotosynthesis?(½ point)

3. ComparethepHchangesinthe“noalgae”vialtothechangesinthelightvialat5cm.

a. Whatdoyounoticeandwhyisthereadifference?(1 point)

4. ComparethepHchangesinthe“noalgae”vialtothechangesinthedarkvialat5cm.

a. Whatdoyounoticeandwhyisthereadifference?(1 point)

Name: ___________________________________ TA: _______________________ Date: ____________


5a. Explainwhich comparisonsamongvialsprovideevidence that photosynthesisoccurred(hereyouaretryingtodistinguishphotosynthesis fromany other biological reaction, sobe explicit aboutwhat youcanlearnfromeachcomparison).(1 point)

b. Howdoyouknowthatphotosynthesisoccurredandnotsimplyanexchangeofgasesbetweentheindicatorsolutionandtheatmosphereinthevial?(1 point)

6. Doesproducingoxygenasabyproductofphotosynthesismeanthatplantscanliveinanoxic(lackingoxygen)soil?Explainyouranswer.Beforeyouanswerthis,lookbackatplantcellstructureonp.25.(1 point)

7. Aplantbody,suchasaredwoodtree,isaverylargemassofcarboncompounds—where does all this carbon come from?(1 point)

8. Whatare“greenhousegases”andwhyshouldtheyaffectplantgrowth?(1 point)

If you want to read a little more about this topic, try the site below or find others with goodexplanations:http://www.npwrc.usgs.gov/resource/birds/greengas/index.htm


Station C. Multicellularity in AutotrophsThings to DiscoverWhatarethedifferentwaystobemulticellular?

Whatproblemsareassociatedwithamulticellularbodyascomparedtoaunicellularform?

Whatadvantagesareassociatedwithamulticellularbodyascomparedtoaunicellularform?

BackgroundThe first autotrophs were unicellular cyanobacteria. Descendants of these single-celled forms persisttoday,alongwithmanymulticellularautotrophs.Whatadvantagesexistfororganismswithlarger,morecomplexbodyplans?Oneobviousadvantagetomulticellularityisthatpartsofthebodycanbespecializedforfunctionssuchasphotosynthesisornutrientabsorption.

How Do You Get a Multicellular Body? Onesimplewaytoincreasebodysizeishaveacolonialbodyplan.Cellsdividebymitosistoproducenewcells,butallthecellsremainattachedtoeachother.Inthiscasethereislittleornodifferentiationamongcells,andtheindividualcellscansurviveontheirownifthecolonybreaksup.

At the station: Examine Volvox and decide whether it is a simple colony or one with specialized cells (1).

Anothersimpleway tobecome larger is to forma filamentousbody.Thisinvolvesrepeateddivisionsalongthesameplane.

YouwilllearninStationKwhyneitherthecolonialnorfilamentousplansresult incomplexorganisms.Complexorganismscanhaveavarietyofbodyshapes and specialized parts, and they are formed by true three-dimensional growthinwhichcellscandivideinanyplane.

The photosynthetic protists (sometimes called algae) illustrate manydirectionsintheevolutionofacomplexbodyplan.

At the station: Classify the algal specimens as colonial, filamentous, or capable of true three-dimensional growth (2).

Basic Plant StructurePlantshavetwobasiccelltypes:thosecapableofperpetualdivision(growth)andthosethateventuallystopgrowing.Thecellsthatstopgrowingarederivedfromtheperpetuallydividingcells.Oncethesecellsstopgrowing,theydifferentiateandbecomespecializedforparticularfunctions(forexample,photosynthesisinleaves;absorptioninroots).Incontrast,thedividingcellsremainundifferentiatedortotipotent,meaningthattheirfateshavenotyetbeendecided.Groupsoftheseplantcells,calledmeristems,arefunctionallyequivalenttoanimalstemcells.Theyarenotevenlydistributedthroughoutplanttissuesbutinsteadoccurinparticularregions,forexample,atthegrowingtipofaplantshootandateachnodeinastem.Wewill

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explainthesetermsandtellyoumoreaboutmeristemsshortly,butfirstyouwillneedtoknowabouttheplantbody,whichismainlycomposedofspecializeddifferentiatedcells.

Amulticellularbodyhasthepotentialforsets of cells to work togetheronparticularfunctions.Groupsofdifferentcellsmightsupportthebodywhileothersuchgroupscollectnutrientsorlight.Agroupofcellsspecializedforaparticularfunctionformsatissue(forexample,animalsandplantshaveepidermalorskintissues),andanorganisacollectionoftissuesthatworktogether(akidneyisananimalorgan).

Therearethree types of vegetative organsinfloweringplants:leaves, stems, and roots.Floweringplantsareincrediblydiverse.Theyrangeinsizefromthetinyduckweedtotalltreessuchasmaples,andoccupyhabitatsrangingfromhot,drydesertstoarctictundraoroceansandlakes.Itisamazingtothinkthat all flowering plants, collectively called angiosperms, are variations of one basic morphology andorganstructure.Thethreebasicpartsoftheplantaredividedintotwobroadcategories:shootsincludetheleavesandstem,androotsaretheundergroundportion.

We recognize certain features typical of each vegetative (nonreproductive) organ:Leavesaretypicallyflatandbroad;theyarespecializedforphotosynthesiswithawaxycuticlethat

protects the inner photosynthetic cells from drying out. Small internal pockets create space for gasexchangeandarevisibleintheleafcrosssectiononthenextpage.

Stemsaretypicallylongandnarrow;theyarespecializedtotransportwaterandnutrientsbetweentheleavesandroots.Stemsalsodisplaytheleavesinpositionsthatmaximizelightexposure.Stemshavenodes, where leaves are attached and lateral shoots may emerge, and regions between nodes, called internodes.Asmallpopulationofmeristematiccellsremainsateachnodeasthestemelongatesduringgrowth (see below). As the plant grows, hormones can trigger different populations of meristem tobecome active and differentiate into new stems and leaves. Nodes and internodes are visible in thediagramabove.

At the station: Identify these structures on a live plant. (3)

©Hamamoto2008.Usedwithpermission.

Figure 2-12


Rootsaretypically longandbranched;theyarespecializedtotakeupwaterandnutrientsandtotransportthosematerialstotherestoftheplantusingthecentralvascular tissue(bloodvesselsareananimalvasculartissue).Afundamentaldifferencebetweenrootsandstemsisthatrootsdonotshowthenodeandinternodestructure.

Roottipscontainmeristematicregionssimilartothestemtips,but,ofcourse,rootstypicallygrowdownward.Asthegrowingroottipadvances,itleavespopulationsofsuppressedmeristemcellswithinthecentralvascular tissue.

Ifthesemeristematiccellpopulationsbegintogrowoutwardfromtheside,theyformlateral rootswhichincreaserootsurfacearea.

Rootsurfaceareacanbeincreasedtoalesserextentbyroothairs;aroot hair formswhenasingle epidermal cellextendsoutwardinatube(seefigureonpage42).

Vascular Tissue:Plantsneedsomewaytomovewaterandnutrientsthroughtheirbodies.Inparticular,theproductsofphotosynthesis,whicharegeneratedintheleaves,needtobeaccessibletotherestoftheplant.Likewise,waterpickedupbytherootsneedsawaytomoveupintotheplantbody.Justlikeanimals,plantsusevascular tissues to move substances throughout the plant body. Thus, vascular tissues are found inroots,stems,andleaves.

Plantshavetwotypesofvascular tissue: Xylemmoveswateranddissolvednutrientsupfromtherootsintotheshoot.Phloemcirculatessugarsandaminoacidsthroughouttheplantbody.Notice thatvasculartissuesarevisible in longitudinalsectionsandincrosssectionsofstemsand

roots.Youwillalsofindthemintheleafsectionabove.Theinsetofvasculartissueinastem(page42,D.lowerright)showsthecellularstructureofthesetissues.

Youmightwonderhowaplantgrows.Thegrowingtissueoccursattheapexofthecentralstem;thisistheapical meristem.Itisaregionofundifferentiatedactivelydividingcells.Asthesecellsdividetheycanhaveoneoftwofates:theycanbecomecellswithfixedfates,forexample,acellintheplantstem,ortheycanbecomemoreundifferentiatedmeristemcells.Overtime,themeristemcellsmaycontinuetodivideortheymayenterasuppressedstatewheretheyremainuntiltheyreceivenewhormonecues.As

©HamamotoandLim2008.Usedwithpermission.Figure 2-13




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thestemelongates,populationsofsuppressedmeristemsareleftateachnode;theycanbeactivatedlatertoproducestructuressuchaslateralbranchesorleaves.Additionally,inmanyplants,populationsofcellswhose fate is limited to producing only vascular tissues remain within the stem; these cells may beactivatedlatertothickenthestem.Seetheplantgrowthdiagramsandmodelsatthestation.Trytofindtheorgansyouhavelearnedaboutineachplant.

What Kind of Support Structure Do Plants Have?Plantcellsdependoninternalwaterforsupport(recallwhathappensifyoustopwateringyourhouseplants),andplantcellwallsaresomewhatstiffenedbythepresenceofcellulose,butsupport for the plant bodyvariesdependingonthegroupofplants.Aquaticplantsrelyonthebuoyancyexertedbysurroundingwatertosupportthebody,whereaslandplantshavespecializedcellsforsupport.Onetypeofsupportthatyouwillrecognizeisthestrandofelongatecellswiththickenedwallsthatmakesupthe“string”ontheedgeofacelerystalk;these“strings”aremodifiedvasculartissues.Thewallsofvascularcellscontainthickenedringsoflignin,astrong,light-weightcarbonpolymer.Reallyfirmsupport,likewood,ismadeup of many layers of a modified form of xylem. There are other kinds of cells that are important insupportingtheplantbody,buttheywillbecoveredinyournextbiologycourse.

Thebasicdesignofafloweringplantisincrediblyversatile.Floweringplantshaveevolvedanamazingrangeofshapesandsizessuitedtoparticularenvironments.Leavescanbelargeasinabananaplantortinyasinacactus.Theplantbodyweseemayrepresentanevolutionarycompromisebetweentheneedfor photosynthetic surface area and the need to reduce evaporation of water across large surfaces.Sometimes,thefunctionstypicallyperformedbyoneplantorganaretakenonbyadifferentorgan.Forexample,incacti,photosynthesisisperformedmostlybythesteminsteadoftheleaves.Someorganshavemultiplefunctions:forexample,rootsareoftenexpandedforfoodstorage(sweetpotatoes)inadditiontotheirfunctioninwateruptake.

At the StationCompletethethreeactivitiesmarkedinboldonpages39and40.Identifytheroot,leaf,andsteminthevariedplantsprovided.Notes at the station explain these ideas and guide observations questions on cards for specific examples.


Station D. Plant Nutrients: Requirements and DistributionsThings to Discover:Nutrientdeficiencieshavevisibleeffectsonplants—canyourecognizethem?

Somenutrientscanbemovedwithinaplanttocompensateforshortageswhenanewpart(e.g.,aleaf)wasmade.Howcanyoutellifanutrientismoveable?

Howcananassociationwithfungiorbacteriahelpaplantcollectnutrients?

Whatismutualism?

BackgroundTwo distribution, or vascular, systems exist in plants: Phloemdistributessugar,aminoacids,andothersubstances throughout thebody(itiscoloredbluehere).

Xylemcarrieswaterandnutrientsuptheshootfromtheroots.Therootsystemminesthesoilforwaterandnutrients,collectingthemacrosstheexpandedsurfaceareaprovidedbyroothairs(itiscoloredredhere).

Intherootcrosssectiontotheright,noticethatthevasculartissueissurroundedbyaringofcellscalledtheendodermis (yellowbandofcellswithapinkstripe).

Water canpenetrate the rootcellsoutside theendodermisby twodifferentpathways:

(1)from celltocellby entering the cytoplasm ofeachcell(symplasticroute) or (2) from cell to cell via the cell walls (called the apoplasticroute).Thisdistinctionisimportantbecausewatercannotpassthroughtheendodermisifitismovingviacellwalls(2a).Theendodermismakesawaxylayercalledthe Casparian strip,whichpreventswaterpassage

Casparian stripin endodermisCasparian stripin endodermis


Figure 2-15

Figure 2-16 Figure 2-17


Station D. Plant Nutrients: Requirements and DistributionsThings to Discover:Nutrientdeficiencieshavevisibleeffectsonplants—canyourecognizethem?

Somenutrientscanbemovedwithinaplanttocompensateforshortageswhenanewpart(e.g.,aleaf)wasmade.Howcanyoutellifanutrientismoveable?

Howcananassociationwithfungiorbacteriahelpaplantcollectnutrients?

Whatismutualism?

BackgroundTwo distribution, or vascular, systems exist in plants: Phloemdistributessugar,aminoacids,andothersubstances throughout thebody(itiscoloredbluehere).

Xylemcarrieswaterandnutrientsuptheshootfromtheroots.Therootsystemminesthesoilforwaterandnutrients,collectingthemacrosstheexpandedsurfaceareaprovidedbyroothairs(itiscoloredredhere).

Intherootcrosssectiontotheright,noticethatthevasculartissueissurroundedbyaringofcellscalledtheendodermis (yellowbandofcellswithapinkstripe).

Water canpenetrate the rootcellsoutside theendodermisby twodifferentpathways:

(1)from celltocellby entering the cytoplasm ofeachcell(symplasticroute) or (2) from cell to cell via the cell walls (called the apoplasticroute).Thisdistinctionisimportantbecausewatercannotpassthroughtheendodermisifitismovingviacellwalls(2a).Theendodermismakesawaxylayercalledthe Casparian strip,whichpreventswaterpassage

Casparian stripin endodermisCasparian stripin endodermis

(in2a).Onlywaterandsolutes moving through the cytoplasmofeachcell(1and2)canreachthevasculartissueandenterthexylem.Thus,cells filter theparticular ionsthatarepermittedtoenterthexylem:potassiummightbeallowedtoenter,butsodiumwouldnot.

At the station: See the demonstration DVD illustrating: a) basic root anatomy; b) ion uptake pathways; c) ion retention in the root

What Do Roots Collect? Thebasicequationofphotosynthesisis:H20+CO2=O2+sugar.ObviouslytheelementsC,H,andOareessentialforplantgrowthanddevelopment,butwhatelsedoplantsneed?Unlikeanimals,whichrequireat least some nutrition in the form of complex organic compounds, plants do not need any organiccompounds(proteins,vitamins,carbohydrates).Plantscansynthesizeanythingtheyneedfromjustafewmineralelements.

Macronutrientsareelementsneededinrelativelyhighamounts—theseincludecarbon,nitrogen,andphosphorus.Fertilizers,suchasmanure,areeffectivewaystoaddnitrogenandphosphorustothesoilandmakethemavailabletoplants.

Micronutrients,suchasboronandmolybdenum,areneededintinyamounts.

Macronutrients: Micronutrients:Hydrogen ChlorineCarbon IronOxygen BoronNitrogen ManganesePotassium ZincCalcium CopperMagnesium NickelPhosphorus MolybdenumSulphur

Whatdotheseelementsdo?

Carbonisamajorcomponentofbiomoleculesincludingcarbohydrates,aminoacids,nucleicacids,andlipids.

Oxygenisacomponentofmanymoleculesandisessentialforcellularrespiration.

Nitrogenisacomponentofaminoacidsandnucleicacids.ThismeansitisanessentialcomponentofproteinandDNA.

Phosphorusisacomponentofaminoacidsandnucleicacids,butitalsoplaysanessentialroleinenergymetabolismbecauseofitspresenceinATP.

Magnesium is an essential part of the chlorophyll molecule. It activates many enzymes needed inphotosynthesis,respiration,andnucleicacidsynthesis.

Ironformspartofcertainenzymesandnumerousproteinsthatcarryelectronsinphotosynthesisandrespiration.

Calciumiscriticalforrootgrowth.Itisacomponentofcellwallsandplaysanessentialroleinsignalingpathways.

Potassium isacofactorforenzymesandisrequiredforbiosynthesis.


Mineral DeficienciesPlantsshowsymptomsofparticulardeficiencies.Thesymptomdependsonthefunctionofthemissingelementandwhethertheelementiseasilymovedfromoldertoyoungerplantparts.Considermagnesium(Mg)—youknow it formspartof thechlorophyllmolecule, so it is critical forphotosynthesis.AnMgdeficiencywillresultinyellowishleaves(chloroticleaves)thatcannotfixcarbon.

However,Mgisrelatively soluble,andplantscanmoveitreadilythroughtheirvasculartissue(phloem).IfaplantisMg-deficient,youwillseeyellowingfirstintheolderleavesbecauseMgistranslocatedfromtheold,alreadyformedleavestothegrowingtissue.Itiscalledamobileelement.Othermobileelementsincludenitrogen,phosphorus,potassium,andchlorine.

Immobile elements includecalciumand iron.Plantsusually showdeficiencysymptoms for theseelementsintheiryoungertissues.

Mutualisms in Plants:How can a relationship with a fungus (a mycorrhizal

association) help roots? Collectingnutrientsfromsoildependsontheamount

ofrootsurfacearea.Plantswithfine rootsthatarecoveredby root hairs have a lot of surface area to mine the soilaroundthem.

The spread of roots across the soil determines howmuchsoilcanbesearched.Someplantshavecoarse rootswithfewhairs—theserootshaveminimalsurfaceareaforuptakeandcannotsearchfornutrientsveryeffectively.

Suchplantsoftenbenefitfromamutualisticassociationwithafungus,calledanAMfungus(arbuscularmycorrhizalfungus;AMF).Thisfunguscolonizes root cellsandsendsoutforagingbranches,calledfungalhyphae,tocollectnutrientsfrom the surrounding soil. The hyphae increase the soilvolumethatisavailabletotheplant,particularlybringingin phosphorus. In exchange, the fungus receives carbon.This is an example of a mutualism. A mutualism is anecological interaction between two different organismswherebothparticipantsbenefit. Ingrasslands,where thedecomposition of plant material is very rapid, it can bedifficult for plants to access enough phosphorus. Manygrassland plants benefit from AM fungal associationsbecausethefungus isbetterataccessingthephosphorustheplantsneed.

Another type of fungi, called ectomycorrhizal or EMfungi(EMF),doesnotpenetraterootcells;insteadhyphaeinteract with plant tissue by moving inward between root cells as shown in the left figure on p. 47. Hyphae alsosurroundtherootandextendoutwardintosoil.Incolderclimates, where decomposition of plant matter proceedsvery slowly, these fungi break down leaf litter and deadplants quickly, allowing nutrients, primarily nitrogen, toreturn to forest trees. In exchange, the trees providecarbohydratestothefungi.



Figure 2-18

Figure 2-19


Ingeneral,plantswithmycorrhizaeareabletoabsorbmorenitrogenandphosphorusthanplantswithout such fungal associations. Overall, the host plant gains mineral nutrients and makes carboncompoundscollectedviaphotosynthesisaccessibletothefungus.

Approximately90%ofgreenplantsmayhavefacultative*mycorrhizalassociations,sometimeswithasmanyas10differentspeciesoffungi.

Some bacteria also form relationships with plants:Amutualisticrelationshipbetweenbacteria(Rhizobium)andplantsinthepeafamily(legumes)also

occurs.Bacteriainvadetheroottissues,formingroundballscallednodules.Insidethenodules,bacteriaareabletofixatmosphericnitrogenfortheplant.Theplantproducesleghemoglobintobindfreeoxygen,whichwould reduce theefficiencyofbacterialnitrogenases. It also supplies respiratoryoxygen to thebacteria.

*Anassociationbetweentwoorganismsmayberequiredforlife(obligate)ormayoccuronlyinsituationswhenitisbeneficialtooneorbothparties(facultative).

At the Station:Examine the bean roots to find the nodules. Splitoneofthenodulesopen;amazingly,thenodulesareredinsideduetothepresenceofleghemoglobin.

Let’s play plant doctor! Therearefivetomatoplantssetoutatthisstation;eachplantwasgrownhydroponically.Theplantlabeled“control”wasgrowninacompletenutrientsolution,buttheothersweregrownwithoutoneessentialnutrient.Selectoneailingindividualandanswerthequestionsinthekeybelowtodecidewhateachpatientneeds.Note: Mottling means that spots are present; chlorosis means the leaves are yellowish instead of green.

#1.Doyouseesymptomsintheolder,lowerleavesontheplant?

Ifyes,goto#2

Ifno,goto#5



Figure 2-20Figure 2-21


#2. Istheplantgenerallyunhealthy,ordoyouseelocaleffects?

Iftheplantisgenerallyunhealthywithlightordarkgreenleaves

andthelowerleavesarebrownedanddry,goto#3

Iftheplantismottledandchlorotic,withorwithoutdeadspotson

thelowerleaves,goto#4

#3.Theplantappearsgenerallyunhealthy.

If it is light green with yellow lower leaves and perhaps has a reddish cast to the veins, it lacksnitrogen.

Ifitisstunted,darkgreenandperhapshasredorpurplepatches,itlacksphosphorus.

#4.Theplanthaslocaldamagewithmottlingorchlorosisontheleaves.

Ifleavesarereddened,orthetipsandmarginsofleavesareturnedupwards,itlacksmagnesium.

Ifithassmallspotsofdeadtissueusuallyattheleaftipsorbetweentheveins,itlackspotassium.

#5.Symptomsaffectnewerorbudleaves.Istheterminalbuddying?

Ifyes,goto#6

Ifno,goto#7

#6.Theterminalbudisdying.

Ifyoungleavesoftheterminalbudbecometwistedandarelightgreenatthebase,theplantlacksboron.Ifyoungleavesoftheterminalbudarehookedanddyingbackatthetipsandmargins,orthestalkbelowthebudisdying,theplantlackscalcium.

#7.Theterminalbudisalive.

Iftheyoungleavesarepermanentlywiltedorcurledbutdonotshowspotsorchlorosis,theplantlackscopper.

Ifthereisyellowingoftheyoungestleaves,especiallyintheareabetweentheveinsoratthebaseofleaves,theplantlacksiron.


Station E. Nutrient Storage SystemsThings to DiscoverPlantscanstoresomeofthecompoundstheymakeforlateruse.Whichorganscanbeusedforstorage?

Howcanyoutellwhichorganwasusedforstorageinapotatooracarrot?

Plant bodies have vegetative parts (roots, leaves, and stems) and reproductive parts (seeds, fruits, orflowers).Isthereanystorageinreproductiveparts?

BackgroundPlantsaccumulateandstoremostoftheirnutrientsascarbohydrates(sugarsandstarches),butproteinsandfatsarealsoimportantstorageproducts.Thestoredproductscanbeusedlaterbytheplant,althoughtheyarealsousedbyheterotrophs,includingpeople.Nutrientstoragecantakeplaceinvegetativeorgansandinreproductiveorgans.

Vegetative OrgansPlantshaveonlythreetypesofvegetativeorgans:leaves,roots,andstems.

©Hamamoto2008.Usedwithpermission.Figure 2-22


Reproductive OrgansFlowers are the sexual organs of plants, but the process of reproduction in flowering plants is verydifferentfromthatofanimals.Whenplantsprepareforsexualreproduction,amulticellularhaploid(1N)stage, called a gametophyte, is made by meiosis. The female gametophyte is housed in the ovule of theflower.Inbothmalesandfemales,thehaploidgametophytedividesbymitosis.Inthefemale,divisionproducesanembryosaccontainingfourpotentialhaploidgametes,oneofwhichwillbecomeanegg.Inthemale,eachgametophyteisapollengrainthatwillcontaintwohaploidnuclei.

Duringfertilization,apollengrainfuseswithafemaleembryosac.Onehaploidnucleusfromthepollengrainfuseswiththeeggtobecomeadiploid(2N)embryo,whiletheremaininghaploidnucleusfuseswith twootherfemalenuclei tomakea triploid (3N)nucleus.Thistriploidnucleusdividesmanytimestomakeatriploidtissuecalledendosperm.Endospermisanutritivetissuethatsupportsearlygrowthof thediploidplantembryo.One familiar exampleof endosperm is thewhite fluffypartofapopcornkernel;anotheristhegroundwheatweusetomakeflourforbaking.Thinkofthisnexttimeyoulookatasliceofbread.

Inseeds,nutrientstorageoccursintheendosperm—wetakeadvantageofthiswhenweeatgrainssuchaswheatandcorn.

In fruits, nutrients are stored in the base of the flower, actually in the ovary that enlarges afterfertilization.Youcanprobablynamemanydifferentfruits,butdidyouknowthatapumpkinisafruit?Lookatthepostedpicturesofthepumpkinflowerandthedevelopingpumpkintoseethatthisisso.Manyfoodsthatwecallvegetablesareactuallyfruits,suchastomatoesandpeas.

At the StationDeterminehowparticularstorageproductssuchaspotatoes,onions,ginger,andothersaremadebytheplant—forexample,isapieceofgingeraroot,ashoot,orastem?

Look at each food item and decide whether it is a root, shoot, stem, or fruit.Thediagramatthestationwillhelpyouidentifythesedifferentorgans.

Therearequestioncardsatthestationtohelpyoudecidehowtheseproductsaremade.


Station F. Interactions between Plants and the EnvironmentThings to DiscoverHowdoplantssenseaspectsoftheenvironment,suchasgravity,light,andwaterlevels?

Doplantsaltertheirbehaviorsinresponsetotheenvironment?

Doplantsaltertheirgrowthforminresponsetotheenvironment?

Whatisphenotypicplasticity?

Whatisthedifferencebetweenacclimationandadaptation?

BackgroundPlantsarefundamentallysessile.Unlikemostanimals,theycannotmovetoafavorablehabitatorleaveanunsuitablehabitat.Aplantmustalteritsphysiologytocopewiththeconditionsitfacesorbecomedormantuntilthingsimprove;ifitcannotdothesethings,itwilldie.Therearetwowaystocopewithextremehabitats:to evolve specializations to match extremeconditions(adaptation)ortohavetheability to alter one’s phenotype (acclimation) as theenvironmentchanges.At this station,weareconcernedwithacclimation,whichhappensoveramuchshortertimespanthanadaptation.Acclimationcanoccurinoneoftwoways:bytheproductionofanewphenotype(changesingeneexpression)orbyamovementorbehaviorinresponsetoastimulus.

TropismsIn plants, movement in response to a stimulus is called a tropism. Movement towards a stimulus iscalledapositivetropism,andmovementawayfromastimulusiscalledanegativetropism.Aresponseto gravity is a gravitropism, and a response to light is a phototropism. Both gravitropism andphototropismcanbepositive(toward)ornegative(away)aswell.

Phenotypic Plasticity:The genetic ability to acclimate by producing different phenotypes in different conditions is calledphenotypic plasticity.Phenotypicplasticitymightinvolvechangesinleafsize—becausethesurfaceareaoftheleafexposedtolightaffectstheamountofphotosyntheticactivitythatoccurs,wemightexpectabranchwithleavesintheshadetohavelargerleavesthanoneinthesun.Butwhenyouthinkabouthowthismighthappen,yourealizethataplantwouldhavetogatherandinterpretalotofinformation.Forexample,aplantwouldhavetodistinguishlowerlightlevelsonabranchfromloweroveralllightonasinglecloudyday.Onceinformationisinterpreted,aplantmustrespondappropriately.

The ability to respond appropriately occurs via the production of new leaves, not by changing existingleaves. Existing leaves are already differentiated. New leaves are made by populations of cells called


meristems; theseare totipotentorundifferentiatedcells similar to thestemcellsofanimals.Whenaplantsensesachangeintheenvironment,itcanusehormonestodirectthedevelopmentofstructuresfrommeristemsinanewway.Themeristemcellsdividemitoticallytoproduceanewstructure.

At this station, you will look for evidence that plants sense three aspects of the environment: the earth’s gravitational pull, the direction from which light comes, and immersion in water.

Didyoueverwonderhowanimalsknowwhethertheyareoriented“rightwayup?”Manyanimalsusestatocysts—acellwithasphereofcalcium(statolith)inaninternalcavitylinedwithsensoryhairs.Inthenormaluprightposition,thesensoryhairsonthebottomofthecavityareactivated.Iftheanimalwereupsidedown,thehairsonthetopofthecavitywouldbeactivated.

Plantsalsohavestatocysts,buttheyaredifferentthan those in animals, and the specific cells thatperceive gravity are different in stems, leaves, androots.Althoughmanyquestionsremainaboutexactlyhowplantssenseandrespondtogravity,muchhasbeenlearnedbystudyingtheprocessinroots.Rootshavearootcap,aspecializedgroupofcellsthatcoverand protect the growing root tip as it grows downthrough the soil. Statocyst cells occur within therootcap.

The root cap statocyst cells that detect gravityhave starch-containing organelles (statoliths—markedbyarrowsinthefiguretotheright),whicharedenserthanthecellcytoplasmandsettleagainstthedownwardsideofthecell.Thisinturnleadstoadifferenceintheamountofaplanthormone,auxin,on the upper versus lower side of the root, whichcausestheroottogrowdownward.

Inthisway,rootscanrespondtogravity,andyouwill shortlydiscoverwhetherotherpartsofaplantarealsoabletorespond.

At the StationPlants have an above-ground portion called a shoot, consisting of the stem and leaves, and a belowgroundportioncalledtheroot.Imagineagerminatingseed.Ifthedevelopingseedlingcouldsensethegravitationalpulloftheearth’smass,itwouldbeveryimportantforrootstorespondpositivelytotheforceandgrowdownward.Doyouandyourpartnerthinkthetipoftheplantshootshouldrespondtogravitationalpullatall?Shouldleavesrespondtogravityiftheycansenseit?



Figure 2-23

Figure 2-24


Light is another critical stimulus for a plant. You might expect plants to be able to respond todifferences in light quality and quantity. Would you expect the leaves, the shoot tip, or the roots toexhibitapositiveresponsetolight?Youwillshortlybeabletotestyourintuition.

We have three Coleus plants for you to study.Coleusisacommonhouseplant.OneColeusplant(A)wasgrowninitsnormalpositionwithlightcomingfromaboveandthepulloftheearth’sgravitationalfieldbeingstrongestattheroot.AnotherColeusplant(B)hasspentthelasttwodayslyingonitsside,sotherelativedirectionofthegravitationalpullhaschanged.AthirdColeusplant(C)hasspentthelasttwodayslyingonitsside,butthedirectionfromwhichlightcomeshaschangedalso.PlantChasbeenlit from belowforthelasttwodays.Thethreesituationsareillustratedincartoons:

Imagine how the plant shoot and leaves might respond to each stimulus (positive or negativephototropismandpositiveornegativegravitropism)andthenseewhatactuallyhappened:

Describe how leaves respond to the direction from which light comes:

Describe how leaves respond to gravity:

Describe how the shoot tip responds to the direction from which light comes:

Describe how the shoot tip responds to gravity:

Did the plants adapt or acclimate?


Figure 2-25


Life Under WaterSomeplantsliveintwoenvironmentssimultaneously.Aplantthatlivesinashallowpondoramarshwillemergefromthewaterifitbecomesverytall.Plantsgrowfromtheshoottip,sowhentheplantisyoungandshort,theleavesitmakeswillbesubmergedinwater(aquaticleaves).Astheshoottipgrowsup,new leaveswill live inair (aerial leaves).How isanaquaticenvironmentdifferentfromaterrestrialenvironmentforaleaf?Tothinkabout this question, refer back to your pre-lab and ask yourself whatfunctions a leaf has to perform and in what ways the environment maylimitthesefunctions.

A leaf has a large photosynthetic surface to expose to light—lightpenetration is reduced in deep water, but these leaves are in relativelyshallowfreshwater.Aleafneedsaccesstoacarbonsourceforphotosynthesis.Yousawfromthepre-labthatCO2dissolvesinseawater,sowecanassumethatitwilldosoinfreshwateraswell.Thedissolvedgasesinairentertheleafthroughopeningscalledstomata—thestomataopentotheoutsidesogasesmayenter,buttheycanalsoclosetopreventdryingout(desiccation).Doyouthinkdryingoutisaproblemforanaquatic leaf?Wouldyouexpect to see stomata?Leaf shape isanother thing thatmaychangeifaleafisunderwaterbecausealeafcouldbetornifitpresentsalargesurfaceareatomovingwater.Whataspectsofleafshapemightmakeiteasierforwatertoflowaroundtheleaf?

Thethoughtexercisesintheparagraphaboveshouldprepareyoutocomparesubmergedaquaticandaerial leaves in the plant Hygrophila difformis. Recall that each leaf develops from a population ofundifferentiated meristem cells at a node. If the plant sends biochemical information about theenvironmentintheformofplanthormonestoeachpopulationofmeristemsbeforetheydevelopaleaf,theneachleafcouldhaveaformappropriatetolocalconditions.

What evidence indicates that Hygrophila can produce different leaves to match a local environment?

How does the form of the leaf differ in each environment?

Do all the leaves from a single node have the same morphology?

Is the ability to respond to each habitat an adaptation or an acclimation?

Do you see evidence of phenotypic plasticity? Justify your answer.

Whenplantsorientthemselveswithrespecttocuessuchaslightorgravity,orrespondtochangesintheenvironment,theseresponsesusuallytakeplaceoverhoursordays.However,someplantscanmoverelativelyquicklyinresponsetotheenvironment.

Touchtheleavesofthe“sensitive”plant(Mimosa pudica). Thisplanthasanextremelyfastresponsetotouch.Theleavesalsocloseeveryeveningandre-openinthemornings.Haveyouevernoticedflowersdoingthis?

Why might this behavior be advantageous?

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Station G. Feeding by Multicellular AnimalsThings to DiscoverForeachanimalbelow,statewhetherthegutisblindorcomplete:

Seaanemone

Flatworm

Earthworm

Snail

Crab

Rabbit

Whydoesthesurfaceareaofagutmatter?

Whatisaprocessinggutandwhydoesithavetobeacompletegut?

Describetwowaysthatgutsurfaceareacanbeincreased.

Nametwosystemstowhichthedigestivesystemconnects.

BackgroundSingle-celledorganismsfeedbyengulfingindividualfoodparticles,thendigestingeachparticleinside the cell (intracellulardigestion).However,mostmulticellularorganismsplace the food inagutcavity (anopenspaceinsidethebody)wheredigestiveenzymesbreakthefooddownintochemicalsubstancesthatarethendistributedaroundthebody.Whenfoodisbrokendowninside a gut cavity,extracellulardigestionoccurs.

Foodcaptureanddigestiontakesplacebyverydifferentmeansfromanimaltoanimal.Wewouldlikeyoutonoticehow food is capturedandconveyedtothemouthandwhat form the digestive system takesinavarietyofmulticellularanimals.

Multicellularanimalshavearangeofspecializedstructuresforfoodcapture.Someanimalscollectfoodontentacles;othershaveamouthattheendofatubethatcanbepushedoutontothefood(aneversiblepharynx).Some ingest soil anddigest thenutrientswithin it, andothers consumeplantsoranimals.

The form of the digestive system also varies among animals. The twin functions of the digestivesystemaretobreakdownfoodandtoabsorbnutrients.Breakingdownfoodrequiresaninternalcavitytoconcentratedigestiveenzymes.Theinternalcavityorgutcanbeasimplesacorasimpleorconvolutedtube.


Saclikegutsarecalled blind guts,ortwo-way guts.Theyhaveoneopening,andanythingthatcannotbedigestedisreleasedviathemouth.Seaanemoneseatsnails,digestthelivingtissue,thenspitthecleanshellsoutoftheirmouths—tocollectseashells,searchatthebaseofseaanemonesintidepools.

Continuous tubelike guts are called complete guts or one-way guts. They have two openings: amouthandananus.Foodmovesinonedirectionfromentrancetoexit.Somegutsarecalledprocessing gutsbecausetheycontainregionsofthedigestivetractspecialized forparticularfunctions.Forexample,an earthworm gut has a region for storing soil, a region for grinding soil, and another region forabsorbingnutrients.

The rate of nutrient absorption depends on the surface areaofthisregionofthegut.Surfaceareais the two-dimensional area across which nutrients pass from the gut into the blood supply. Theabsorption of nutrientsfromthedigestedfoodtakesplaceacrosstheliningofthegutcavity.Lookatthesaclikegutwehaveopenedflatheretothinkaboutabsorption.Howcouldonehaveasaclikegutwithmoresurfaceareaforagivenvolume?

Whenyouexaminethebodyplanssetoutatthisstation,keepinmindthatmoresurfaceareameansmoreopportunitytoabsorbnutrients;youwillseesomeinterestingwaysthatanimalsexpandthegutsurfaceareaavailableforabsorption.Ofcourse,largeranimalscanhavelargerguts,andthereforelargersurfaceareas,butyoushouldlookforinnovationsingutarrangementsthatdonotnecessarilyrequiremakingtheanimallargertoincreasethesurfaceareaofthegut.

Insmall,simpleanimals,nutrientsabsorbedacrossthegutsurfacediffuseintothebody.Therearenocirculatoryorrespiratorysystemsintheseanimals;theyrelyondiffusiontomovesubstancesintoandoutofthebody.Largerormorecomplexanimalshavearespiratorysystemtocollectoxygen,acirculatorysystemtodistributeoxygenandnutrientstocellsinthebody,andanexcretorysystemtocleansewastesfromtheblood.

At the StationExamine the diagrams and models of guts. Match the models with theanimalthathasthisdigestivepatternandthenfindtheliveanimalwiththesameguttype.Live organisms are in the flumes and at the station. Trythequestionsonthecards;answersareonthebackofeachcard.

Look at the simple tubelike guts. They are limited by the size of the body around them, but ourmodelsshowatleasttwo ways that gut surface area can be increased withoutincreasingthesizeofthebody.Howisthisdone?

Draw a picture of one way to increase the surface area of the gut without increasing the size of the body.


Station H. What Is a Heterotroph?Things to DiscoverHowcanyoutellwhetheranorganism’sdigestionisprimarilyintracellularorextracellular?

Whataretheadvantagesofbeingasingle-celledheterotroph?

Whatproblemsareassociatedwithbeingasingle-celledpredator?

Whatadvantageswouldamulticellularbodyprovideforaheterotroph?

Describephagocytosisinyourownwords.

Whatisacytostome?

Whatisamixotroph?

BackgroundUnlikeautotrophs,heterotrophscannotmaketheirownfood.Aheterotrophmustobtainatleastsomebasicnutrientbuildingblocksbyeatingotherorganisms.Somenutrientscanbesynthesizedinthebody,whereasothers,calledessential nutrients,cannotbesynthesizedandmustbeobtainedfromthediet.Essentialnutrientsvaryfromorganismtoorganism,dependingonwhichnutrientsandaminoacidstheorganismcansynthesizeonitsown.Whenyoueata“completeprotein,”youareeatingasourceofallhumanessentialaminoacids.

Heterotrophsfeedbyconsumingotherorganisms.Heterotrophsaredescribedwithdifferenttermsdependingonwhattheyeatandhowtheycollectit.

Animalscanbeclassifiedbywhat they eat:herbivoreseatplants,carnivoreseatanimals,omnivoreseatplantsandanimals,detritivoreseatdecayingplantoranimalmatter,scavengers eatrefuseorpreyitemsabandonedbypredators


OR

byhow they collect food:predatorscatchandkillliveprey,suspensionfeederscollectfoodparticlessuspendedinwater(thefoodparticlescanbeplantsoranimals),deposit-feederspickupfoodthathasfallen(beendeposited)ontoasurface;thisincludesfoodparticlescollectedfromthesoilsurfaceorthebottomofanocean,lake,orstream.Thefooditselfcanbeplants,animals,ordetritus(decayingplantoranimalmatter).

Organismscanbesinglecelledormulticellular.Hereweshowyousingle-celledheterotrophs.Single-celledorganismstakefoodintotheirbodiesbyengulfingitviaphagocytosis,orbyhavingapermanentcellopeningthatfunctionslikeamouth.Thecellopeningiscalledvariouslyacytostome,anoralgroove,oragullet.

At the StationExaminethepostedpicturesofsingle-celledheterotrophsandthenlookattheliveorganismsusingthedissectingmicroscope.Determinehoweachonetakesinfood.

Ifyouseeaspecimenthatisfeeding,noticewhatiteats.

Observephagocytosisandbeabletodescribeit.

Arethereotherwaysfoodcanenterasingle-celledorganism?Examinetheorganismshereandseeifyoucanobservewherefoodenters.

Youknowthetermsheterotrophandautotroph,butwhatdoyouthinkamixotrophis?Euglenaisamixotroph,soreadaboutitslifestyleandseewhatoccurstoyou.


Station I. Natural Selection on Plant Morphs for Lab 4Things to DiscoverIn lab #4, we will ask you to discover whether there are costs and benefits to possessing a particulargenotype.Youwillworkwiththetwogeneticallydifferentplantmorphsofwildmustard(Brassica)ondisplayhere.Examinetheplantsforafewminutes.Thinkabouttherelativeabilitiesofeachmorphtosurviveundernaturalconditions.

Doyounoticeanydifferencesinvigorbetweenthetwomorphswhentheyaregrownseparately?

Doesitseemthateachmorphallocatesitsenergytobodytissueinthesameway?

Basedontheinformationprovidedwiththeplants,describeoneaspectofplantgrowththatisdeterminedbythegenestheplantcarries:

Describe two aspects of plant growth that you would guess are largely affected by the plant’senvironment:

Plantshavemore thanonepigment.Whydoplantscontaindifferent types of pigmentsusedtoharvestlight?(youmightgethelpwiththisquestioninotherstations)

Theyellow-greengenotypehasapurplecolorationonitsstems, leaftips,andotherparts.Thepurplecolorindicatesthepresenceofthepigmentanthocyanin.Makeaneducatedguessastohowpigment typescouldinfluenceplantgrowthandsurvival.(Again,youmightwaittoanswerthisquestionuntilyouhaveseenotherstations.)

Theyellow-greenBrassicahasatleastsomesmallhairs,calledtrichomes,onthestemsandleavesoftheplantbody.Examinethetomatoplantatthestationtoseetrichomes,andthenseeifyoucanfindanyontheBrassicaplants.

BackgroundHerewepresentplantsthatyouwillstudyinlabtwoweeksfrom now.Theplantsaremustardscalled Brassica rapa; they belong to the crucifer family of plants (Brassicaceae). Crucifers have been bred byhumansformanycenturiestoprovidedifferentfoodstuffs(mustards,turnips,cabbage,andcauliflower,to name just a few). Crucifer populations contain considerable naturally occurring variation amongplants(stemandleafcolor,height,plantshape,etc.)thatresearchershavemaintainedbybreedingand


selectioninthelaboratory.Theplantsyouwillstudybelongtotwo particular genetic types:astandard morphwithdarkgreenleavesandstems,andayellow-green leaf morphwithyellow-greenleavesandpurplepigmentinthestems.Theyellow-greenleafmorphhastwoidenticalcopiesofarecessivecolorgene(homozygousrecessive).

Intwoweeks,youwillassesscostsandbenefitsofeachmorphbyaskingaboutthelikelihoodthataplantwill survive to reproductive age.Youwill compare the twomorphs in termsof thenumbersofoffspringproduced.Inotherwords,wewillaskyoutolookforevidenceofnaturalselectioninaction.Bythetimethislabtakesplace,youwillhaveheardagreatdealaboutnaturalselectioninlecture.However,today’slabincludessomebasicplantbiology,soitprovidesagoodopportunityforyoutothinkaboutplantgrowth.

Theplants you seeare versionsof Brassica rapa developedbyWisconsinFastPlants® togrowandflowerquickly.

Thestandardmorphwillcompleteitsentirelifecycleinabout40days.Germinationoccurs2to3days after planting, and plants will flower within 14 days. The flowers need to be pollinated, but ifpollinationhasbeensuccessful, seedswillbeproduced in17to35days.Seedscanbeharvestedfromnatureseedpodsat40days.

The yellow-green leaf morph has leaves that are much paler in color than those of the standardmorph.Youshouldbeabletomakeaneducatedguessaboutthefunctionalsignificanceofleafcolorbytheendoflabtoday.Theyellow-greenleafmorphisdeterminedbyasinglegene.Thealleleforyellow-greenleavesisrecessive,soyellow-greenmorphshavetwocopiesofthisallele.

The plant stems and leaf tips have a purple coloration due to the presence of a new pigment,anthocyanin. The amount of coloration is determined by growth conditions, so plants grown wherethereisalotoflight,orlowlevelsoffertilizer,havestrongerpurplecoloration.

Therearesmallhairs,calledtrichomes,onthestemsandleavesofsomeoftheseplants.Thenumbersoftrichomespresentdoesnotvarywiththeconditionsunderwhichtheplantisgrown.

Theyellow-greenleafmorphsflower inabout15daysandtake35to45daystocompletethelifecycle.

At the StationExaminetheposteddiagramsandtheflowersontheliveplants.Theseare“perfect”flowerscontainingbothmale(stamen)andfemale(carpel)parts.Themalestamenincludesboththefilament(stalk)andtheantherwherepollenismade.Thefemalecarpelincludesthestigma,style,andtheovarywithovulesinsideit.Findthemaleandfemalereproductivepartsshown.Fromwherewilltheseedpodsdevelopifpollinationissuccessful?

Learntorecognizethepurpleregionsontheplantstemsandleaftips.

Lookfortrichomesonthestemsandleavesoftheseplants.Imagineyourselfasanherbivoreofferedayellow-greenleafmorphorastandardmorph.Whatfactorsmightaffecttheplantonwhichyoufed?

Comparethetwomorphs.Doyouhaveanyideahowtheymightlookiftheyweregrowntogetherwhere they had to compete for resources such as nutrients or light? Could there be consequences todifferences intissueallocationtopigmentsor trichomes?Youdonotneedtoanswerthesequestionstoday,butwhenyouseetheplantsagainintwoweeks,youwillneedtothinkabouttheissuesjustraised.Trytoanswerthequestionsposedinthesectionentitled“Thingstodiscover.”


Station J. Specializations for Feeding on Live OrganismsThings to DiscoverHowdoesasilkwormprocessitsfood?

Howdoesasnailprocessitsfood?

Whatisthedifferencebetweenhomodontandheterodontdentition?

Howdoesoneidentifyavertebrateasanherbivore,omnivore,orcarnivore?

BackgroundHeterotrophsconsumeotherorganismstogetthenutrientstheyneed.Eatinglivingthingsoftenrequiresspecialized feeding structures, but these structures differ in vertebrates (animals with backbones) orinvertebrates(animalswithoutbackbones).

Shortlyyouwilllookatvertebrateskulls.Thehardpartsusedforfeeding(teeth)areeasytoseehere,butwewantyoutolearntonoticemoredetailedaspectsofmorphologyassociatedwithfeeding.Whenlookingatsetofteeth,thefirstquestionshouldbe“aretheyallthesame?”

Animalswithhomodont dentitionhaverowsofidenticalteeth.Theskullsofthegar,thefrog,andthesnakehaveonlyasinglerowofverysimpleteethoneachsideofthejaw.Animalswithteethlikethisdon’tchewbutswallowtheirpreywhole.

Theskullofafishcalledacarphastwo sets of teethintwodifferentlocations;theyarevisiblewhenexamined from the side and from the back. The front set will be obvious, but there is a second set(pharyngealteeth)thatappearjustbeforethethroatbegins.Thecarpcrushespreywiththeseteeth.

Animals with heterodont dentition have a variety of specialized teeth for shearing, crushing, orgrinding.Thekindsofteethpresentdependonthekindoffoodthatwillbeconsumed.

Animals that specialize on certain types of food can be categorized into three groups: carnivores,herbivores,andomnivores.Inmammalsandreptiles,thestructureoftheteethoftenprovidesinformationaboutdiet.

Carnivores eat other animals and thus need to tear through skin and rip flesh off a carcass. Ingeneral,carnivoreteethareconicalandbladelike,sharpandstrong,andfewinnumber.Somemammalshavespecialteethusedforshearingtendonsandskin.Theseanimalsgenerallybitewithaslicingratherthanagrindingmotion.Sharpcanines(“fangteeth”)areusedtograspandholdontoprey.

Herbivores eatplantsandmust copewithhard-to-digest celluloseandotherplantmaterial.Theplantmaterialiscutandthengroundup.Thisrequiressharp incisorsatthefrontofthemouthandheavy grinding teethatthesidesandbackofthemouth.Theincisors, towardthefrontofthemouth,clipoffpiecesofplantmaterial.Themolars, in theback,are relativelyuniform,withbroadsurfaces thatareoftenfoldedtoincreasethegrindingsurfacearea.

Herbivorecuttingteethcanworkinseveralways.Rabbitshavetwopairsofincisors,onebehindtheother.Rodentshaveenamelonlyonthefrontsideofthetooth:thebackedgehasnoenamelandwearsmorequicklythanthefrontside,keepingthetoothedgesharpatalltimes.


Inherbivores,thereisagapinthejawwheretherearenoteeth.Thisisthediastema,aregionwherecaninesandpremolarswouldbeinotheranimals.The jaw of an herbivore can only be used for one function at a time:plantmaterialiscutbytheincisors,heldinthediastema,andthentransferredtocheekteethforgrinding.

Omnivoreshaveverybroaddietsandeatbothplantsandanimals.Whatkindofteethdoyouthinkthey have? You were right if you said a few of everything! Omnivores generally have teeth that areintermediate between herbivores and carnivores, with less specialization. Generalized teeth handle arangeoffoods.

At the StationObserveasilkwormeatingmulberryleaves(naturaldiet)oracommerciallyprepareddiet(whenmulberryleavesarenotinseason)andnoticehowitchews.Usethedissectionmicroscopetogetacloserview;yourTAcanhelp.

Observeascorpion—itmayormaynotbeeatingacricketwhenyouseeit.Lookaroundthecageanddiscoverwhetherscorpionsconsumeentirecrickets.

Look at the skulls:Findtheanimalswith homodont dentitionanddistinguishthemfromtheanimalswithheterodontdentition.Theposteddiagramswillhelp.

Find two sets of teethon the skullof thecarp.Thepharyngeal teetharecoloredhot-pink in thediagram.

Matchthefourkindsofteethshownonthediagramofthedogjawwiththoseontheactualdogjaw.Noticethevariationinshapeandsizeoftheteeth.

Examine the fox skull and be sure you know how to tell it is a carnivore.Don’tbefooledbyskullsize.Theskullsmaybelarge,asinpolarbearsthateatseals,orsmall,asinbatsthatfeedoninsects.

Look at the sheep skull and be sure you know how to tell it is an herbivore.Again,don’tbefooledbyskullsize—miceandelephantsarebothherbivores.


Station K. Nutrition in Saprotrophs and ParasitesThings to DiscoverWhydobiologistssaythatcarbonandothersubstancesare“tiedup”inthebodiesoflargeplants,suchasredwoodtrees?

Whatroledofungiplayinmakingnutrientsavailabletoothercomponentsoftheecosystem?

Canfungimove?

Howdoesafunguspenetrateitshost?

Whatisthetextureofwoodifcellulosehasbeenconsumedandligninremains?

Howdoesaseastarapplydigestiveenzymestoapreyitem?

Whyaretheresmalldriedbodiesoffliesinaspiderweb?

Howcanasoft-bodiedsquidconsumealargecrab?

Whyisthedigestivetractofaleechsimpleascomparedwiththatofanearthworm?

Whataretheprosandconsofingestingnutrientsdirectly,thatis,without“eating”?

BackgroundWhenananimalfeeds,itoftentakeslargechunksoffoodintoaninternaldigestivecavitywhereenzymesbreakdownthefoodintosmallmolecules,whicharethentakenintocellsforfurtherdigestion.Thus,digestionoccursinthebodyofthefeedingorganism.However,anotherwaytofeedistodigestthefoodbeforeitistakenintothebody.Inthiscase,onlysmallmoleculesoffoodaretakenin.Wewouldnotexpectorganismsthatfeedinthiswaytohavedistinctivefeedingstructuressuchasjawsorteeth.Instead,weexpectthemtohavestructurestoexcretedigestiveenzymesandtotakeuptheproductsofdigestion.


Manyorganismsthatfeedthiswayarecalledeithersaprotrophs,whichfeedondecayingorganisms,orparasites,whichconsumepartofalivingorganismwithoutkillingit.

Structuralsupportinterrestrialplantsisprovidedbycelluloseandlignin,bothcomponentsofthecell wall. Both compounds are very resistant to digestion by most organisms, but some specializedorganismscandigestthem.Onesuchorganismisafungus.Thereareagreatmanykindsoffungi:someweknowasediblemushrooms,otherscauseathlete’sfoot,buttheyallfeedbysecretingdigestiveenzymesfromtheirbodiesonto,or into, food items.Youmightknow fungiby their reproductivebodies—themushroomcapwithitssporesunderneath—butthefeedingpartofthebody,themycelium,composedofagreatmanystrandsofhyphaemaybenewtoyou.Ifwelookmorecloselyatthemycelium,wenoticethatitiscomposedofelongatedbranchingstrandscalledhyphae. Hyphaegrowoutwardastheydigesttheirfoodsource.Theydonotmove,buttheyelongateastheygrow.

Fungiincludemushrooms,molds,andvariousagentsofrot.Twocommonlyobservedtypesoffungiarewhiteandbrownwoodrots.Whitewoodrotscanconsumeallcellcomponents,butsomeconsumeligninearlyinthedecayprocess,leavingcellulosebehindaspocketsofverysoftstringywood.Brownrotfungiconsumecarbohydratesincellwallsquickly,leavingremnantsoftheplantrichinlignins.

Fungirequirenitrogenforproteinsynthesisandothermetabolicpathwayssuchaschitinformation.Nitrogencanbetakenupinseveralforms,includingnitrate,ammonium,andorganicnitrogen.Sulphur,phosphorus,potassium,andmagnesiumarealsoneeded,alongwithtraceelementssuchasiron,zinc,andcopper,andsomevitamins.

Plantsproduceinsolublecompounds,suchascelluloseandlignin,thatmostanimalscannotbreakdown.Withoutdigestionbyfungi,theselong-lastingplantproductswouldbeveryresistanttodecay,andtheirconstituentmoleculeswouldbeunavailabletootherorganisms.

At the StationSome fungi are true saprotrophs (feeding on decaying organisms). Examine the specimens andpicturestofindouthowfungiforage.

Certainanimalsfeedinasimilarmannerinthattheyfeedonbodyfluidssuchasblood,ortakeinpredigested food. Parasitic leeches pump blood meals from their hosts, and some predators, such asspiders,seastars,andoctopuses,takeinpredigestedfood.Examinethespecimensandpicturestoseehowliquid-feedinganimalsmanagetodigesttheirprey.Wehavespiders, leeches,andseastarsat thestationbutnoliveoctopus.

Youmayknowaboutanimalparasitessuchasleechesandmosquitoesthatharvestblood,butdidyouknowthereareparasiticwaspsthateatpreyfromtheinsideout?

Notes at the station explain these ideas and guide observations, along with questions on cards for specific examples.


Station L. Respiratory and Vascular SystemsThings to DiscoverWhydoorganismsneedmoistmembranesforoxygenpickup?

Whatistheroleofsurfaceareaintherespiratorysystem?

Howdorespiratoryneedsaffectthebodyplanofanaquaticorterrestrialorganism?

You should also be able to make intelligent guesses to answer the following questions on any organism:

1. How/whereisoxygenbroughtintothebody?

2. Howarethemembranesusedforoxygeninputkeptmoist?

3. Howisoxygentransportedtoeachcell?

4. Whichsystemsinteracttocollectanddistributeoxygenthroughouttheorganism?

1. Background for Surface AreaBothplantandanimalcellsneedoxygen;youshouldknowwhythis istruefromthepre-labexercise.Organisms take in oxygen and expel carbon dioxide in a process called gas exchange. The metabolicprocessesofmostcellsareaerobic(requiringoxygen),soeachcellneedsanoxygensource.Plantsproduceoxygen in their leaves but not in their roots (Stations A and B provide more details). Organisms usesourcesofnaturallyoccurringoxygenintheirenvironment.Oxygenispresentinairataconcentrationof21%andinwateratamuchlowerconcentrationofabout1%.

Oxygen dissolves readily in water, so oxygen from the surroundings (air or water) enters cells bymovingacrossthewetcellmembrane.Theamountofoxygendiffusingacrossthismembraneislimitedbythesurface areaofthemembrane.Oxygendiffuses over very short distances (atmost0.5mm),andsobodyarrangementsmustbringeverycellwithin0.5mmofanoxygensource.

Atfirstglancethisseemslikeaprettylimitingrequirementforanorganism.Iftheorganismismadeofonlyonecelland is spherical, itcan’thaveadiameterofmorethan1mm,even if it iscompletelysurroundedbywater;ifitdoes,theoxygenwon’tdiffuseallthewaytothecenterofthecell.Mostanimalsandplantsaremulticellular,sothecellsmustbearrangedinawaythateachcellcangetoxygen.Ifthecellsarepiledontopofeachother,cellsinthecenterofthepilewillnotgetoxygen,evenifedgecellsarecompletelysurroundedbywater.Onesolutionistomaketheorganismahollowballofcellswithwateronthe insideandoutside.Therearecolonialprotozoanswiththismorphology (lookat theVolvox atStationC),butmostorganismsdon’thavethisshape.

Comparing Surface Area at the StationThedemandforoxygenisdeterminedbythevolumeoftissue(composedofcells)thatneedstheoxygen.Oxygenissuppliedtoacellortissueacrossitssurfacearea.Sowecancalculatearatioofsupply(surface


area)todemand(cellortissuevolume)foroxygen.Thissurface-area-to-volume ratiotellsushowmanyunitsofsurfaceareaareavailabletosupplyoxygenperunitbodyofvolume.Wewillcalculatethisratioforthegreenballonthetable:

Calculation of Surface Area of a Sphere Area(A)=4r2whereristheradiusofthesphere.Theradiusofthegreenball=4.567cm.Thesurfaceareaofthegreenball=4(4.567cm)2=262.1cm2

Calculation of Volume of a SphereVolume(V)=4/3r3(risstilltheradiusofthesphere)V=4/3(4.567)3]=399cm3

Sothevolumeoftheballis399cm3.

Thesurface-area-to-volumeratiooftheballis262.1/399,whichreducestoabout0.66/1.0.Thisratiotellsusthatforeveryunitofvolume,thereare0.66unitsofsurfaceareatosupplyoxygen;

thisislessthanoneunitofsurfaceperunitofvolume.Anorganismwithasurface-area-to-volumeratioof3wouldhave3unitsofsurfaceareatosupplyoxygentoeachunitofvolume,soitwouldbebetteratgettingoxygentoitstissues.

Mostorganismsarenotspherical.Whatisthesurface-area-to-volumeratioforanorganismwithtwolayersofcellsandalotofsurfacearea?Wecanrepresentsuchatwo-layeredanimalwithwoodenblocksofdifferentshapes.Compare thesurface-area-to-volumeratio for thesphere to theratios for the twowoodenblocksatthisstationbyfollowingtheinstructionsbelow.

Calculate the surface-area-to-volume ratio for each wooden block according to directions below. Usethediagramoftheblockbelowtohelpyouwiththemeasurements.Theactuallength,height,andwidthoftheblocksarewrittenoneachblock.

Calculation of Surface Area for the blocksSurfaceareaoftheblockisthesumoftheareasofallsides.Thiscanbecalculatedas:

Surface Area (SA)=2(X×Y)+2(Y×Z)+2(X×Z)

Notethatablockhas6sides,sosurfaceswithsidesXandY,YandZ,andZandXoccurtwiceandareasmustbemultipliedby2.

Calculation of Volume for the blocks

Multiplythethreelengthstogether.Volume (V)=X×Y×Z

Calculation of Surface-Area-to-Volume ratio

Dividethesurfaceareabythevolume.

Ratio=SA/Vor2(X × Y) + 2(Y × Z) + 2(X × Z)] X×Y×Z


Figure 2-27

emac2

Sticky Note

Where is opening bracket?


Record your calculations in the table below:Sphere: Block 1: Block 2:

Surfacearea:262.1cm2 _________________ _________________

Volume:399.0cm3 _________________ _________________

Ratio: Ratio 1: Ratio 2:262.1/399.0=0.66/1

A high ratio means that there are more units of surface area to absorb oxygen for every unit of volume.

Whichmodelhasthehighestsurface-area-to-volumeratio?______________________________________

Whichmodelwoulddiffuseoxygenfaster? ____________________________________________________

Whichmodelwoulddryoutfaster?___________________________________________________________

Didyounoticethatallthreemodelshadthesamevolumebutdifferentratios?

Whathaveyoulearnedaboutshapechangeandsurface-area-to-volumeratios?

Presumablyyounoticedthatathinflatorganismhasmoresurfaceareaavailableforoxygenpickupthanarectangularorsphericalanimalwiththesametotalvolume.Sothinflatorganismswithonlyafewcelllayerscouldsurvivebyabsorbingoxygenacrosstheskinsurfacearea.

What about the requirement that an organism keep its skin wet?Aquaticanimalswouldhavenoproblemwiththis,butonlandananimalwouldbeunabletokeepthesurfaceofeverycellwetforverylongbecausethehigherratiomeansitwillalsodryoutfaster.

How does this apply to actual organisms?The surface-area-to-volume ratio is useful for determining the rate at which oxygen diffuses into anorganism,butitalsohasmanyotherapplications.Movementofanythingacrossasurfaceeithertoorfromtheinteriorvolumeisaffectedbythesurface-area-to-volumeratio.Therateofheatabsorptionbyanobjectplacedinthesunwillvarywiththesurface-area-to-volumeratio.Whenwaterevaporatesfromanobject,therateofevaporationalsowillbeafunctionofthesurface-area-to-volumeratio,aswilltherateatwhichdigestedfoodisabsorbedintothegut.Keepinmindthatthereisnosuchthingasagoodorbadratio;theabsorptionofheatandthelossofwaterwilloccurmorequicklyinastructurewithhighsurface-area-to-volumeratiothaninastructurewithalowsurface-area-to-volumeratio,butthedesiredrate of movement will change depending on the environment of the organism. For example, a seaanemoneoutofwaterneedstoreducetherateatwhichitloseswater,soitclosesuptoreduceitssurfacearea. In the water, anemones do not have to worry about drying out, so they extend their tentacles,increasing their surfacearea, to capture food.Anyparticular shape represents a compromisebetweenconflictingpressures(i.e.,toabsorbheatortoconservewater).

Figure 2-28


2. Background for RespirationWeknowthatmanyorganismshavemorethantwocelllayers,andsomeoftheseorganismsalsoliveonland.Howdotheyrespire?Theanswerliesin the use of special structures forrespiration. Respiratory systemshave two major parts: one part for oxygen pickup (gills or lungs) andanotherpartforoxygen distribution(usuallythebloodinthecirculatorysystem).

The diagram at the right illus-trates three ways oxygen can bepickedupbyanorganism:

(1) itcandiffuseacrosstheskin;

(2) it can be picked up from waterthrough the expanded surfaceareaprovidedbyblood-filledflapsexposedtowater(gills)

(3) itcanbepickedfromairheldininternalcavities(lungs)linedwithbloodvessels.

Anaquaticanimalusesthinflapsoftissue(gills)asthewetmembraneacrosswhichoxygendiffuses.Therefore,gillsneedtohavealotofsurfaceareathatisexposedtothewater;theycanbeflatsheetsorhighlybranchedstructures.Thegillsarefilledwithblood,sooncetheoxygendissolvesacrossthegillsurface,itisboundbycarriermoleculesintheblood(hemoglobinisaredcarriermolecule,butthereisalsoagreencarriermoleculecalledchlorocruorin).Theoxygenatedbloodisthenpassedaroundthebodythrougha finelybranched systemofbloodvessels thatpassby eachcell in thebody.The circulatorysystemoftenhasacontractileregion(asimpleorcomplexheart)thatpumpsregularlytokeepthebloodmovingfromthegillstothetissuesandbacktothegills.

Inanopencirculatorysystem,illustratedbelowbyagrasshopper,thebloodrunsintubesinsomepartsofthebody,butnotinallpartsofthebody.Insomeregions,thebloodslopsaroundinsideacavity.Inaclosedcirculatorysystem,illustratedbytheearthwormbelow,thebloodrunsintubesthroughouttheentirebody.


Grasshopper

Earthworm

©Lim2008.Usedwithpermission.

Figure 2-29

Figure 2-30


Respiration at the StationThere are respiratory system models at this station. Pair each model with the living or preservedorganismwiththesamebodyplan. Try the questions on the quiz cards; the answers are on the back of the cards.

There will be a discussion on interactions among plants, animals, and fungi at the end of class. Therewillbea30-minutediscussionattheendofthelabwhereyoumayclearupanytopicsofconcernandput thenewmaterial intocontext.Membersof theclasswill contribute to theconstructionofaconceptmaptosummarizeallthematerialcoveredhere.

Population GrowthGoals and ObjectivesAt the end of this laboratory you should be able to:

1. Describehowpopulationsizeatanytimeperiodisestimated.

2. Explainhowbirthratesanddeathratesinfluencepopulationsize.

3. Describethefiniterateofincreaseforapopulation.

4. Describehowpopulationdensityisestimated.

5. Explainintraspecificcompetitionanditsrelationshiptodensity.

6. Explainwhatitmeansforaprocess,suchasbirthrate,tobedensity-dependent.

7. Developatestablehypothesisandexplainhowitcouldbetested.

8. Describethefoursectionsofascientificreportandtheircontents.


In lastweek’s labyoudiscoveredhoworganismsacquiretheresourcestheyneedtosurviveandgrow.Acquired resources are also used in reproduction. Organisms that fail to reproduce do not place anycopies of their genes in future generations—their genetic lineage ends when they die. The genes ofreproducingorganismsarepresentinthenextgeneration,andthesegenescontinuethroughtimeaslongasthedescendantorganismscontinuetoreproduce.

Reproductioncanbeassimpleasdividingthebodyinhalf(asexualreproduction),oritcanrequireamorecomplicatedprocessofproducinghaploidgametesandmating(sexualreproduction).Thekeypoint is thatnew individualsareproduced; thebirthofoffspringconstitutes thebeginningofanewgeneration. Intheabsenceofpopulation-limitingfactors (suchasstarvationordisease),apopulationstartedwithasmallnumberofindividualstypicallywillincreaseinsizewitheachgeneration.Biologistshave several ways of keeping track of population growth over time—you will explore some of themtoday.

Lab 3

71

72 Lab3: PopulationGrowth

Now,assumethatevery individual produces one offspringforeachtimeperiodthatitisalive.Thetablebelowshowsthefirsttwogenerations:offspringareshowninredandarrowsindicateproductionofnewoffspring.Itmightseemoddtolistnewindividualstotheleft,butyouwilllaterseewhyitisdonethisway.AtT1thepopulationhastwoindividuals,thesurvivorandoneoffspring.AtT2,boththesurvivorandoffspring1haveproducednewindividuals,sothereare4organisms.

Figure 3-1

Ntindicatesthenumber of organisms aliveateachtimeperiod(forNtthesubscripttindicatesthetimeperiod)andincludesbothsurvivorsandnewbirths;N0equals1organism(thismeansthatattimezerothereisoneorganism),N1equals2organisms,andN2equals4organisms.Thetwo columns on the right of the table are for keeping track of the number of individuals in the population (accountingcolumns).

Tokeeptrackofbirthsinfuturegenerationsweneedtwo new accounting columnsinourtable.Onecolumnisforthenumberofindividualspresentinthepreviousgeneration:weneedthisbecausetheseareorganismsthatwillgivebirthtonewindividuals.Theothercolumnistokeeptrackofthenewbornindividuals.Nt–1 representsthenumberofindividualsinthepreviousgeneration(t-1indicatesthetimebeforethepresent),andBrepresentsthenumberofbirthsbetweenNt-1andNt..

WeseethatNt-1 + B = Nt.Toputthisequationinwordsyouwouldsaythatthenumber of individuals in the previous generationplusthenumber of birthsequalsthenumber of individuals in the present generation.

Figure 3-2

Figure 3-3

Imagineoneindividualthatlivesforthreetimeperiods(days,months,oryears,dependingontheorganism).WedesignatethestartingtimeperiodasT0,andthesubsequentperiodsasT1,T2,T3,andsoon.Wecanrepresentthelifeofthefirstindividualwithatablewherea1 indicates survivalateachtimeperiodandXindicatesdeathinT3.

Lab3: PopulationGrowth 73

Noticethatfournewcolumns(*)wereaddedtothetabletoaccountforthebirth of new individualsfromthefoursurvivinganimalsatT2.Noticealsothatthefirstindividualdied,asindicatedbyanX.

We see that Nt–1 + B – D = Nt. To put this equation in words you would say that the number ofindividualsinthepreviousgenerationplusthenumberofbirths,minusthe number of deaths,equalsthenumberofindividualsinthepresentgeneration.

To explore population growth, follow the directions below to complete the population worksheet and the accounting table on page 78. Afteryoucompletetheworksheet,youwilluseyourresultstoanswerquestionsontheelectronic worksheet.

Yourtaskistorecordthebirthsanddeathsinthispopulation,startingfromoneindividual,overtimefromT0toT6.Thestepsaresimple,butyouneedtobemeticulousasyourecordbirthsanddeathsateachgeneration.

T0andT1havebeendoneforyou,sopulloutpage78andfollowalong—thefirstindividualisshownontherightsideofthechart.ItlivesforthreetimeperiodsandthendiesinT3,asshownbytheX.ThefirstindividualgavebirthtoitsfirstoffspringbetweenT0andT1,asshownbythearrowfromthefirstindividual to the offspring marked in the yellow box in T1. The original individual is considered a“survivor”andismarkedwitha“1”foroneindividual.

Eachnewbornshouldbemarkedinawaythatmakesiteasytosee.Soeachtimeanorganismisborn,circleitorwriteitwithadifferentcolor.Thennoteitspresenceassurvivorfortwomoretimeperiodsbywriting1sbelowthenewborn.Survivorsarenotcircledorcolored.MarkadeathbyplacinganXinthefourthtimeperiod.Exceptforcirclingnewborns,thisprocesshasbeencompletedinyourtableforthefirstfourorganismsinthepopulation.

AtT1,therearetwoindividuals—eachonewillsurvivetoT2andgivebirthtooneoffspring.Fill in the accounting table for T2,butignorethelastthreecolumnsfornow.Alwaysincludeallindividualspresentatatimeperiod,whethertheyaresurvivorsornewborns,inyourcountsforNtandNt-1.

Didyougetthesequence2,2,0,4fortheaccountingsectionatT2?Ifso,youareontherighttrack;ifnot,pleasetryagain.

Now,usingarrowstoindicateabirth,diagramthebirthsforT3.Markabirthbyplacingnewbornsinto the next yellow space to the leftoftheorganismgivingbirth(thespacesaremarkedinyellowsothattherewillbeenoughspaceforallbirthsanddeathsinT0toT6).Youwilldrawanarrowforeachbirth—therearetwoarrowsmarkedintothechartforthetwobirthsinT2.Use a ruler to make sure you are in the right time period.

Continuetocirclethenewbornsoruseadifferentcolortodistinguishthem.Markthelifespanofeachorganismasitisbornbyputting1sinthecolumnbelowit.Countthenumberofcircled1s(oronesofadifferentcolor)ateachtimeperiodtofindthenumberofbirths.MarkeachdeathusinganX.CountthenumberofXsateachtimeperiodtofindthenumberofdeaths.

Fill in the left four columns of the accounting table for T3. You will fill in the remaining threecolumnsasyoudotheelectronicworksheet.

ProceedinthesamemanneruntilyoufinishT6(thiswilluseallsquares)fortheworksheetandtheaccountingtable.

Ourimaginaryorganismlivesonlythreetimeperiods.Howdowetakedeathintoaccount?Wecanaddanothercolumntotherightsideofthetablefordeaths(D)andanotherrowtothetableforT3.

Figure 3-4


Pre-lab Questions based on the population worksheet and accounting table. Record your answers on the online version.

Examinetheleftfourcolumnsofyourcompletedaccounting table.

1. Comparethecolumnofnumbersrecordedforthepopulationattheprevioustimeperiod(Nt-1)withthecolumnofnumbersforbirths(B)after T0.Whyarethetwonumbersequencesthesame?

2. Comparethecolumnsofnumbersyourecordedforbirth(B)anddeath(D).Lookforashift in the startofthesequencebetweencolumns;suchashiftiscalledatime lag.Howlongisthetimelagandwhyisitthere?

3. Without drawing in any more generations on the worksheet, complete the left four columns in theaccounting table for thenext twotimeperiods,T7andT8by using what you noticed about thenumbersequencestopredictpopulationsize(projectpopulationgrowthintothefuture).

(a) HowmanyindividualsshouldbeborninT8? __________________________________________

(b)HowmanyindividualsshoulddieinT8? __________________________________________

(c) Whatisthepopulationsize(Nt)forT8? __________________________________________

Clearly,itwouldbetedioustokeeptrackofgrowthinlargepopulationsusingthesemethods;laterin your academic career you will learn about mathematical models used to follow populationgrowth.

4. Wecanplotpopulationsizeasafunctionoftime.

(a) UsedatafromyouraccountingtabletodiscovertherelationshipbetweenpopulationsizeandtimeinFigure 3-5 onthenextpage.UsedatafromT0toT8.

List of Terms and Symbols

Term Definition

T0 Timeingenerationswherethesubscript0indicatesthefirsttimeperiod.Forexample, T1isnextgeneration,thenT2.

Nt Numberoforganismsaliveateachtimeperiodrepresentedbythesubscriptt.

B Numberofbirthsbetweenthepreviousgenerationandtheparticulargeneration beingexamined(fromNt-1toNt).

Nt-1 Numberofindividualsinthegenerationprior tothegenerationbeingexamined, wherethesubscriptt-1indicatesthetimeperiodbeforethepresent.

D Numberofdeathsinthecurrentgeneration.

Nt+1/Nt Finiterateofincreaseortheper-individualgrowthrateacrossanytwotimeperiods.

Density Numberofindividualspresentinacertainarea.


5. ExamineFigure 3-5.Noticehowquicklythepopulationincreasedandhowmanyindividualsweredescendedfromoneorganismineightgenerations.

(a) Doesitseemasthoughrealpopulations(imaginerabbits,bacteria,orcornplants)increasethisquickly?

(b) Whatevidencedoyouhavethatsomepopulationsdonotgrowlikethis?

6. Densityisthenumberofindividualspresentinacertainarea.Imagine that the population you projected from T0 to T8 lived in an area of 10 m2. Atfirst,thepopulationhadadensityof1organismper10m2(1/10)or0.1.Bythesecondtimeperiodthedensitywas0.2.Calculatingthedensityallowsyoutoestimatetheaverageamountofspaceanorganismhasinwhichtolive,collectfood,andproduceoffspring.

(a) CalculatedensityfromT0toT8;recorditinthelastaccountingcolumn.

(b) Onaverage,how much spaceisallottedtoeachorganismatT1,T3,andT8?

T1:______________________; T3:______________________; T8:______________________

7. Whenorganismscompete with members of their own species for resources,suchasfoodorlivingspace,wecallthisintraspecific competition.

(a) HowwouldintraspecificcompetitionlikelychangefromT0toT8,giventhedensitiesyoucalculated?

(b) Wouldtheinitialamountoffoodpresenthaveaneffect?Explainyourreasoning.

8. Biologiststalkaboutcertainprocessesasbeingdensity-dependent;bythistheymeanthattherates,suchasbirthrate,deathrate,ornetrateofchangeinpopulationsize,areaffectedbythedensityoforganismsalreadypresentateachtimeperiod.(Forexample,foodishardertofindasmoreanimalslookforit,sodeathbystarvationiscommonathighdensity.)

Typically,ifdensity-dependentprocessesact,apopulationwillgrowmoreslowlyduringlatertimeintervals[whereN(populationsize)ishigh]thanitdidduringearlytimeintervals(whereNwaslow).ExamineFigure3-6:notice thattherateofpopulationgrowthslowsafter30timesteps.Populationsizelevelsoffasdeathratescanceloutbirthratesathighdensity.

9. Howdoesthiscomparewithwhathappenedintheplotfromourimaginarypopulation(Figure3-5)?

Figure 3-5

(b) WhattypeofgrowthdoyouseeintheplotofNtacrosstime?


10. Youraccountingtablecontainsacolumnforthenumber of individuals in the next generation(Nt+1).FillinNt+1 bylookingaheadonetimesteptoseehowmanyindividualswillbepresentuptoT8. Then, divide Nt+1 by Nt and place this newnumber(Nt+1/Nt)intotheappropriatecolumninyourtable.

Nt+1/Nt is called the finite rate of increase becauseitcalculatesthenumberofnewindividualsproducedperexistingindividualduringthattimeperiod.Youhavejustcalculatedthefiniterateofincrease for several one-day time intervals.However, the time interval can be any unit (i.e.,oneday,oneweek,oneyear,threeyears,etc.)

(a) Usingyourworksheetdata,calculatethefiniterateofincreaseoveran8-daytimeintervalfromT0toT8:______________________

(b) Usingyourworksheetdata,calculatethefiniterateofincreasefortwo3-daytimeintervals,fromT0toT3andfromT3toT6.

T0toT3:__________________________;T3toT6:________________________

Whatdoyounoticeaboutthesevaluesforfiniterateofincrease,eventhoughtheycomefromthesamesetofdataonthesamepopulation?

Tocomparefiniterateofincreasevaluesthatwerecalculatedoverdifferenttimeintervals,wecancalculateaper-day finite rate of increase. Forexample,hadyoucollecteddataevery5days,youwouldcalculateaper-dayfiniterateofincreasebyraising(N5/N0)tothe1/5power(i.e.,(N5/N0)

1/5).YourscientificcalculatororMicrosoftExcelshouldbeabletodothiscalculationforyou.

You have just calculated three differentfiniteratesofincrease—thefirstonewasfor8days,andthesecondtwowerefordifferent3-dayperiods.

(c) Usingtheper-dayfiniterateofincreasetocompare the finite rates of increase forthese three time intervals. Show your work.

(d) Dotheratesdiffer?Whyorwhynot?

(e) What would be the finite rate of increaseforapopulationthathasleveledoffatitscarryingcapacity(K)?

11. Wecancalculatethe finiterateofincreaseasafunction of population size for the imaginarypopulation.

(a) Prepare Figure 3-7 by placing the finiterateofincrease(Nt+1/Nt)onthey-axisandthepopulationsize,Nt,onthex-axis.

(b) Doestherateofincreaseslowdownasthispopulationgetslargerandlarger?

Figure 3-6

Figure 3-7


12. Iftherateofincreasedidslowaspopulationsizeincreased,itwouldindicatethatdensity-dependentprocesseswereacting.Density-dependentprocesseswereactinginFigure3-6,soifweuseddatafromthispopulation tomakeanewversionofFigure3-7,wecouldseehowtheplotwould lookwithdensitydependence.

(a) MakeFigure3-8withNt+1/Ntonthey-axisandNtonthex-axisonthispage.Todothis,youmustestimatepopulationsizefromFigure3-6forthetimestepslistedinthetablebelow.Toestimatepopulationsizes,make your best guessofpopulationsizefromthegraphforeachtimesteplistedinthetable.Twotimestepshavebeenfilledinforyou.

Time Step Estimated Nt Nt+1/Nt

10days 5 10/5=2

15days 10

20days

25days

30days

35days

40days

45days

50days

(b) ComparetheshapesofthecurvesinFigure3-7andFigure3-8.Inyourownwords,describethemaindifferencebetweenthetwoplotsandexplainwhytheyaredifferent.

The laboratory exercise begins on page 79.

Figure 3-8


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79

Lab 3: Population GrowthToday you will design and carry out a study on population growth. Background knowledge for yourstudywasprovidedinthepre-lab,butyoumaystillhavequestionsaboutsometopicssuchasdensity-dependentgrowth,ortherelationshipbetweenfoodsupplyandratesofbirthanddeath.Labwillbeginwithadiscussionwhereyoumayaskaboutissuesfromthepre-labanddiscussthequestionsbelow.

Questions for Group-Brainstorming1. Whatfactorscontrolbirthratesinnonhumanpopulations?

2. Whatfactorscontroldeathratesinnonhumanpopulations?

3. Howmighttheamountoffoodavailableateachgenerationinfluencepopulationsize?Fill inthetablebelowwithyourpredictionsfornonhumanorganisms.

Food level Acts on birth rate? Acts on death rate? Acts on life span?

Increasing

Decreasing

Constant

4. Howcanhumansrespondtochangesinresourcesinwaysthatotherorganismscannot?

5. Giveanexampleshowinghoworganismscompeteforfoodorspace.

6. Giveanexampleshowinghowdeathratesaredensity-dependent.

7. Giveanexampleshowinghowbirthratesaredensity-dependent.

Donotallowthediscussiontoenduntilyouhaveabasicunderstandingofpopulationgrowthratesoverseveralgenerations.Youshouldknowhowbirthanddeathratesaffectpopulationgrowth,andyoushouldbeawareofthewaythatresourcelevels(foodandspace)affectbirthanddeath.Youshouldalsohaveabasicunderstandingofdensitydependenceandintraspecificcompetition.

Todayyouaregoingtodevelopa testable hypothesisonsomeaspectofpopulationgrowth.Youwillusetheavailablepopulationsofsingle-celledorganismstotestyourhypothesis.Anhypothesisproposesa relationship between an event and its cause. For example, suppose you are interested in how fastpopulationsgrow.Itseemslikelythatagrowingpopulationwillrunoutoffoodeventually,causingittogrow more slowly. One hypothesis is that the rate of population increase will be slower in olderpopulations than in younger populations because old populations will have less available food. Thishypothesisistestable—youcanmeasuretheratesofincreaseinoldandyoungpopulationsandseeiftheydifferinthewayyourhypothesispredicts.

TodayyouwillbeworkingwithaprotozoancalledBlepharisma. Blepharismaisasingle-celledeukaryote.Itsbodyiscoveredwithciliaforswimmingthroughwater.Itfeedsonbacteriafromthewater—bacteriaaredrawnintothe“mouth”(buccalcavity)anddigestedinfoodvacuoles.

ApopulationofBlepharismaincreasesaseachorganismdividesinhalf—wecallthisprocessfission.Fission represents asexual reproduction because the new physiologically independent individual isgeneticallyidenticaltotheold.Incontrast,sexualreproductionproducesnewindividualsthatarenot


genetically identical to their parents (new genotypes). Blepharisma isabletoproducenewgenotypesundersomeconditions,butthefocusfortoday’slabisonpopulationincreaseacrossmanygenerationsbyfissionalone.Blepharismadivideevery8hoursgivenadequatefood.

In lab today, we have cultures of Blepharisma that were started with the same number of “founder” individuals but have been growing for different lengths of time (cultures are 4, 7, 10, 14, 21, and 30 days old). For each age, we have cultures that were started at three nutrient levels (low, medium, and high).NutrientlevelsintheculturemediumaffectthegrowthofbacteriaandthustheamountoffoodavailabletoBlepharisma.

Thepre-labexerciseshouldhavepreparedyoutothinkingeneraltermsaboutpopulationgrowthratesinrelationtofoodlevels,spacelimitation,density-dependentbehaviors,andintraspecificcompetition.Herearesomeofthequestionsyoushouldbeconsidering:Howwouldyouexpectthepopulationstogrowatdifferentnutrientlevels?Willthere be a period where population size increases rapidly (andequivalently)acrossnutrientlevels?Willthepopulationeverreachasteadystatewherebirthequalsdeath?

However,inadditiontothe“basic”populationdynamics,thereisaveryunusualformofintraspecificcompetitionthatoccurs inBlepharisma. Inculturesthathavebeengrowinga longtime(oldcultures),some Blepharisma develop into large cannibal morphs (types) that feed on smaller Blepharisma. Thecannibalmorphsareabouttwicethesizeofatypicalindividual(seethepictureabove).Theswitchtoacannibalmorphcouldbearesponsetopopulationdensityitself,oritcouldresultfromfoodshortage.Perhapsthesetwohypothesescouldbeteasedapartusinganexperiment.

Designing an ExperimentWhataspectofpopulationgrowthinterestsyou?Wouldyouliketoseehowquicklypopulationsincrease,orwhetherpopulationgrowtheverslowsdown?Doyouwanttoseehownutrientlevelsaffectpopulationgrowth?Doyouwanttounderstandthemechanismsthatcausecannibalmorphstodevelop?Yourtasktodayistochooseanareaofinterest,developaworkinghypothesisaboutthisareaofinterest,makeapredictionbasedonyourhypothesis,andthentesttheprediction.

Asanexample,supposethatyouareinterestedinknowingwhetherpopulationgrowthslowsdownasorganismsrunoutoffood.Fromthepre-labyourememberthatNt-1 + B – D = Nt,soiffoodshortageleads to an increase in the death rate in the longest-lived cultures, population size (N) will not increasemuch,infactitmightevendecrease,inoldcultures.YoucouldmeasureNbycountingtheprotozoanspresentafterdifferenttimeperiods.

Atwhichculturesshouldyoulook?Youareinterestedinfindingoutwhetherfoodshortagecauseschangesinpopulationgrowthrate,soyoucouldcompareold(long-lived)cultures,wheremuchofthefoodhasbeeneaten, toyoungcultures thatshouldstillhave food. It isalsopossible tocompare lownutrientculturestohighnutrientcultures.Thelownutrientculturesbeganwithverylittlefood,sotheymight reach starvation levels sooner. Both comparisons could be useful—it depends on what you arestudying.Below isone exampleofhypothesisdevelopmentand testing.Keep inmind thatyouwillbeformulatingyourownscientificreportincludingthefollowingcomponents:

A working hypothesis is: Newlyestablishedpopulationswillhaveamorerapidnetrateofgrowththanolderpopulations.

Youwillalsoneedanull hypothesis—astatementofwhatyouwouldseeifyourhypothesisisnotcorrect.Itisastatement of no differencebetweenthevariablesyouaregoingtomeasure,orastatementofnochange,ornoeffect.Here, thenullhypothesis is:net ratesofpopulationgrowthare thesamefornewlyestablishedandolderpopulations.

Figure 3-9

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As you formulate your hypothesis, look at the types of Blepharisma cultures that we provide—you have to have a hypothesis you can test using the given cultures.

A prediction of the hypothesis is: Newlyestablishedpopulations(4,7,10,and14daysold)willhavegreaterfiniteratesofincreasethanolder(21or30daysold)populations.

Methods to test the prediction: Youpredictedthatyoungpopulationswill increasemorequicklythanoldpopulations.Howwillyoumeasurethis?Tomeasureratesofpopulationincreaseyouhavetocomparepopulationsizeoverseveraltimeperiods.Ideally,youwouldrepeatyourcountsofindividualswithinasetofpopulationsovermanydays,butthisisimpracticalwithinourshortlabperiods.Instead,youwillcountindividualsfromasetofpopulationsthathavebeengrowingfordifferentnumbersofdays,andyouwillassumethatthis“snapshot”samplefromsimilarpopulationswillgiveyouthesameresultaswouldmonitoringasinglepopulationovertime.

Recallthatthefiniterateofincreaseperdayis(Nt+1/Nt),raisedtopowerof1overthenumberofdays.Toestimate the finite rateof increase, youwillneed toknowthepopulation sizes (Nt) for each timeperiod. You could count protozoans to find these numbers. So, if you count protozoans to estimatepopulationsizeat4,7,10,14,21,and30days,youcouldcalculatethefiniterateofincreasefor5timeintervals:4to7days,7to10days,10to14days,14to21days,and21to30days.

Rewrite your prediction in mathematical terms sothatyouwillknowexactlywhatnumbersyouwillneedandhowyouwillusethem.Forourexample,we predict that per-day rates of finite increase will decline over time. Numericallythismeansthat(N7/N4)1/3 > (N10/N7) 1/3 > (N14/N10) 1/4.

Ifthispredictionismet,wewillacceptourworkinghypothesis.Ifthispredictionisnotmet,wewillrejectourworkinghypothesis.

Our null hypothesis is that (N7/N4) 1/3 = (N10/N7) 1/3 = (N14/N10) 1/4. Our null hypothesis is a

mathematicalstatementthatthereisnodifferenceamongrates.Methods:Describeexactlywhatyouintendtomeasuretodayinenoughdetailthatanotherstudent

atyourskilllevelcouldrepeatyourmethod.Oneexampleis:IwillestimatethenumberofBlepharismain1 ml samples taken from 4-day-old cultures, 7-day-old cultures, 10-day-old cultures, and 14-day-oldcultures.Toestimatethisnumber,Iwillcountprotozoansin0.03mldropsexaminedunderadissectingmicroscopeat10×magnification.Iwillcalculatetheper-dayfiniterateofincreaseasNt+1/Nt

1/daysfor3timeintervals(4to7days;7to10days,and10to14days).

Youmayworkonyourexperimentsinpairsoringroupsofuptosixpeople.However,large groups must work on large projectsthatrequirealotofsamplestobecounted.Youmustdescribethepartoftheexperiment you did and list data collected by each contributor. See the section on replication underdesigndetails2onpage85.

Note:Countingprotozoansistheeasypart,butifyoumeasurethewrongthing,youwillnotgetananswertoyourquestion.Timespentthinkingabouteachstepintheexperimentaldesignandanalysiswill be worth it, so discuss your experiment with classmates and the TA before you count anything.Informationonhowtocountprotozoansisbelowinthesectionondesigndetails.BringquestionstoyourTA.

Experimental results: In this section you will report the results of your study by presenting thenumbersandbymakingagraph.Forexample,iftheper-dayratesoffiniteincreaseforN7/N4,N10/N7,andN14/N10were2.5,1.2,and0.7,respectively,youcouldplottheratesagainsttime.Youwouldreportyourprediction:N7/N4

1/3>N10/N71/3>N14/N10

1/4.Youwouldstatethattheresultswereaspredicted,so

youaccepttheworkinghypothesisandrejectthenullhypothesis.Youwouldnotknowwhetherfoodshortagecausedthispattern.Ifyouhadadifferentsetofresults,youmightrejecttheworkinghypothesisandacceptthenull,orrejectbothhypotheses.Youwouldrejectboththeworkinghypothesisandthenullhypothesisifthepatterninyourresultswasnotconsistentwitheitherprediction.

Theratesofincreaseintheexampleseemquitedifferentfromoneanother,butsupposetherateswere2.1,1.9,and2.0.Youmightwonderhowtotellifthenumbersarereallydifferentfromeachother.Are thedifferencesyouobservedsignificantor trivial?Eventually, youwill takeaclass in statistics toanswerthisquestionquantitatively,butfortoday,reporttheresultsattheirfacevalueanddiscussanyquestions you may have about significance with your TA. We will return to the issue of elementarystatisticaltechniquestodeterminewhetheradifferenceissignificantinlaboratory4.


Discussion:Inthediscussionsectionofthewrite-up,resultsaresummarizedandconclusionsmaybedrawn.IftheratesoffiniteincreaseforN7/N4,N10/N7,andN14/N10were2.5,1.2,and0.7,respectively,youcouldsay:“RatesoffiniteincreasedeclinedwithageinpopulationsofBlepharismagrownincultureupto14days.Thechangewasinthedirectionpredictedbythehypothesisandwasconsistentwiththeideathatfoodshortagesreducegrowthratesovertime.”

Itisimportanttonoticethatresultsfrom your dataareconsistentwiththeideaoffoodshortage,butyoudo not know how much food was leftbecauseyoudidnotmeasureit.Youdonotknowwhetherthelowerratesoffiniteincreasewerecausedbymoredeathorfewerbirths(fissionevents),inolderpopulations,butyourinitialhypothesiswassupported.Ifyouwantedtostrengthenyourstudy,youcouldcountthebacterialpopulationsintheculturesateachtimeperiod(thereisnotenoughtimeforthisinclasstoday).In your discussion, you could mention that you assume that food levels were lower in older culturesbecausenonewfoodwaseveradded.Ifyourresultsleadyoutorejecttheworkinghypothesis,youcouldproposeamodifiedworkinghypothesiswithnewpredictionsforthefuture.Outlinethisbrieflyinyourdiscussion.

Using the Microscope to Collect DataTodayyouwilllearntouseadissectingmicroscope.Itmightbeagoodideatobookmarkthispagesothatyoucanlocateiteasilylateron.

Figure 3-10PhotobySamWoo,courtesyofUCDMediaworks


Rules

a. Alwayscarryanymicroscopewithtwohands.Microscopesareheavyandexpensivepiecesofequipment.

b. Neverturnamicroscopeupsidedown.Theeyepieceswillfallout!!

c. Alwaysuselenspaperandacleaningsolution(e.g.,Windex®)tocleanlenses.NeveruseKimwipes®(theywillpermanentlyscratchthelens).

Practice with the Dissecting Microscope

1. ObtainapieceofpaperwithawordwrittenonitfromyourTA.

2. Placethepieceofpaperonthecenterofthestageofthedissectingmicroscope.

3. Settheeyepieceadjustmentringonthelefteyepieceto0.

4. Turnontheilluminator,andadjustthetwoendssothattheyshineonthepaper.

5. TheSMALLKNOBontherightsideofthemicroscopeisusedtoadjustthelevelofmagnification.Initially, set this knob at 1. It is much easier to focus initially on a small object at lowmagnification.

6. UsetheLARGEKNOBSoneithersideofthemicroscopetofocusthemicroscopeontheword.

7. Whenyoucanseetheword,movethetwoeyepiecestogether(orapart)untilthetwoimagesmerge,andyoucanseethewordwithBOTHeyesatthesametime.

8. Ifyoufindthatoneeyeisinfocusbuttheotherisnot,closeyourlefteye,andlookthroughtherighteyepiecewithyourrighteye.Usethefocusknobtobringthewordintosharpfocuswithyourrighteye.Thenopenyour left eye, and turn theeyepieceadjustment ringon the left eyepieceuntil theimageisalsoinfocususingthelefteye.Atthispoint,youshouldseeasinglesharpimageusingbothofyoureyes.Ifyoudonot,repeatsteps3through8again,untilyoudo.

Thewordonthepieceofpaperis ___________________________________________________

9. Tochangethemagnification,lookthroughthemicroscope,turnthemagnificationknobawayfromyou(to0.9)andtowardyou(to4).Theimageofthewordshouldremaininfocus,eventhoughyouarechangingthelevelofmagnification.

Measuring Body Size in BlepharismaDistinguishingthecannibalmorphsfromtheregularorganismstakessomepractice,soyouwilllearntocomparebodysizesamongspecimens.Recallthatthecannibalmorphsareapproximatelytwicethesizeoftheregularmorphs.Tomeasurethecells,youwilllearntouseatinyrulerintherighteyepieceofyourmicroscope.Thetinyruleriscalledanocular micrometer.

Using the Ocular MicrometerThemicrometermustbecalibratedbeforeitsfirstuse.Themicrometeris100unitslong,numberedatintervalsof10units. Inthisexercise, theabbreviationcu (forcalibrationunits) refers tounitsonthemicrometer.


1. Calibration

a. Setthemagnificationknobat4.PlaceapieceoftapelooselyovertheknobtoremindyouNOTtochangethemagnificationduringcalibration(ifyoudo,youwillneedtostartover).

b. Placeapieceofwhitepaperonthestage,andthenplacethemetalruleronthepaper,sothatthelettering on the ruler is right side up. The bottom of the ruler is marked with black lines atintervalsof0.5mm; this is the sideof the ruler that youwilluse for calibration.Because thecenterofeachoftheblacklinesoccursat0.5mmintervals,thedistancefromthebottomleftcorneroftherulertothecenterofthefourthblacklineonthebottomoftheruleris

__________mm

c. Close your left eye. Looking through the righteyepiecewithyourrighteye,shifttheruleronthestageuntilthebottomoftherulerisparallelwiththemicrometerscale,andthebottomleftcorneroftherulerislocateddirectlyabovethe0markonthemicrometer.Youshouldnowseesomethingsimilartothediagramontheright:

Without moving the ruler, locate the fourthmark from the left on the bottom of the ruler,and estimate the position of the center of thisfourth mark. Record the number of calibrationunitsatthislocation:__________cu

d. To complete the calibration process, computethefactorrequiredtoconvertunitstomillimeterswith this particular microscope, at this level ofmagnification.

Conversionfactor=c= lengthoftheobjectinmm

lengthoftheobjectincu

Inthiscase,thedistancebetweentheleftcorneroftheruler

andthefourthlineonthebottomoftheruler=___________mm,

andthissamedistanceincalibrationunits=___________cu,

soforthislevelofmagnificationandthismicroscope,c=___________.

Onceyouhavecomputedtheconversionfactor,youcanconvertcutomm.Toobtainthelengthoftheobjectinmm,youwillmultiplythelengthofanobjectincubyc.

Forinstance,ifmm/cu=c=0.035,andifanobjectis50cuinlength,thenthelengthoftheobjectinmm=50cu×0.035mm/cu=1.75mm.

2. Measuring an object using a micrometer

a. Removetherulerfromthestageandreplaceitwiththepieceofpaperwiththewordonit.

b. Measurethelengthofthewordusingthemicrometer.Thiswordis___________micrometerunitslong.

c. Calculatethelengthoftheword,inmm,basedontheconversionfactorabove.Thewordis___________mmlong.

d. CheckyouranswertoquestioncwithyourTA.AskyourTAforhelpifyouarehavingproblemswithcalibration.

©Stamps2008.Usedwithpermission.

Figure 3-11


Measuring BlepharismaTakeadropoftheyoungBlepharismacultureandmeasurethreeorganismstogetanaveragelength.

Set the magnification knob on the dissecting microscope to 4 and tape it in position. Use theconversionfactorabove:

c=______________________

EstimatebodylengthforthreeyoungBlepharisma:

______________________; ______________________; ______________________Takeasamplefromtheoldestcultureandfindthelargestanimals.Measurethelengthofthreelargeanimals:

______________________; ______________________; ______________________

Basedonyourbodysizeestimates,weretheseorganismsthecannibalmorphs?_______________________Ifnot,searchfurtherbecauseyoumustlearntocorrectlyidentifythetwomorphs.

A Few Design Details

1. Youaregoingtocountprotozoansfromsmallsamplesoftheculture.Beforeyoutakethesesmallsamples,homogenizetheculturebyinsertingthepipetteintothecultureandsqueezingitvigorouslyaboutseventimes.Takeasamplefromtheculturewithapipetteandplacedropsofsolution(seelabnotesondropletsizesothatonedropisequalto0.03ml)onaPetridish.Examineeachdropletunderthedissectingmicroscopeat10×.Countalltheindividualspresent.YouwilleventuallyneedtoknowthenumberpermlorN/ml.Notes on the lab bench will guide you if the sample is too dense to count.YourTAhasdatarecordingsheets.

2. Howmany samples fromeachcultureareneeded togetagoodestimateofN/ml?Youcangetafeelingforthisbylookingatvariationamongsamples—supposeyoutook3samplesandthecountswere 35, 42, and 28 protozoans each. The average would be 35 protozoans per ml. Alternatively,supposeyoutook3samplesandthecountswere75,5,and25protozoanseach.Theaveragewouldstillbe35protozoansperml,butthesamplesweremuchmorevariableinthiscase.Youmaynothaveagoodestimateoftheaveragewithonlythreereplicates(repeatedmeasurements).ReplicationisneededtomakesurethatyoursamplesarerepresentativeofthetruecountsofBlepharismapresentintheculture.Youshoulduse15to20replicatesofeverycounttoday,butbeawareof(andrecord)thevariabilityyousee.

Whenyoutakeastatisticsclass,youwill learnhowtodecidehowmanyreplicatesareneededformakingastrongcasebasedonyourdata.Taketheaveragevalueofyourreplicatestogetwhateverestimatesyouneedfortoday.Iftherewereseveralculturesforeachtimeperiodandnutrientlevel,itwouldbebesttotakesamplesfromseveralofthese.Wehaveonlyonecultureofeachtypeinlabthisweek.

3. Youmaychoosearelativelysimpleworkinghypothesis(forexample,thenumberofcannibalmorphsshouldbehighestinthemostdensecultures,orinthecultureswiththeleastfood,orintheoldestcultures),oryoumaychoosesomethingmorecomplex(forexample,thetimecourseofpopulationincreaseshouldvarywithfoodlevel,orgrowthwillslowbecauseofdensity-dependenceintheoldestcultures) depending on your interest. Whatever you choose, be sure you have a clear numerical prediction to test.


4. As mentioned above, groups may consist of two, four, or six people, but individual workloads(samples/replicates counted) must be listed. All group members must contribute to the projectequally.CheckinwithyourTAifyouforeseeproblems.

5. Wehaveprotozoanculturesforeverytimeperiod(4,7,10,14,21,and30days)atthreefoodlevels,sotherewillbemanyculturesinlab.Itisworthsayingagain—know what you are sampling and why you are sampling it, long before you count the first protozoan.

6. Youwillspendthefirsthouroflabpreparingyourhypothesisandplanningthestudy,youwillspendthesecondhouroflabsampling.Thereshouldbetimetocompletetheexperimentalwrite-up(onpage87)beforeyouleavelabtoday,butyourTAmaygrantanextensiondependingonclassprogress.Thewrite-upwillbeturnedinforagrade.

A Little Philosophy Amid the SciencePerhapsyouhavebeenwonderinghowlonganindividualprotozoanlives.Theanswerisnotatallclear.Dividing asexually by binary fission (mitosis) introduces its own set of complexities to the notion oflifespan.OnewaytolookatitiswellcapturedbyTomRobbinsinintroducinghisnovel,Even Cowgirls Get The Blues.YoucanseetheamoebafromRobbins’perspectiveinatextpostedinlab.

The format for the lab write-up is on the next page.

87

Report on Population Growth(7 points total, plus 3 points for questions on p. 91)

Introduction (1 point)

Thegeneralproblemthatinterestsmeis:

My working hypothesis is: (½ point)

My null hypothesis is: (½ point)

A prediction of the hypothesis is: (½ point)

My prediction in mathematical terms is: (½ point)

MethodsTotestthepredictionIwill:(1 point)

ResultsPresentyourresultsinatableorgraph(1 point)

InmathematicaltermsIdiscovered: (½ point):

Wastheworkinghypothesisacceptedorrejected? (½ point)

Name: _______________________________ TA: __________________________ Date: ___________


DiscussionWhatareyouabletoconclude?Werethereproblems?Whatwouldbethenextquestiontoexplorebasedonyourconclusions?(1 point)

89

Group Work SummaryComplete this sheet at lab’s end and turn it in.

Group members names: Work completed:

1.

2.

3.

4.

5.

6.

Circleone:

Didyourgroupworkwelltogether? Yes No

Wasyourprojectcompletedsuccessfully? Yes No

Didyoudoyourfairshareofwork? Yes No

Didothersdotheirfairshareofwork? Yes No

Who,ifanyone,did notmakeanadequatecontributiontothegroup?

Who,ifanyone,madeanexcellentcontributiontothegroup?

Signature

91

HomeworkYour TA will assign one or more of these homework or study questions (3 points).

1. IfahurricaneseriouslydamagesapopulationoflizardsonanislandintheCaribbean,isthislikelytobeadensity-dependentprocess?Brieflyexplain.

2. Imagineadiseasethatkills50%ofthemaleroachesinapopulation.Doyouthinkthiswouldaffectbirthrate?Brieflyexplain.

3. Ifadiseaseinsteadkilled50%offemales,woulditlikelyaffectbirthrate?Brieflyexplain.

4. Youobservedhowquicklyapopulationcanincreasewhenyouprojectedpopulationgrowthontheworksheet.Imaginethreenewsituations:

a) Howwouldthenumberofindividualsalive at T4changeifhalftheoffspringweremaleandcouldnotgivebirth?

b) IfhalftheoffspringproducedatT4weremales,howcouldreproductionbytheotherhalfofthepopulationcompensateforthemales?

c) Howwouldthenumberofindividualsalive at T4havechangedifeveryindividualgavebirthtotwooffspringatatime?

d) Howwouldthenumberofindividualsalive at T4changeiforganismshadtowaituntiltheywereage2beforereproduction?

5. Suppose a starting population contained six individuals. Each pair of individuals had a differentgeneticmakeup(genotype)fromtheothers(genotypes:G-1,G-2,andG-3.)Thegenotypesvarywithrespect to reproductive rate and offspring survival. Suppose G-1 has 8 offspring that survive toreproduce, G-2 produces only 4 surviving offspring over its lifespan, and G-3 leaves 12 survivingoffspring.

a) Iftheaverageindividualproduceseightoffspringbytheendofitslifespan,willthereproductivedifferencesamonggenotypesalterthepopulationgrowthrate?

b) HowwouldthereproductivedifferencesamonggenotypesalterthegeneticcompositionofthepopulationbyT9?

c) SupposethatoffspringdescendedfromG-3survivebetterandreproducemoresuccessfullythanall other genotypes in every generation. What might happen to the relative frequency of thisgenotypebyT500?

6. Supposethatthegenotypeshavethesamebirthratesasshownaboveatlowdensity,butgenotypes1and3experiencegreatereffectsofdensitydependenceonbirthratethan2.WhatmighthappentotherelativefrequencyofthisgenotypebyT500?

Name: _______________________________ TA: __________________________ Date: ___________

Competition and Natural SelectionGoals and ObjectivesAt the end of this laboratory you should be able to:

1. Describephenotypicandgenotypictraitsoforganisms.

2. Distinguishpurelygeneticfrompurelyenvironmentaleffectsonphenotype.

3. Explainhowheritabilityisestimated.

4. Describethethreeconditionsfornaturalselection.

5. Explaintheconceptofrelativefitnessinthecontextofnaturalselection.

6. Explainhowrelativefitnessmaybeinfluencedbytheenvironment.

7. Explainhowrelativefitnessmaybeinfluencedbyintraspecificcompetition.

8. Explainhowdifferencesinrelativefitnessleadtodifferencesinallelicfrequenciesacrossgenerations.


Thisweek’slabwillgiveyouanopportunitytothinkaboutnaturalselection.YouwillbeworkingwithtwoformsofmustardplantsinthegenusBrassicathatyouwereshowninlaboratory2.Didyouknowthatoneplantspecies,Brassica oleracea,hasproducedmanyofthevegetablesweeatsuchascauliflower,cabbage,brusselsprouts,broccoli,kohlrabi,kale,andcollards?Allthesevegetablesbelongtoonespecies,buthumanshave selecteddifferent lineages forparticularmorphologies.Cauliflowerwas selected fortightflowerbuds,whereaskalewasselectedforlargeleaves.Ifhumanscanaltermorphologysodrasticallybyartificialselection(controllingthetypesthatareallowedtomateandtheformsoftheoffspringthatsurvive),aretheresimilarprocessesthatgooninnature?Itseemslikelythatthenaturalselectionwillfavormorphologiesdifferentfromthosehumansprefer—youwillhaveachancetoseehowtwoformsofonespeciesdifferandtodecidewhichformislikelytopredominateinnature.

Asyouthinkaboutnaturalselection,youmustdrawoninformationfrompastlabs.Inlab2,youlearnedsomebasicplantbiology.Inlab3,youdiscoveredthatthegrowthrateofapopulationdependsontheaveragebirthanddeathratesofindividualswithinit.Birthanddeathratesareinfluencedbytheavailabilityoffoodandspaceinwhichtolive.Theavailabilityoffoodandspaceisdeterminedbythequalityoftheenvironment,butitisalsoaffectedbylocalpopulationdensity.Inthisweek’slabyouwill

Lab 4

93

94 Lab4: CompetitionandNaturalSelection

observetheeffectsofdensityonplantgrowthanddevelopment,andyouwillinferhowcompetitionislikelytoaffectnaturalselectionbyactingonthesurvivalandreproductionofdifferentgenotypes.

In all naturally occurring populations, individuals vary in genetic makeup (genotype) and in theresultingphysicalmakeup(phenotype).Whenindividualsmakegametes,theypasssomepartoftheirgeneticmakeupontotheiroffspring.Theexactgeneticmakeupoftheoffspringdependsontheversionsof genes (alleles) present in a pair of fusing gametes. The parental generation contains a range ofgenotypes and phenotypes. After the processes of meiosis and mating, the offspring generation willcontaindifferentgenotypesandarangeofphenotypes.

Inanonevolvingnaturalpopulation,thefrequencyofallelesandgenotypesintheoffspringgenerationwillbethesameasitwasintheparentalgeneration.Inanevolvingpopulation,allelefrequencieschangeacrossgenerations.Theallelefrequencieschangebecauseparental individuals,carryingparticularalleles,varyintheirnumbersofsurvivingoffspring.Oneparentalorganismthatleavesmoreoffspringthatsurvivetoreproductiveagethananothersuchparentwillhaveincreasedthefrequencyofitsparticularallelesinthenextgeneration.

Thisweekinlabyouwillbestudyingnaturalselection,whichisthedrivingforcebehindadaptation.Natural selection is inevitablewhen thepossessionofaparticulargenotypehas consequences for thesurvivalandreproductionofindividuals.Youwillnotbemeasuringparticularallelefrequencies,butyouwillbelookingforvariation in parental contributions to the next generation.

Inpreparationforthislab,weplantedmustardseedswherethegeneticmakeup(genotype)atthebodycolorlocuswasknown.Thegenotypeofeachseedproducesaplantofeitherastandard green body (morph) ora yellow-green body (morph).Theyellow-greenplantsarehomozygousrecessive(yy)atthebody-colorlocus.Thecentralquestionwewantyoutoansweristhis:Does body color (genotypeatthebodycolorlocus) influence a plant’s expected survival and reproduction?Ifso,thenallelescausingtheyellow-greenphenotypearelikelytochangeinfrequencyfromgenerationtogenerationduetonaturalselection.

What kinds of differences among plants are likely to be important? We are interested in differences thatmatter evolutionarily. If evolution requires a change in the frequency of alleles and genotypes from

Figure 4-1©ThienMai2008.Usedwithpermission.

Lab4: CompetitionandNaturalSelection 95

parentstooffspring,thenwemustlookfordifferencesinperformancethatwillcausesuchchanges.Solet’sthinkaboutwhatthismeansinrealterms.Is ittruethattherewillbenoevolutionarychangeinallelefrequenciesifgreenandyellow-greengenotypesachieveexactlythesametotalweight?Whatiftheyproduceexactlythesamenumberofseeds?Beforeyouanswer,considerthesubtlewaysinwhichplantsmayalterthewaystheygrowandreproduce.

Whatifonegenotypegrewlessandmadeasmallerplantbutallocatedmoreenergytoflowersthantheothergenotype—coulditstillsetasmanyseeds?Whatifonegenotypemadefewerflowersthantheotherandsetfewerseeds,buteachseedwaslargerthanseedsoftheothertypeandhencemorelikelytogerminate?Onegenotypecouldproduceasmallernumberofrelativelymorevigorousoffspringthantheothertype,couldn’tit?Thebottomlinehereisthattoreallyassesstherelativesurvivalandreproductivevalueofthesetwoparentalgenotypesatthebodycolorlocus,wewouldwanttoknowhow many offspring of each parental plant genotype will survive long enough to produce “grandchildren.”Bythetimethegrandchildrenareborn,wewillhaveagoodideawhetherthetwogenotypesaremakingequalcontributionstolatergenerations.Iftheyare,thenneitherallelicfrequenciesnorgenotypicfrequenciesareexpectedtochangefromgenerationtogeneration. If theyarenotmakingequalcontributionstosucceedinggenerations,thenallelicandgenotypicfrequencieswillchangeandevolutionisoccurring.

Thetermfitnessdescribesthenumberofanindividual’soffspringthatsurvivetoreproduce.Thus,anindividual’sfitnessismeasuredwhenheorshehasoffspringthatreproduce.However,theactualnumberofoffspringisnotveryimportant—making100offspringsoundslikeitmightresultinhighfitness,butifeveryorganisminthepopulationmakes100offspring,thenproducingthisnumberoffersnoadvantage.Whatmatterstoevolutionisanindividual’srelative fitness—the number of its offspring that survive to reproduce(makegrandchildren)in comparison to the population average(mean).Ifanindividualhashighrelativefitness,thentheallelesitcarrieswillbecomemorefrequentthanotherallelesinthenextgeneration.

Many characteristics of plant growth, such as height, flower number, flower size, and seed size,contributetotherelativefitnessof individualplants.Thesecharactersareconsideredcomponents of fitness.(Notethatevenflowersthatdonotproducefruitmaycontributetofitnessbyfertilizingotherflowers with their pollen.) In one lab, we cannot follow individual plants until “grandchildren” areproduced, but we can compare some components of fitness for two genetically determined body color forms or morphs of thewildmustardplant,Brassica rapa.Brassica rapa isanannualplant thatgrows, reproduces once, and then dies, making it easy to measure lifetime seed production as animportantcomponentoffitness.

Wewouldlikeyoutodiscover whether there are costs and benefits in terms of relative fitness to possessing a particular genotype. Inotherwords,weare askingyou to look for evidenceofnaturalselectioninaction.

Recallthethreeconditionsforevolutionbynaturalselection:

1. Individualsvarywithrespecttophenotype.

2. Phenotypicvariationisheritable.

3. Phenotypicvariationresultsindifferentialreproduction(inotherwords,itresultsindifferencesinrelativefitnessamongorganisms).

Pre-lab Questions—Record Your Answers on the Online Version 1. The twoBrassicamorphsused in this labdifferat thebodycolor locus.List threeother aspectsof

phenotypicvariationthatyoumightreasonablyexpecttoseeamongplants:

1.

2.

3.


2. Inyourownwords,defineheritability:

3. Humansartificiallyselectmanykindsofplantsandanimalsthroughcontrolledbreedingprograms.Whyisheritabilitycruciallyimportantforasuccessfulbreedingprogram?

4. Iftheplantmorphsyoustudyaregoingtorespondtonaturalselection,theymustvaryinphenotype,andthisvariationmustbeheritable.Thisvariationmustalsoresultindifferencesinrelativefitnessamongthemorphs.Forexample,thesizeofleavesaplantproducescouldaffectrelativefitnessbydetermining how much energy a particular plant collects. The amount of energy collected couldaffect investment in reproduction. Leaf size is measurable trait. Suggest three other measurableplanttraitsthatcouldaffectrelativefitnessandexplainthembriefly.

1.

2.

3.

Physiological Attributes of the Two Genotypes

Inthesecondlabofthisquarter,youspentsometimethinkingaboutresourceacquisitioninplants.Asyou recall,plants collect energy throughphotosynthesisbyusingpigments toharvest light.Youmayneed to look back at your notes from lab #2 in this manual, or in your text, to answer some of thequestionsbelow.

5. Whatistheprimarypigmentthatmakesleavesgreeninlandplants?

6. Theremaybemorethanonepigmentinaplant.Forexample,theyellow-greenBrassicamorphthatyouwillstudythisweekhasapurplecolorationonitsstems,leaftips,andotherparts.Thepurplecolorindicatesthepresenceofthepigmentanthocyanin.Whatdoesaplantgainfromhavingmorethanonepigment?

7. Developasimplehypothesisregardinghowthepresenceofanthocyaninaffectstherelativefitnessofthetwo Brassicamorphs.

The yellow-green Brassica morph often has small hairs, called trichomes, in varying numbers on thestemsandleavesoftheplantbody.Severaldifferentfunctionshavebeenproposedfortrichomes,soseetheWebsitelistedbeloworfindanotheroneontheweb:

www.biologie.uni-hamburg.de/b-online/e05/05a.htm#trichome

8. Listtwopotentialadvantagesofhavingtrichomes.

1.

2.

9. Aplanthastodivideitsenergyamongmanydifferentfunctions.Brieflyoutlineonereasonthataplantmaynotproducetrichomes.


Adaptations

Variationamongindividualsarisesthroughsexualreproductionandmutation.Insexualreproductionthe processes of crossing over and independent assortment during gamete formation result in newcombinationsofallelesinzygotes.Asthezygotebecomesanorganism,eachnewgeneticcombinationisexposedtoanenvironmentwhereitsrelativefitnesswillbetested.

Wecan think about the fitness valueof the entire collectionofgenetic traits,orwe can considerindividualtraitssuchasbodycolor.Atraitmighthavenoeffectonfitness(aneutraltrait),itmighthaveapositiveeffectonfitness(abeneficialtrait),oranegativeeffectonfitness.Ifthetraithasaconsistentlynegativeeffectonfitness,thenorganismspossessingthetraitwillleavefeweroffspringthanothersinthepopulation,andtheallelesforthetraitwilldeclineinfrequency.Wesaythistraitis“selectedagainst.”

Ifthetraithasapositiveeffectonfitnessthenorganismspossessingthetraitwillleavemoreoffspringthanothersinthepopulation.Theallelesunderlyingthistraitwillincreaseinfrequency.Werefertoatrait that increases fitness as an adaptation. Natural selection should result in a high frequency ofadaptations within each population. This is the basis for what biologists often observe as a good fitbetweenorganismsandtheirenvironment.

10. Assumethattrichomesappearin1%oftheoffspringofaparticularpopulation.Outline a situationinwhichtrichomeswouldbeadaptivebasedonanalysisofthebenefitsandcostsoftrichomes.Describewhat you expect to see in terms of genotypic and phenotypic frequencies over the next 50generations.

11.Usingyouranalysisofthebenefitsandcostsoftrichomes,outline a situationwheretherewouldbeselectionagainstthepresenceoftrichomes.Describewhatyouexpecttoseeintermsofgenotypicandphenotypicfrequenciesoverthenext50generations.

12.Aretrichomesanadaptation?Explainwhyyouthinkso.

Environmental Effects on the Measurement of Fitness

Thegoalofthisweek’slabistoevaluatetherelativefitnessofthetwomorphsofBrassica rapaindifferentenvironments.Atrait that results in high fitness in oneenvironmentmaynotconferhighfitnessinanotherenvironment.

ThestandardgreenmorphofBrassica rapa is indicatedbyY.Theyellow-greenmorphofBrassica rapaisindicatedbyy.Each morph will be grown in threedifferent environments (treatments): itwillbegrownalone;itwillbegrownwithneighbors of its own morph; it will begrown with neighbors of the othermorph.Youwillhavesixpotsofplantsto study, one for each morph in eachenvironment, as shown to the right. Ineach pot, you will measure one targetplantorfocal plantcircledinFigure4-2.Ifotherplantsarepresent,theyconstitutetheenvironmentforthefocalplant.

Figure 4-2©Chu2008.Usedwithpermission.


Supposewewereinterestedinoneofthecomponents of fitnesssuchasnumber of seeds per plant.Wewouldmeasurethisonthefocalplant ineachpot.Ifwereplicatedtheplantingscheme12timesandmeasured 12 focal plants, we could get the average number of seeds for each morph in each environmentbytakingthemeanofour12replicatesforeachmorphineachtreatment.

13. Whenplantsaregrownwithneighbors,itispossiblethattheywillcompeteforresources.Theycancompeteattherootlevelortheshootlevel.Lookingbacktolab2ifnecessary,placetheresourcesfrom the left side of Figure 4-3 in the boxes for the shoots or the roots depending on wherecompetitionforeachislikelytooccur.

©Hamamoto2008.Usedwithpermission.Figure 4-3

There are different kinds of competition. Intraspecific competition occurs when an organismcompetesforresourceswithmembersofitsownspecies.Interspecificcompetitionoccurswhenanorganismcompetesforresourceswithmembersofanotherspecies.Competitionmightbethemostintenseifanorganismiscompetingwithotherswhoneedexactlywhatitneeds.Competitionmightnotbeintenseiftheneededresourcesarequiteabundant.

14. InourBrassicaexperimentwedonothavetwospecies,butwedohavetwomorphs.Weknowthatthemorphsdiffergeneticallyat thebodycolor locus,sowecouldhaveintramorphand intermorphcompetition.Inoneofthetreatments,eachfocalplanthasnocompetition.Placethelabels below onthecorrecttreatmentsonpage97.

Figure 4-4©Chu2008.Usedwithpermission.


15. Supposewewantedtoknowwhethertheyellow-greenmorphproducedmoreseedsthanthestandardgreen morph under the best possible conditions. We have data on the average number of seedsproducedbyeachmorphineachtreatment.Whichdatasetwouldweuse?

(1) theaveragenumberofseedsperplantforeachmorphcollectedfromTreatment1



16. Whatisthenull hypothesisfortheexperimentaltestdescribedinQuestion15?

(1) theaveragenumberof seedsperplant for theyellow-greenmorph isgreater than theaveragenumberofseedsforthestandardgreenmorph

(2) theaveragenumberofseedsperplantfortheyellow-greenmorphislessthantheaveragenumberofseedsforthestandardgreenmorph

(3) theaveragenumberofseedsperplantfortheyellow-greenmorphisequaltotheaveragenumberofseedsforthestandardgreenmorph

17. Supposeyouwantedtotestwhethercompetitionhadanyeffectontheyellow-greenmorph.Whatisthecorrectnull hypothesisforthisexperimentaltest?

(1) theaveragenumberofseedsperyellow-greenplantgrownaloneequalstheaveragenumberofseedsperyellow-greenplantwhengrownwithneighbors

(2) theaveragenumberofseedsperyellow-greenplantgrownaloneislessthantheaveragenumberofseedsperyellow-greenplantwhengrownwithneighbors

(3) theaveragenumberofseedsperyellow-greenplantgrownwithintramorphneighborsequalstheaveragenumberofseedsperyellow-greenplantwhengrownwithintermorphneighbors

18. Supposeyouwantedtotestwhetherintramorphcompetitionwasgreaterthanintermorphcompetitionforthestandardplants.Whatisthecorrectnull hypothesisforthisexperimentaltest?

(1) theaveragenumberofseedsperstandardplant inTreatment1equals theaveragenumberofseedsperstandardplantinTreatment2

(2) theaveragenumberofseedsperstandardplant inTreatment2equals theaveragenumberofseedsperstandardplantinTreatment3

(3) the average number of seeds per standard plant in Treatment 1 is greater than the averagenumberofseedsperstandardplantinTreatments2and3

19. Suggestamechanismthatwouldallowaplanttogrowbetterifithadintermorphneighborsthanifithadintramorphneighbors.Thinkaboutthedegreeofgeneticsimilaritybetweenthefocalplantanditsneighborsandmakealinkbetweengeneticsimilarityandresourceuse.

20.Whenbiologistsstudynaturalselectiontheyarelookingforchangesingenotypeorallelefrequenciesacrossgenerationsintherealworld.Youwillseethisinmoredetailinlabnextweek.However,basedonthethinkingyouhavejustdoneaboutplantgrowth,density,andneighbors,whichexperimentaltreatment(s)is(are)mostlikelytogiveyouinformationaboutrelativefitnessofthetwomorphsthat would apply to nature?Why?


21.Below are several plant attributes that could be measured. For each attribute, provide one briefdescription of how it could affect the outcome of competition.Thereismorethanonereasonableanswerforeachaspectofgrowth.Thefirstaspecthasbeenfilledinasanexample.

Plant shoot height—tallplantswouldbecompetitivelysuperioratlightcollection(andthusmakemoretissueandeventuallymoreoffspringthanshortplants).

Total root weight

Root to shoot ratio

Number of flowers

Number of seeds per flower

Weight of individual seeds

Seed hardness

Speed with which seeds germinate


101

Lab 4: Competition and Natural SelectionInlab2youwereshownthetwoBrassicamorphsthatyouwillstudytoday,butyouwerenotasinformedaboutplantsorexperimentaldesignasyouarenow.Youaregoingtotestthehypothesisthattherearedifferencesintherelativefitnessofthetwomorphs.Withyourpartner,discussthepossiblecomponentsoffitnessthatyoucouldmeasureanddecide on threethatareofinteresttoyouboth.

Measurable Components of FitnessWritethecomponentsoffitnessthatinterestyouontheblackboardandseeifanyoneelseisgoingtotakethesamemeasurementsontheplants.Youwillneedasamplesizeofatleastsixplantstogetagoodestimateof theaverage for eachcomponentof fitness, sodiscussyour interestswithothergroupsofstudents.Make sure that five other groups will estimate values for each character that you want to use.YourTAwillhelptheclassnarrowdowntheconceptstobemeasuredifthereareproblems.Listthecharactersthatyouwillmeasurebelow.

Component1:

Component2:

Component3:

Measurement MethodsDiscusshowyouwillmeasureeachcomponentwiththeothergroupsthatwillmeasureit.Make sure your methods are the sameanddiscussanymeasurementconcernsyoumayhavewithyourTA.Outlineyourmethods(e.g.,counting,wetweights)brieflybelow.

Component1:

Component2:

Component3:

Take the measurements you need and compile your data, along with those of other groups, in the table on the next page.

103

RESULTS: Data Compilation, Tables 1a, 1b, and 1c

Table 1a. Treatment 1 Data

Table 1b. Treatment 2

Name: _______________________________ TA: __________________________ Date: ___________

Replicates:

Yours 1 2 3 4 5 6 Mean SD SE

Component 1

Standardmorph

Yellow-greenmorph

Component 2

Standardmorph

Yellow-greenmorph

Component 3

Standardmorph

Yellow-greenmorph

Replicates:


Component 1

Standardmorph

Yellow-greenmorph

Component 2

Standardmorph

Yellow-greenmorph

Component 3

Standardmorph

Yellow-greenmorph


Table 1c. Treatment 3

RESULTS: Data AnalysisCalculatethemeanvalueforeachtraitineachtreatment:

WewanttoknowwhetherthereisadifferenceinrelativefitnessofthetwomorphsofBrassica rapa.Ournullhypothesisisthatthereisnodifferenceinrelativefitnessbetweenthetwomorphs.Wecantestforadifferenceusinganycomponentoffitness.Wecantestforthedifferenceunderdifferentenviron-mentalconditions.

Forexample,wemighttest whether the two morphs differ in a particular component of fitness when they do not have competitors, sowewoulduse the means from treatment 1.

Thenullhypothesisisthatthereisnodifferenceinthemeanvaluesforthiscomponentoffitnessbetweenmorphs(SM=standardmorphandYG=yellow-greenmorph).Writtenmathematically,thenullandalternatehypothesesare:

Component1:Null:x̄smx̄yg Alternate:x̄smx̄yg.

Placeyour estimates of the means for each of the three components of fitness from Treatment 1intothetwoequationsinTable2onthenextpage.

Replicates:


Component 1

Standardmorph

Yellow-greenmorph

Component 2

Standardmorph

Yellow-greenmorph

Component 3

Standardmorph

Yellow-greenmorph


Do you think you can reject the null hypothesis and accept your data as evidence that there aredifferencesbetweenthetwomorphsintheabsenceofcompetition?Areyoufeelinguncertainabouthowmuchofadifferencebetweenthemeanscountsasasignificantdifference?

Youhaveuncoveredtheneedforstatisticsagain.Inlab3thesameproblemarose,butwedidnotteachyouanystatisticaltechniquestoresolveyourdilemma.TodayyouwilllearntomakeaStudent’st-test to distinguish between two means (Student was the pseudonym of the mathematician, W. S.Gossett,whoinventedthetest).Theideabehindthetestisthatweusethevariabilityinourestimatesofthemean(howdifferentthesixestimatesyouhavearefromeachother)todecidewhetherthetwomeanswearecomparingwerelikelytobetakenfromthesamedistributionofestimates.Ifthetwomeansarelikelytohavecomefromthesamedistribution,thenwemustacceptthenullhypothesis.

Sofirstweestimatethevariabilityaroundthemeanusingameasurecalledthestandarddeviation,SD. The main term in the equation is the difference between each estimate of the component of fitness and the mean value of the component.

Thisdifferenceisthensquared,andallthedifferencesaresummed.Nisthenumberofsamplesused.Foreachmean,n=6,soyousumall6differences,divideby(n–1),andtakethesquareroot.

Calculatethestandarddeviationforeachmeanusingtheformulaabove.Placethestandarddeviationnexttoeachmeaninyourtable.Youwillneedthesetermstocalculateavalueoftforthet-test.

The formula for calculating t is:

TodeterminewhetheryourvalueoftisstatisticallysignificantyoulookupthevalueinaTableofCriticalValues.Youwillneedtoknowthedegreesoffreedom(df)foryourvalueoft.Forthisstudy,thedegreesoffreedomarecalculatedasthenumberofsamplestaken(heren=12forthetwomeans)minus2.Sowehave12–2=10degreesoffreedom.

Componentof Alternate Fitnessyou Nullhypothesis hypothesis Rejectnull? measured x̄SM x̄YG x̄SMx̄YG

SD = 1n − 1

xi − x ( )2

i =1

n

∑

t = x SM − x YG

SDSM2

nSM

+ SDYG2

nYG

Table 2


We find the critical value of t in a table of confidence limits for 10 degrees of freedom. If thecalculated value of t fortheexperimentis greater than the critical value of t listedinTable3,thenwe reject the null hypothesisandacceptthealternativehypothesis.

Table 3 Criticalt-value

90%confidence 95%confidence 99%confidence df limit limit limit

6 1.94 2.45 3.71 7 1.90 2.37 3.50 8 1.86 2.31 3.36 9 1.83 2.26 3.25 10 1.81 2.23 3.17

Weneed todecidehowmuchconfidencewewant tohave thatweare correctly rejecting thenullhypothesis.Wecanusea90%confidencelimit,whichmeansthatweexpectthatwewillrejectthenullhypothesiscorrectly9outof10timesandbewrong1outof10times.Wecouldalsouse95%confidencelimitsor99%confidencelimits.Noticethatthecriticalvalueincreaseswiththeamountofconfidencewewishtohave.Mostscientistsusea95%confidencelimit.

Belowisasampledatatablewithimaginaryresults;completethetableonthenextpageusingyourowndata.

Table 4. Sample Data as an Example of Statistical Analysis Measured Null Alternate t Reject Component hypothesis hypothesis value null? Treatment ofFitness x̄SMx̄YG x̄SMx̄YG

1.Nocompetition Plantheight 16.412.4 16.412.4 2.35 Yes,at95% Flowernumber 3237 3237 2.40 Yes,at95% Seednumber 7274 7274 1.65 No

107

Name: _______________________________ TA: __________________________ Date: ___________

Table 5. Class Data Measured Null Alternate t Reject Component hypothesis hypothesis value null? Treatment ofFitness x̄SMx̄YG x̄SMx̄YG

1.Nocompetition

2.Intra-morphCompetition

3.Inter-morphCompetition

Graphical Presentation of ResultsTablesaresuitableforpresentingcertainkindsofdata,butoftenresultsaremucheasierforotherstovisualizeifyoupresenttheminafigure.Below you will learn how to make a plot showing your results for each of your measured components of fitness. Plotthemeanvaluesfortheselectedcomponentoffitnessforeachmorphateachtreatment—useadifferentsymbolorcolorforeachmorph.Foreachmeanvalue,calculateanerrorbarusingthestandarderror,SE.

SEiscalculatedas:SE=SD/√nwherenisthesamplesize(ifn=6,√n=2.45).

ThereisspaceforrecordingSEinTables1a,1b,and1conpages103and104.


Tomakeastandard error bar,letthetopendofthebarbethemeanplusoneSE,andletthebottomendofthebarbethemeanminusoneSE.Youwillneedtomakeindividualerrorbarsforeachpoint.

The width of the standard error barillustrates how much confidence one hasthattheestimatedmeanvalueofyoursampleis close to the true mean value for thepopulation from which the sample wasdrawn. To get some intuition about this,imaginethatyouhadasetofhighlyvariablesamples (for example: 75, 5, and 25) andanother set of much less variable samples(forexample:35,42,and28).Bothsampleshave a mean of 35, but there would be amuch larger standarderror for the first setof samples than for the second set. Youwouldbemuchlessconfidentthatthetruemeanofthefirstsetofsampleswas35thanyouwouldthatthetruemeanofthesecondsetofsampleswas35.Themeanof35onthefirst set of samples would have a standard error bar of plus or minus 20.8, whereas themean of 35 on the second set of sampleswould have a standard error bar of plus or

minus 4.04.Whenyouseeahugeerrorbar,youknowthattherewasalotofvariabilityinthesamplesusedtoestimatethemean.Ingeneral,wecanbe95%confidentthatthetruemeanofeachsetofsampleslieswithinplusorminus(1.96×SE)ofthemeanweestimated.

Whatisthisnotionofthe“true”mean?Ameanoraverageisanestimatebasedonthesamplesyouhavetaken—ifyouwantedtoknowthetruemean,youwouldhavetomeasurealltheorganismsinyourstudypopulation.Wewanttoknowwhetherthetruemeansofthetwopopulationsaredifferent,andwedecidehowlikelytheyaretobedifferentbasedonthevariabilityinourestimatesofthemean.Forthepopulationwherethesampleswere75,5,and25,themeanwas35,but ifthetruemeanliesbetweenminusandplus1.96×SE,thenthetruemeanissomewherebetween–5.77and75.77.Thisisnotaverygoodestimate!Forthepopulationwherethesampleswere35,42,and28,thetruemeanissomewherebetween 27.08 and 42.92. This is much narrower estimate for the true mean, reflecting the fact thesamplesusedtomaketheestimateweremuchclosertogether.

Whatdoerrorbarstellus?Examinetheplotabove—itwasmadewithimaginarydataandshowstwosamplemeansforplantheightineachofthreedifferenttreatments.LookatTreatment1;themeansareclosetogetherandtheerrorbars(plusorminusoneSE)donotoverlap.Iftheerrorbarsweremadewithplusorminus1.96×SE(basicallytwiceaslarge),theywouldoverlap,tellingusthatwecouldnot be sure the means were different.Thus,wewouldexpectourcalculatedt-valuetobemuchsmallerthanthecriticalvalues for any of the confidence intervals shown—the small t-value means that we accept the nullhypothesisofnodifferencebetweenthetreatmentmeans.Thet-valuetellsyouwhethertoacceptorrejectthenullhypothesis,butyoushoulddevelopasenseofthesignificanceoftheresultfromlookingataplotwitherrorbarsoneachdatapoint.

Lookbackattheplotofimaginarydataabove.ForTreatment1,thestandardmorphplantshadameanheightofabout9.5cm,whereastheyellow-greenmorphplantshadameanheightofabout8cm.Theerrorbarsshownareplusorminusone SE.Imagineerrorbarsthatincludethetruemean(±nearlytwoSE).Doyouexpecttorejectthenullhypothesisforthiscomparison?Isitappropriatetosaythatthestandardmorphplantsaretallerthantheyellow-greenmorphplants?

Theanswertothelasttwoquestionsis“no.”Fromwhatweseeintheplot,weexpectthet-valuewecalculatetoleadustoaccept the null hypothesisthatthereisnodifferenceinplantheightbetweenthetwomorphsinTreatment1.Youmusttakethisintoaccountwhenyouwritetheanalysisofyourexperiment.

Figure 4-5


You cannot say that the standard morph plants were significantly taller than the yellow-green morphplantsinTreatment1eventhoughtheestimateofmeanheightforthestandardplantswas9.5andtheestimateofmeanheightfortheyellow-greenmorphwas8cm.Thestatisticalanalysissaysyoucannotdistinguish thesemeansand everything that youwrite in your reportmust reflect this.There isonetreatmentintheimaginarydatasetwhereitislikelythatyouwillrejectthenullhypothesisandfindasignificantdifferencebetweenthetwomorphs.(Canyouseewhichoneitis?)

Foryourresults,makeaplotofthetreatmentmeanswithSEbarsfor each component of fitnessthatyoumeasured.USEARULERtomaketheplot.RememberthateachmeanislikelytohaveadifferentSEvalue.Ifyouuseagraphicsprogramtoplotyourdata,ratherthanplottingbyhand,youwillhavetofindawaytoinputtheindividualSEvalues.SomeofthegraphicsprogramsusethesameSEvalueforeverypointasadefaultvalue.Ifthishappens,yourplotswillbewrong.

Report of Discussion and ConclusionsInlabnextweek,turnareportthatincludes:

Table1(a,b,c);

Table5;

threeplotsofyourdatashowingmeansanderrorbars;(3 @ 1 point each)

answerstoquestions1to5onpage111;(5 @ 1 point each)

thesummarydescribedinitem6onpage111.(2 points)

Name: _______________________________ TA: __________________________ Date: ___________

1.Explainhoweachcomponentthatyoumeasuredrelatestothefitnessofplants.

2. Did you find evidence that one morph has higher relative fitness than the other morph? Use numerical and statistical data to justify your answer. Payattentiontosignificancelevels.


3. Underwhichtreatmentcondition(s)doesthegreatestdifferenceinrelativefitnessbetweenthetwomorphsoccur?Explain.

4. Howmightwhatyouobservedinclassdeterminetherelativefitnessofthetwomorphsiftheplantsweregrowingoutinafield.Explainwhatyouwouldexpecttoseeintermsofsurvivorshipand/orreproductioninnatureforthetwomorphs.

5. Ifa fieldpopulationstartedwith50%standard(YY)plantsand50%yellow-green(yy)plants,howwouldyouexpectfrequenciesofthetwoalleles(Yandy)tochangeinfuturegenerations,basedonyouranswertoquestion#4?(Hint:Ifyoucanpredictsurvivorshipandreproductionfortheparentalorganisms,whatwill thatmeanabout theabundanceofparticularversionsofgenes (alleles) thattheseparentspassontothenextgeneration?)


6. Createasummaryparagraphforthisexperiment.Thefirstfivelineshavebeendoneforyou,buttheparts in boldshouldeitherbecompleted by, or replaced by, your own words.Youcannot add more than nine lines tothissummary,sochooseyourwordscarefully.

Create an abstract for your research project.Brassica rapaoccursintwogeneticallydistinctmorphs,ahomozygousrecessiveyellow-greenmorphandahomozygousdominantstandardgreenmorph.Tocomparetherelativefitnessofthetwomorphs,Imeasured three components of fitnessunder conditionsofno competition, intramorphcompetition,andintermorphcompetition.Theplantsweregrowninagreenhouse.The three components of fitness I measured were:

I observed differences in relative fitness between the two morphs . . .

I predict that under natural conditions these differences will alter the frequency of the two morphs (or the alleles they carry) in future generations so that . . .

Mendelian and Population GeneticsGoals and ObjectivesAt the end of this laboratory you should be able to:

1. Explainhowtheunitsof inheritance(genesandchromosomes)aredistributedamonggametes inmeiosisandamongoffspringaftergametefusion.

2. Explain how genetic information is transferred across generations (from parents to offspring) viaMendeliangenetics.

3. Describephenotypicandgenotypictraitsinorganisms.

4. Distinguishamongphenotypicfrequencies,genotypicfrequencies,andallelefrequencies.

5. Calculate phenotype frequencies by observing populations, calculate genotype frequencies fromobservedphenotypefrequencies,andcalculateallelefrequenciesusingdataongenotypefrequencies.

6. CalculateexpectedallelicandgenotypicfrequenciesusingtheHardy-Weinbergequilibriumequations.

7. CompareobservedallelicandgenotypicfrequencieswiththoseexpectedunderconditionsofHardy-Weinbergequilibriumandnotedeparturesfromexpectations.

8. Distinguishbetweendiscreteandquantitativegenetictraits.


In lab3youdiscoveredthatthegrowthrateofapopulationdependsonthebirthanddeathratesofindividualswithinit.Inlab4youdiscovereddifferencesamongindividualsintermsoftheirobservablephysicalcharacteristics(thephenotype)andtheirgeneticmakeup(thegenotype).Thisweekyouwillexploretheinheritanceofthesecharacteristics.

As we will see in lectures and labs throughout the quarter, understanding why individuals differ from one another and how these differences are passed from parent to offspring is crucial to understanding evolution.

Your first task will be to understand how the genotype determines the phenotype. Then you willdiscoverhowthegeneticmakeupofamotherandafatherdeterminesthegenotypesandphenotypespossibleintheiroffspring.TheinheritanceoftraitsfromparentstooffspringfallsundertheheadingofMendelian genetics.Whenweextendthisanalysisfromfollowingthegenotypesandphenotypesofasingle

Lab 5

113

114 Lab5: MendelianandPopulationGenetics

family across a generation to following the genotypes and phenotypes of an entire population acrossgenerations,wearestudyingpopulation genetics.

YouwillstudyMendelianandpopulationgeneticsusingfruitfliestoday.ThescientificnameofthefruitflyisDrosophila melanogaster,anditistheequivalentofthewhitelabratforgeneticists.Whatmakesfruitfliesworthstudying?Theirgeneticmakeupisreasonablywellunderstood—theyhaveonly4differentchromosomescarrying13,601genes.Theycanbegrowninlargenumbersforstudiesofevolution.Therearemanyobviousphenotypicvariationssuchasredeyesversussepiaeyes,orebonybodiesversusbrownbodies.Onecantesthypothesesaboutthegeneticbasisofthesephenotypictraitsbydoingexperimentalcrosses(matingparticularfemalestocertainmales).Offspringphenotypestakeonly10daystodevelop,soscientificquestionscanbeansweredquickly.Therearemanygeneticsimilaritiesbetweenthehumanand Drosophila genomes; for example, we learned about the genetic causes of human aniridia fromstudyingtheeyelessmutationinDrosophila.Allinall,fruitfliesareverysuitableorganismsforexperimentalbiology.

Inashortwhile,wewillaskyoutothinkaboutfruitflyphenotypesandgenotypes,butbeforewedothat,takeaminuteandmakesureyouareclearonsomebasicgeneticconcepts.

Anorganism’sgenotypeisthemolecularinformationthatspecifiesmanycharactersofthephenotype.Wesay“manycharacters”becausesomefeaturesofanorganismareinfluencedbytheenvironmentandothersresultfrominteractionsbetweengenesandtheenvironment.Considerasinglegenethatproducesa protein product visible in the phenotype, like eye color. Fruit flies are diploid organisms, so theycontaintwocopiesofeverygene,oneinheritedfromthemother,theotherfromthefather.Eachflyhastwocopiesoftheeyecolorgenebutnotnecessarilytwoidenticalcopiesofthisgene.Mostgenesexistinseveralversionsoralleles;onealleleproducesared-eyecolorproduct,whereasanotheralleleproducesasepiaproduct.Sohowmanyeyecolorgenotypescouldtherebeifonegenehastwoalleles?Ihopeyousaidthreegenotypes:onehavingtworedalleles,anotherhavingtwosepiaalleles,andathirdgenotypehavingoneredalleleandonesepiaallele.Havinginheritedtwoidenticalallelesfromtheirparentsmakestheorganismhomozygous(asinthered/redandsepia/sepiagenotypes),whereasinheritingadifferentallelefromeachparentmakestheorganismheterozygous.

Doeseverygenotypeproduceadifferentphenotype?Itdependsontheparticularallelesinquestion.Analleleisasetofinstructionsforaproteinproduct,sohomozygousorganisms(homozygotes)producetwocopiesofthesameproductwhenthegeneisexpressed,butheterozygotesoftenmaketwodifferentproducts.Whathappenstothephenotypeinheterozygotes?Again,itdepends.Insomecasesoneproteinproductisabletomasktheeffectoftheotherproteinproductortheotherproductisnonfunctional;inthatcase,wesaythatthemaskingalleleisdominantandweseeonlyitseffectsonthephenotype.Theallelethatdoesnotaffectthephenotypeisrecessive.Iftheproductsofbothallelescombinetoactonthephenotype(forexample,whenared-floweredplant iscrossedwithawhite-floweredplantandtheoffspringhavepinkflowers),wesaytheallelesarecodominantorresultinintermediate inheritance.

Acrossawholespecies,theremaybemanyallelespresentforagivengene;forexample,thereare900variantsofthehemoglobingeneinhumans,althoughnotalloftheirproteinproductsareequallygoodatcollectingandholdingoxygen.

Howdowerepresentgenesandalleles?Mostgenesareidentifiedbyasingleletteroraseriesofletters.ThetypicalDrosophilaeyecolorisred,sotheeyecoloralleleis“S.”Ahomozygousred-eyedflyhastwocopiesofthisallele,soitsgenotypeis“SS.”Thesepiaeyealleleisrecessiveanddesignatedwith“s,”soasepia-eyedflyis“ss.”WhatabouttheSsheterozygote?Itseyesarered.The dominant allele is labeled with a capital letter when there are only two alleles.Obviously, there isnowaytoextendtheuseofcapitalandsmallletterstomorethantwo(multiple)alleles.Whentherearemultiplealleles,wetypicallynumberthem(S1,S2,S3,S4,etc.).Thereisnowaytoknowwhichallelesaredominantwhenallelesarenumbered—youmustbetoldbasedonexperimentalresults.

Forpractice,let’sworkwithbloodgroups.Threeallelesexistforhumanbloodtypes.TheywerenotnumberedbutinsteadmarkedwiththelettersforA,B,andO.Thelabelingconventionis:IA,IB,andIi.IAandIBarecodominantalleles,whereasIiistherecessiveallelefortypeO.Humansarediploid;everyindividualcarriesonlytwoalleles.

Lab5: MendelianandPopulationGenetics 115

Pre-lab Questions—Record Your Answers on the Online Version 1. Listallsixpossiblebloodgroupgenotypes: (1)IAIA (2)____________ (3)____________

(4)____________ (5____________) (6)____________

2. Howmanykindsofhomozygotesarethere?______________________________________

3. Howmanykindsofheterozygotesarethere?______________________________________

4. Thereareonlyfourblood group phenotypes (A, B, AB, and O). Basedonthe informationgivenhere,listthegenotypesthatproduceeachphenotype.

a. typeA:

b. typeB:

c. typeAB:

d. typeO:

Sofaryouhavebeenworkingwithonlyasinglegenedescribingagenotype.Drosophilahas13,601genes.Whatifwewanttoincludemorethanonegeneinourgenotypenotation?Let’saddthebodycolorgene(orlocus)toourfruitflystudy.Innaturemostfruitflieshavebrownbodies,sowecallbrownthe“wildtype”alleleanddesignateit“E.”Somewhereintheevolutionaryhistoryoffruitflies,aversionofthegenethatmadeanebonypigmentaroseviaagenemutation,soebonyiscalledthe“mutant”alleleanddesignated“e.”Therearethreegenotypesforbodycolor:EE,ee,Ee,butflieshavebotheyesandbodies.Thegenotypeofaflywithanebonybodyandsepiaeyesis“eess.”

Recall that “e” (ebony) and “s” (sepia) indicate recessive alleles; dominant alleles are “E” (brown) and “S” (red).

5. Theflypicturesbelowarelabeledforeyecolorandbodycolor.

List all the possible genotypes of each fly below the picture.Whydowesaypossible?Rememberthatthered and brown alleles are dominant—this means that the same phenotype might be produced bymorethanonegenotype.Notethatredeyesareclearlyredincolor,whereassepiaeyesmayhaveareddishtintbutarenotbrightred.Thefirstsethasbeendoneasanexample.

Genotypes:

SseeorSSee __________________ ________________ _________________

Drosophilahavesexchromosomesthatcarrygenesresponsibleforproducingsexdifferencesinanumberofdifferentphenotypictraits.Theseincludesexdifferencesinthestructureofthegenitalia(sexorgans)andthepresence/absenceof thetraits thatoccur inonlyoneof thetwosexes.Forexample, thegenetic lociresponsiblefortheproductionofsexcombsresideonthesexchromosome,whichiswhymalefruitfliespossesssexcombsbutfemalesdonot.Thefliesaboveareallmales.Therearetwowaystotellthis.

©Zumalt2008.Usedwithpermission.

Figure 5-1


Thefirstistolookattheforelegsofeachflyandnoticeadarkfuzzyareacalledasexcomb(seefigureatbottomleft).

Thesecondwayistonoticetheshapeofthebackendoftheabdomen—itisroundedinmales(shownatbottommiddle)andpointedinfemales(shownatbottomright).

Inlabthisweek,youwillhavetosortfruitfliesaccordingtosex.Rememberthattherearenosexcombsinthefemalesandremembertolookforamorepointedabdomeninfemaleflies.

Meiosis and Gamete FormationAssuming you have now grasped the relationship betweenphenotypeandgenotype,let’sexplorethenotionofgenotypeby returning to the chromosomal basis of inheritance. Totheleftisacartoonofacellwithfourpairsofchromosomeswithinit.Mostofthetime,DNAisuncoiledaslongstringswithin the nucleus, but as a cell prepares to divide, thechromosomescondenseasshown.Nisthenumberofkindsofchromosomes,andhereN = 4.Thiscellhas2 copies of each chromosome (1 inherited from each parent),sowedescribeitas2N = 8.Thecellshownisdiploid,asarebodycells(somaticcells)ofmostorganisms.

When diploid organisms reproduce, they producehaploid cellsthroughaprocesscalledmeiosis.Thehaploidcells(N=4)willbecomesexcellsorgametes,specificallyeggsorsperm(inplants,ovules,andpollen).Whenaneggfuseswith a sperm, the resulting zygote (first cell in a neworganism) has the same number of chromosomes (2N) asdideachofitsparents.Clearly,ifgametesarehaploid,thenthezygotewillgethalfitsgeneticmakeupfromitsmotherand half from its father. Assume that the cell pictured isfromamale fruit fly andwillundergomeiosis toproducesperm. Notice the position of the two genes we discussedpreviously—thebodycolorgeneisontheuppercentralpairofchromosomesandtheeyecolorgeneisonlowercentralpairofchromosomes.Neitherofthesechromosomesaresexchromosomes—sex chromosomes have special inheritancepatternsthatwewilldiscusslater.ThegenotypeofthiscellforbodyandeyecolorisEESs.Ifthiscellmakessperm,each

©Santos2008.Usedwithpermission.

Figure 5-2


Figure 5-3


Figure 5-4

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.

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spermwillhaveone copyofeverychromosometypeinthecell(forexample,eithertheonewithSortheonewiths).Whenthespermfuseswithanegg,thediploidchromosomenumberwillberestoredandthezygotewillhavetwocopiesofthebodycolorgene,twocopiesoftheeyecolorgene,andtwocopiesoftheother13,599genespresent.Ineachcase,onecopywillbeinheritedfromthemother,theotherfromthefather.

Beforemeiosis,everychromosomewillbeduplicated.Howchromosomesaredividedintofoursex cells during meiosis is discussed in your basic genetics handout from lecture. If you are notcomfortablewiththisprocess,re-readthehandoutnowandpayspecialattentiontothesectiononindependent assortment. You must be able to visualize how the genes on chromosomes behaveduringmeiosisbecausefromthispointon,wewillnotdrawanymorecells,wewill justwritethelettersforthegenotypesofparents,gametes,andoffspring.ThegenotypeofthecelljustshownisEESsandforthebodysizeandeyecolorgenes,itcouldproducetwokindsofhaploidgametesviameiosis:ESorEsgametes.

To diagram the expected offspring genotypes from a mating, use a Punnett square where themaleparents’gametesareplacedononesideofthesquare,thefemale’sgametesontheothersideofthesquare,andtheunionofeachpossiblepairofgametesproducesadiploidoffspringgenotype.Ifanyofthisisunclear,returntothebasicgeneticshandoutagain.

6. Fortheproblembelow,translatetheflydescriptionsintoparentalgenotypes,andthencontinueasinstructed.

Consideracrossbetweenfemaleflieshomozygous for brown bodies and red eyesandmaleflieshomozygous for ebony bodies and sepia eyes.Givethegenotypesandthepossiblegametesofeachsetofparents.

Femalegenotype:_________________ Gametespossible:__________________

Malegenotype:___________________ Gametespossible:__________________

7. The offspring of this cross, known as the F1 generation, will have the following genotypes andphenotypes:

Genotype:_______________________ Phenotype:________________________

8. WhatarethefourpossiblegametesproducedbyfliesfromtheF1generation?

______________________ ______________________ ______________________ ______________________

9. TheF2generationisproducedbycrossingmalesandfemalesoftheF1generation.RepresenttheF2generationwithaPunnettsquare.FirstplacethefourpossiblegametesoftheF1parentsalongthetoprowandinthefirstcolumn.Next,unite gametes to form zygotesbycombiningeachgameteofoneparentwitheachofthepossiblegametesoftheotherparenttofillinthePunnettsquarebelow.For example, the zygote (offspring) resulting from a cross between an ES and an ES gamete hasalreadybeenplacedintheappropriatesquare[EESS].

Zygoteoroffspringfromgametefusion

Gametesfromthe

otherparent

Gametesfromoneparent

ES

ES EESS


10. ListallpossiblegenotypesfromthePunnettSquareabovethatcorrespondtothefourphenotypesintheF2generation.

Phenotype Genotype

brownbody/redeyes

ebonybody/redeyes

brownbody/sepiaeyes

ebonybody/sepiaeyes

11. Whatistheexpectedratioofthefourphenotypes?

[Togettheratio:Countthenumberofzygotesfromabovethatcanproduceeachphenotypeandlistthemasin4:2:2:1.Thetotalshouldequalthenumberofpossiblezygotes]

Calculating FrequenciesAfrequencyiswrittenasthenumberoftimesaneventoccursdividedbythenumberoftimestheeventcouldhaveoccurred.Ifyouflippedacoin10timesandgot6headsand4tails,thenthefrequencyofheadswas6/10.Itistypicallywrittenasf(heads)=6/10or0.6.

Forourpurposes,anexpected frequencyisdefinedastheproportionoftimesaneventshouldoccurbasedonwhatyouknowabouttheevent.Forexample,headsandtailsarefoundonoppositesidesofacoin,soeachevent(gettingaheadorgettingatail)shouldoccur50%ofthetimeiftheflipsarerandomevents.Thustheexpectedfrequencyofheadsis5/10or0.5.

Wecancomparetheexpectedfrequencytotheobservedfrequencyandseeiftheymatch.Inthiscase,theobservedfrequency(0.6)doesnotmatchtheexpectedfrequency(0.5).

Doesthismeanthatthecoinflipperwascheating?Notnecessarily—whenweobserveonlyasmallsampleofevents,wearemuchmorelikelytoseechancedeviationsfromtheexpectation.Soinsampleof10flipsgetting6headsisnotthatsurprising,butinasampleof1,000flips,wewouldnotexpecttoseeasmanyas600headsveryoften.Ifweran10trialswith100coinflipseach,andourhypothesisthatthecoinisfairandunbiasediscorrect,theobservedfrequencyforheadsshouldbeveryclosetotheexpectedfrequency.

12. Lookbackatquestions9,10,and11.Foreachphenotype intheF2generation,givetheexpectedfrequencybasedonyourresultsfromthePunnettsquare.(Togettheexpectedfrequency,dividethenumberofoccurrencesofaparticularphenotypebythetotalnumberofzygotesproducedintheF2generation.Forexample,if1phenotypeoccurred9timesandtherewere16possiblezygotes,theratiowouldbe9/16.You would convert this to adecimalnumber.)Showcalculations to two decimal places(nofractions).

brownbody/redeyes

ebonybody/redeyes

brownbody/sepiaeyes

ebonybody/sepiaeyes


13. Ifyouscored200flies,howmanywholeflies(fractionsoffliesdonotexist,soroundyournumberupordownasappropriate)wouldyouexpect toexhibiteachof the fourphenotypes?Showyourcalculations.

brownbody/redeyes

ebonybody/redeyes

brownbody/sepiaeyes

ebonybody/sepiaeyes

Paternity Exclusion in Fruit FliesAfamilyoffliesisshowninFigure5-5onpage120.Usetheobservedphenotypestoanswerquestions14to18.

14.Phenotypeofmother: ________________________ (body/eyes)

Phenotypeofpresumedfather:________________________ (body/eyes)

15.Genotype(s)whichcouldproducethephenotypeofthepresumptivefather:

16.Offspringphenotypes—listallof thedifferentphenotypesthatyouobserved inthis family. (Note:Thenumberofdifferentphenotypesobservedwillvaryfromquartertoquarter,soyoumaynotfillintheentiretablethisquarter.)

Number of offspring Genotypes which could Phenotype with this phenotype produce this phenotype (e.g., ebony/sepia) (e.g., 4) (e.g., eess)

1.

2.

3.

4.

17. Arethereanyoffspringphenotypesthatwouldbeimpossibletoobtainifthepresumedfatherwerethe

onlyfatherofthisfamily?(Yes/No)_________________

18. If“Yes,”whatarethesephenotypes?_________________________________________


©Zumalt2008.Usedwithpermission.

Figure 5-5


Population GeneticsIn population genetics, we count genes and alleles present within individuals, but we analyze thepopulation as a whole.Wefocusonapopulationratherthanonindividualsorfamilies,becauseevolutionoccursatthepopulationlevel.Evolutioninvolveschangesinalleleandgenotypefrequenciesfromonegenerationtothenext.Thus,ifwecomparethealleleandgenotypefrequenciesatageneticlocusinonegeneration with the allele and genotype frequencies at that same locus in a later generation, we candeterminewhether these frequencieshave changed, andhencewhether evolutionhasoccurred. If thealleleorgenotype frequencieshavechanged,wecanaskwhy, i.e.,whatprocesseswere responsible forproducingthechangesinfrequenciesacrossgenerations?

You might think that Darwin provided the answer to this question (natural selection causesevolution), but this answer is too simple. Two statisticians (Hardy and Weinberg) independentlyapproached the question and derived a clever solution by identifying the conditions under which no evolutionwouldoccur.Undertheseconditions,allelefrequencieswillnotchangefromgenerationtogeneration.Instead,thefrequenciesreturntowhatisnowknownasHardy-Weinbergequilibrium.Thisequilibriumstatepersistswhenallfiveofthefollowingconditionshold:

1. Chanceeventsdonotalteralleleorgenotypefrequencies;thisismorelikelytobethecaseinverylargepopulations.

2. Matingbetweenindividualsisrandomwithrespecttothetraitinquestion.

3. Mutationdoesnotchangeallelefrequencies.Thatis,thechancesthatanynewallelewillbecreatedbymutationequalthechancesthatamutantallelewillmutatebacktotheolderform.

4. There is no gene flow: individuals do not migrate out of the population (emigration), and newindividualsdonotmigrateintothepopulation(immigration).

5. There is no selection acting on the individuals in the population, so each genotype has an equalchanceofsurvivingandreproducing.

The theorem allows us to generate a set of expected genotypic frequencies from known allelefrequencies.These frequencies are expected in the absence of evolution.Wecancomparegenotypicfrequenciesexpectedwhenevolutionisnotoperatingwiththegenotypicfrequenciesactuallyobservedinapopulation.Iftheobservedfrequenciesmatchthoseexpectedunderahypothesisofnoevolution,thenevolution isnotoccurring.However, if theobserved frequenciesdo notmatch those expectedunderahypothesisofnoevolution,thenevolutionisoccurringandwecanexamineeachoftheHardy-Weinbergconditionstoseewhatiscausingthepopulationtoevolve.Hardy-Weinbergequilibriumservesasanullhypothesis for evolutionary change: when evolution is not occurring, the genotypes occur in Hardy-Weinbergproportions(frequencies)anddonotchangefromgenerationtogeneration.

Notethataccordingtothelistofconditionsneededforequilibrium,selection(whichincludesbothnaturalandsexualselection)isonlyoneoffiveprocessesthatcanproducechangesinalleleandgenotypefrequenciesacrossgenerations.

Inlargepopulations,smallfluctuationsinsurvivorshiporreproductionamongindividualorganismsareunlikelytoaffectalleleorgenotypefrequencies inthepopulation. Ifoneorganismcarryingararealleledies,anotherorganismalsocarryingthatallelemighthaveenoughoffspringtocompensateforthefirstdeath.However,insmallpopulations,thiskindofcompensationislesslikely;chanceeventsmayresultinchangesinalleleandgenotypefrequenciesacrossgenerations.Inlecture,youheardaboutthreesituationswheresmallpopulationsthatarevulnerabletotheeffectsofgeneticdriftwouldbecommonlyencountered: (1) continuously small populations with very little available habitat for expansion, (2)populationbottleneckswheretherehasbeenashort-termreductioninpopulationsize,and(3)foundereffectswherepopulationswerestartedbyoneorafewindividuals.Foundereffectsoccurwhenoneorafewindividualsfromasourcepopulationmigratetoanewlocationwheretheyestablish(i.e.,found)anewpopulation.Becauseonlyafewindividualsfoundedthenewpopulation,itisunlikelythatthenewpopulationwillhavethesameallelefrequenciesasthesourcepopulation.


19. ImaginealargeSource(S0)populationof500fliesataparticulartimethatwewillcallT0.Theflieshavebrownorebonybodies.Basedontheinformationgivenabove,howmanytotalallelesforbodycolorexistin the population?

20.Supposethat200fliesareee,150fliesareEe,and150fliesareEE.

Calculatethefrequenciesofthe3genotypesforthispopulation:

f(ee)=_________________ f(Ee)=_________________ f(EE)=_________________

21. Using the counting methodexplainedbelow,calculatetheallelefrequenciesforthepopulation:Asanexample of the counting method, let’s calculate the frequencyof theeallele.A frequency is the

numberoftimessomethingoccurreddividedbythenumberoftimesitcouldhaveoccurred.ImagineatotalofNfliesinthesample.NoticethattheealleleoccursineefliesandinEeflies.TherearetwocopiesoftheealleleineefliesandonecopyinEeflies.Sowefindthefrequencyofeefliesandmultiplyby2andbyNtogetthenumberofeallelesineeflies.WefindthefrequencyofEefliesandmultiplyby1andbyNtogetthenumberofealleles inEeflies.Weaddthemtogethertogetthe total number of e allelesoccurring:[(f(eeflies)×2×N)+((f(Ee)flies×1×N)].Wedividethisnumberbythetotalnumberofflies(N)multipliedby2becauseeachflycarries2alleles.

Theformulais: f(e)=[(f(ee)×2×N)+(f(Ee)×1×N)]/2N

NoticethatNcancelsfromeverytermandyouareleftwith: f(e)=[(f(ee)×2)+(f(Ee))]/2

You must be able to use this method to calculate any allele frequency from known genotypefrequencies.Calculatethefrequencyofeachallele.

f(e)= f(E)=

22. Assumethatyou justcalculatedbaselineallele frequencies for thispopulationS0 at time, T0.Supposetheseflieswereallowedtomateandproduceoffspringforthenexttwoweeks.Thepopulationislarge,ratesofmutationatthebodycolorlocusarelow,andfliescannotenterorleavetheSourcepopulation.

If the allele frequencies were: f(e) = 0.55 and f(E) = 0.45, and you waited 2 weeks (one flygeneration)totakeanothersampleofflies(S1fliesat T1),whatgenotypefrequencieswouldyouexpecttosee?

Tocalculateexpected genotypefrequenciesfor S1fliesatT1,useaProutSquare(forpopulations).Placetheobservedallelefrequenciesineachofthemarginalsquares,thenmultiplythemtogethertofillinthecenterfourvalues(A, B, C,andD).

f(e) = 0.___ f(E) = 0.___

f(e) = 0.___Af(ee)=0.____

Bf(Ee)=0.____

f(E) = 0.___Cf(eE)=0.____

Df(EE)=0.____

Theupperleftcentralsquare(A)tellsyoutheexpectedfrequencyoftheeegenotype.Thelowerrightcentral square (D) tells you the expected frequency of the EE genotype. To get the frequency of theheterozygotes,addthevaluesfromsquaresBandCtogether.


Whataretheexpectedgenotypicfrequenciesunderthehypothesisofnoevolution?

f(ee)=_____________;f(Ee)=__________________;f(EE)=___________________

23.Suppose you were following the fly population studied in Question 22 and you observed thefollowing frequencies in the second fly generation, T1: f(ee) = 0.30; f(Ee) = 0.50, and f(EE) = 0.20.Comparetheobservedfrequencieswiththoseexpectedunderthehypothesisofnoevolutionusingthetablebelow.

f(ee) f(Ee) f(EE)

ExpectedfrequencyatT1

ObservedfrequencyatT1

Differencebetweenexpectedandobserved

Examinethedifferencebetweentheobservedandexpectedvalues.Doyouthinkthisflypopulationisevolving?

24.Re-readthedescriptionofthecultureconditionsdescribedinQuestion22andtherequirementsforHardy-Weinberg equilibrium. Which equilibrium conditions were not described in Question 23?Whatcanyounowconcludeabouttheseconditions?

25.SupposethatonerandomlyselectedfertilizedfemalefruitflywasselectedfromtheSourcepopulationattimeT0.Thisone pregnant femalewasplacedinaseparatevialandallowedtolayhereggsinthemedium.Herhatchingoffspringmakeupthefirstfilialgeneration(F1)forthatfemale.Becausethefemalewas themotherofallof theoffspring inhervial, shewas the“foundress”of thisnew,F1,population(seeFigure5-7onpage131foranillustration).

Giventhewaythatthispopulationbegan,howwouldyouexpectthefrequencyofgenotypesinthe F1 generation to differ from the frequency of genotypes in the T1 generation as described inQuestion22?

26.Whatisthelikelycauseofanydifferencesyouexpecttosee?

27.AreyoumorelikelytoobserveevolutionbetweenT0andT1intheSourcepopulationorbetweenT0andT1intheFounderpopulation?____________________Explainyourreasoning.

Quantitative GeneticsAll thephenotypic traitsyouhaveconsideredsofararesinglegene,ordiscrete, traits.Here,onegeneaffectsonecharacterinthephenotypeandthephenotypeiscategorical(redeyesversussepiaeyes).

Thereareotherwaysthatgenescanaffecttraits.Forexample,pleiotropicgenesaffectmorethanonetrait in the organism (the color pattern and vocalizations of Siamese cats provide an example—bothaspectsofthephenotypearedeterminedbyonegenewhoseproductactsthroughoutthebody).Oryoumayfindthattheallelesatanumberofgenes interactwitheachothertoproducenearlycontinuousvariationinaphenotype.

Inlabthisweekyouwilllookatquantitativetraits,wherethephenotypeisaffectedbythecombinedproductsofseveraldifferentgeneticloci.Quantitativetraitshaveseveralothernames,includingmetric,additive, or dosage-dependent traits. These names all refer to the idea that each gene is contributingsomethingtothephenotype.Contributionstothephenotypearesometimesdescribedas“doses”ofgene


product.Asmoregenescontributetoatrait,thegreaterthepotentialamountofgeneproductpresent,andthemoreextremethephenotype.Forquantitativecharacters,weseeawiderangeofphenotypes,notjustoneortwocategories.

InthecellshowninFigure5-6,youseetwolocimarked:theSlocusandtheQlocus.Supposeeachlocuscontributedtotheheightoftheorganismthatpossessedit.AssumethatS1allelecontributestwodoses of a growth product that increases height and S2 contributes four such doses, whereas Q1contributes1doseandQ2contributestwodoses.

29.Howmanydosesofgrowthproductwillbepresentintheanimalwiththegenotypeshown?_________________________________

30. Ifapopulation of animalscontainsonlythoseallelesdescribedaboveforeachlocus,whatisthemaximumnumberofdosesofproductthatananimalinthispopulationcouldhave?____________

31. If a population of animals contains only those alleles described above for each locus, what is theminimumnumberofdosesofproductthatananimalinthispopulationcouldhave?

32.Howisitpossiblethatthispopulationcouldcontainanimalswith6,7,8,9,10,11,or12dosesofgrowthproduct?

33. If each dose of growth product increases the height of the organism, how many phenotypes forheightcouldexistinthepopulation?



Figure 5-6

125

Lab 5: Mendelian and Population Genetics

In-Class Exercise 1: Mendelian Genetics of a Model OrganismToday’slaboratorybeginswithasimpleexerciseinwhichmodelsofchromosomesareusedtohelpyoubridge the conceptual gap between chromosomes and Mendelian genetics. You will use modelchromosomes to produce gametes, combine these gametes with those of your lab partner to makeoffspring,andthenguessyourpartner’sgenotype,basedonyourknowledgeofyourowngenotype,thegenotypesoftheoffspringyouhaveproducedtogether,andtherulesofMendeliangenetics.Thisexercisewillbemoreeffective(andmorefun!)ifyoudon’tknowyourpartner’sgenotypebeforeyouguesswhatitis,soworkinvisualisolationfromyourpartneratfirst.

A. Getting Started

1. Selectalaboratorypartnerandsitinawaythatensuresthatyouandyourpartnercannotseeeachother’swork.

2. Pickupasomaticcell(=plasticbag)fromyourTA.Thiscellrepresentsasingleindividual(organism).Insidethecellarechromosomes,representedbystringsofbeads.Eachsmallbeadrepresentsadifferentgeneticlocus,andbeadsofdifferentcolorsrepresentdifferentallelesatthesamegeneticlocus.

3. Removethechromosomesfromthebag,andmakesurethatyourorganismhasfourchromosomes—twocopiesofthe“R”chromosome(identifiedbyalargeroundblackbeadatoneendofthechromosome)andtwocopiesofthe“S”chromosomes(identifiedbya largesquarepurplebeadatoneendofthechromosome).TellyourTAifyoudonothavetwocopiesofeachtypeofchromosome.

Fromthepre-lab,recalltheterminologyusedtoidentifydominant,recessive,andcodominantalleles.Onegeneticlocusinyourorganism,theHlocus,producesaproductthataffectsheadshape.Twoallelescanoccuratthisgeneticlocus:H1 (indicated by a red bead) and H2 (pink bead).ThegenotypeH1H1producesaroundhead,genotypeH2H2producesapointedhead,andtheheterozygote(H1H2)producesanovalhead.NotethatH1andH2arecodominant.

Asecondgeneticlocus,theFlocus,producesaproteinwhichaffectsfoottemperature.Fourallelescanoccuratthisgeneticlocus:F1 (green), F2 (gold), F3 (light blue), and F4 (dark blue).

Inthehomozygouscondition,F1F1produceshotfeet,F2F2produceswarmfeet,F3F3producescoolfeet,andF4F4producescoldfeet.AlleleF1isdominanttoallelesF2,F3,andF4;alleleF2isdominanttoallelesF3andF4;andalleleF3isdominanttoalleleF4.Insummary,therelationshipsbetweengenotypesandphenotypesareasfollows:

Head ShapePhenotype Genotype Genotype indicated by

Round H1H1 tworedbeads

Oval H1H2 oneredbead,onepinkbead

Pointed H2H2 twopinkbeads


Foot TemperaturePhenotype Genotypes Genotype indicated by

Hot F1F1,F1F2,F1F3,F1F4 atleastonegreenbead

Warm F2F2,F2F3,F2F4 nogreenbead;atleastonegoldbead

Cool F3F3,F3F4 nogreenorgold,atleastonelt.bluebead

Cold F4F4 twodarkbluebeads

Lineupeachpairofyourhomologouschromosomesnexttoeachothersothatmatchinggeneticlociaredirectlyadjacenttooneanother.

Whichchromosome(RorS)carriestheHgeneticlocus?______________________________________

Whichchromosome(RorS)carriestheFgeneticlocus?_______________________________________

Thegenotypeofmyindividualis__________________________________________________________ . [e.g.,H1H1F2F4]

Thephenotypeofmyindividualis_________________________________________________________ . [e.g.,roundhead,warmfeet]

B. Meiosis and Gamete FormationYourorganismreproducessexually,anditproducesgametesusingaprocessanalogoustomeiosis.

Basedonthegenotypeofyourindividual,howmanydifferenttypesofgametescoulditproducewithrespecttotheHandFloci?______________________________

Inthespacesbelow,indicatethegenotypesofallofthegametesthatyourorganismcouldproduce.(Forinstance,ifthegenotypeisH1H2F3F3,itcouldproducetwotypesofgametes:H1F3andH2F3)

_______________________ ________________________ ________________________ _______________________

Making GametesPutallfourofyourchromosomesintothegonad(=clothbag).Usingyoursenseoftouch,findandpulloutoneSchromosomeandoneRchromosomefromthebag.Thispairofchromosomesconstitutesonegamete.Recordthegenotypeofthisgametebelow.Replacethechromosomesinthebag,andthenrepeattheprocess,makingasecondgamete.Recordthegenotypeofthissecondgametebelow,andthenreplacethechromosomesinthebag.

Genotypeofgametenumber1: ____________________________________

Genotypeofgametenumber2: ____________________________________

Eachofyourgametesshouldbehaploid(containingonlyonecopyofeachautosome).Anymistakesingameteformation(e.g.,twoRchromosomes,orthreechromosomes)arelethal,somakesurethateachgametehasone(andonlyone)copyofeachtypeofchromosome.Becauseyourandomlyselectonecopyofeachtypeofchromosomewhenyoumakeagamete,geneslocatedonthetwotypesofchromosomeswillassortindependentlyofoneanother.


The procedure you have just used to make gametes is analogous to the stage in meiosis in whichhomologouschromosomesarepulledapartfromoneanother.BesureyoucanexplainhowtheprocessthatoccursduringthisstageofmeiosisresultsinMendel’s law of independent assortment(Mendel’ssecondlaw).

C. Using Genetic Information to Guess your Partner’s GenotypeInthisportionofthisexercise,youandyourpartnerwilleachmakegametes,andthencombinethemtoformanewdiploidindividual.

1. Recordyourgenotypeinthetableprovidedbelow.

2. MakeagametebyrandomlyselectingoneRchromosomeandoneSchromosomefromyourbag.Recordthegenotypeofyourgameteinthetablebelow.

3. Atthesametime,yourpartnerwillrandomlyselecttwochromosomesandmakeagamete.Recordthegenotypeofyourpartner’sgameteinthetablebelow.

4. Combineyourgametewithyourpartner’sgamete to formanew individual,Offspringnumber1.RecordthegenotypeofOffspringnumber1inthetablebelow.

5. Retrievebothofyourchromosomes fromOffspringnumber1andput themback intoyourbag.Check your genotype of the gamete (recorded below) to make sure that you have your ownchromosomesandnotthoseofyourpartner!

6. Repeatsteps1through5fourmoretimes,makingatotaloffivedifferentoffspring.

ResultsMygenotypeis______________________________________________________ .

My Gamete Partner’s Gamete Offspring Genotype

1. ______________________________ ______________________________ ______________________________

2. ______________________________ ______________________________ ______________________________

3. ______________________________ ______________________________ ______________________________

4. ______________________________ ______________________________ ______________________________

5. ______________________________ ______________________________ ______________________________

Basedontheresultsindicatedinthetableabove,yourpartnercouldhavethefollowinggenotype(s):[NOTE:morethanonegenotypeispossible!]

Finally,askyourpartnertoshowyouallfourchromosomesfortheirorganism.

Whatisyourpartner’sgenotype? __________________________________


In-Class Exercise 2: Population Genetics of a Model OrganismTheexerciseyoujustcompletedtaughtyoutouseinformationfromgenotypesandphenotypesofsinglefamilies(parentsandoffspring).ThisisthebasisofMendeliangenetics.Normally,afamilyoforganismsispartofalargerinterbreedingpopulationoforganisms.Inpopulation genetics,westudytheentirecollectionofindividualsbyrecordingthefrequencies of the genotypesandthefrequencies of the alleleswithinthepopulation.Whenalleleandgenotypefrequencieswithinapopulationchangefromonegenerationtothenext,evolutionisoccurring.Soweneedtobeabletocalculategenotypicandallelicfrequencieswithinpopulationsacrossgenerationstolookforevolution.Youwilllearntomakethesecalculationsnow.

A. Calculating Allele Frequencies from Genotype FrequenciesTo determine the frequency of alleles in a population, we count them. One way to do this is to usemolecularmethodsthatallowustovisualizedifferentallelesdirectly.Anotherwayistomeasuregenotypefrequenciesandthencountallthealleleswithineachgenotypiccategory.Todemonstratethecountingmethod, we will assume that the model organisms with the bead chromosomes represent a smallpopulation. We will first calculate genotypic frequencies for this population and then move fromgenotypicfrequenciestocalculationsofallelefrequencies.

Usingmodelchromosomes,therearethreepossiblegenotypesattheheadshapelocus:H1H1,H1H2,andH2H2.ReportthegenotypeofyourmodelorganismtotheTAsoitcanbecompiledincolumntwoofthechartbelow.

Onceyouknowhowmanyindividualshaveaparticulargenotype,youcancalculatethefrequencybydividingbythetotalnumberofstudentsinyourclass.Forexample:f(H1H1)=#H1H1genotypes/totalnumberofgenotypes.

Numberofstudents= _________________________

Placethefrequenciesofthethreegenotypesincolumn3ofthetablebelow.

Numberofindividuals FrequencyofGenotype withthisgenotype thisgenotype

H1H1

H1H2

H2H2


FromthefrequenciesoftheH1H1,H1H2,andH2H2genotypesinyourclass,yourTAwillshowyouhowtocalculatethefrequenciesoftheH1andH2allelesusingthecountingmethod.Thereisspaceforyournotesbelow

CALCULATIONS of Allele Frequencies from Genotype Frequencies

Results

ThefrequencyoftheH1alleleinyourclassis_________________________________________ .

ThefrequencyofH2alleleinyourclassis_____________________________________________ .

You will use the procedure you just learned to calculate genotype and allele frequencies in a fruit fly population. Ask any questions you may have now.


In-Class Exercise 3: Scoring Fruit Fly Phenotypes under the Dissecting MicroscopeTo find the genotypic frequencies at the eye color locus in a fruit fly population you must learn todiscriminatefruitflyeyecolorphenotypes(thisiscalledscoringeyecolor).Youwillusethedissectingmicroscopeskillsyoulearnedlastweek.

Sorting the FliesObtainavialcontainingfruitfliesfromyourTA.ThisvialwillbeidentifiedasS0orS1,dependingonitscolor;thesetermswillbeexplainedshortly.Fornow,removethefliesfromthevial,andwritethevialyouhave(S0orS1)intheboxtotheright.PlacethefliesinaPetridish,andthenputthedishunderthedissectingmicroscope.

Inthisexercise,weusefruitfliesthathavebeengeneticallyengineeredtoproduceasimplecodominantMendeliansystemthatallowsustodiscriminateflieswiththreedifferentgenotypes:

C1C1(redeyes) C1C2(pinkeyes) C2C2(whiteeyes)

Becausetheseeyecolorsaremucheasiertoscoreinfemalesthaninmales,youneedtosexthefliesbeforeyoucountthem.Asyourecall fromthepre-lab,fruitfliescanbesexedusingsexcombsortheshape of their genitalia. In males, the abdomen ends with a horseshoe-shaped ring of small bumps,whereasthefemaleabdomenends inapointedstructure.Picturesofsexcombsandmaleandfemalegenitaliaareprovidedateachlabtable.

Sortmalesandfemalesusingsexcombsandgenitalia.

Youwillneednomorethan35 femalesforthepopulationgeneticsstudy.Putthe35femalesattherightsideofthedish.Putallremainingfliesbackintothevial.

Collecting Data on the 35 FemalesPicturesofflieswiththethreedifferentphenotypes(redeyes,pinkeyes,andwhiteeyes)areavailableforeachlaboratorytable;consulttheseifyouhavedifficultydiscriminatingeyecolors.

Recordthenumberoffemaleflieswitheachphenotypebelow:

PopulationCounted(circleone)S0orS1

Phenotype NumberofFemaleFlies

RedEyes

PinkEyes

WhiteEyes

Record the number of female flies with each eye color from your sample onto the blackboard in the appropriate S0 or S1 column.


Youshouldknowhowtousethesecountstocalculatetheactualallelefrequencyforapopulation(youjustdiditintheclassbeadexercise).FromtheallelefrequenciesinS0,youcancalculatetheexpectedgenotypicfrequenciesforS1.

Why Are We Making These Counts?Evolutionisdefinedasanychangeingenotypeorallelefrequenciesacrossgenerations.Canweobservesuchchanges?ToobserveevolutionwewillcomparetheexpectedgenotypicfrequenciesunderHardy-Weinbergconditionswiththeobservedgenotypicfrequenciesinalatergeneration.

In-Class Exercise 4: Population Genetics of Fruit FliesToday’sstudyisananalysisoftwopopulations.ThefirstisalargeSourcepopulationoffruitflies,andthesecondisasmallFounderpopulation.TheSourcepopulationlivesinthelaboratory,passingfromonegenerationtothenextasindividualsmate,reproduce,anddie.

TheSourcepopulationcontainsmorethan300flies representingall threegenotypes (C1C1,C1C2,C2C2).Wehavetakenmanysmallsamplesoffruitfliesfromthispopulation,andthestudentswillcountthe flies with different phenotypes. The class will pool its data to estimate the observed genotypicfrequenciesfortheSourcepopulationatthefirstgeneration(S0).

WehavetakenasecondsetoffruitflysamplesfromthesameSourcepopulationonegenerationlater(theS1samples).Theclasswillpoolitsdatatogettheobservedgenotypicfrequenciesforthisgeneration,S1.

Thebasicdesignisshownontheleftsideofthediagrambelow.Youwillusethegenotypefrequenciesfor the Source population at T0 to calculate the allele frequency at T0. From this, you will calculateexpectedgenotypicfrequencies.YouwillcomparetheexpectedgenotypicfrequencyfortheS1populationatT1withwhatyouobserveatT1toseeifevolutionisoccurring.

Lookbacktopage121 atthe5conditionsnecessaryforHardy-Weinbergequilibrium(noevolution)tobemaintained.

Do you think the Source population meets these conditions, or are you likely to see it evolve in the lab?

Thesecondpartoftoday’sstudyistoanalyze the genotypic and allelicfrequenciesinasmallFounderpopulation.LookattherightsideofthediagraminFigure 5-7—notice that a single matedfemalestartsanewFpopulation.

Obviously, when the female wasfirst put into her Founder vial, thegeneticdiversityofherpopulationwasverylow.Thefemalecarriedthespermoftheoneortwomaleswithwhomshemated before she was removed fromthe Source population. So at itsinception, the Founder populationcontained the genes of only two tothree individuals. However, becauseeach female subsequently producedmanyoffspring,eachFoundervialnowhasarelativelylargepopulationintheF1generation.


Figure 5.7


FounderPopulation

Phenotype NumberofFemaleFlies

RedEyes

PinkEyes

WhiteEyes

Do you think the Founder population meets the conditions necessary for Hardy-Weinberg equilibrium, or are you likely to see it evolve? We can look for evidence of evolution by comparing the observed genotypic frequencies in F1 with those expected from the allele frequencies in S0. Do you think the observed and expected frequencies will be the same?

[YourTAmayshowyouthedistributionofallelefrequenciesinthevariousFounderpopulations,soyoucanseehowthesepopulationsdivergefromoneanother.Ifyouhavetroubleunderstandinggeneticdrift,askaboutthis.]

Collecting Data on the Founder Population

1. ObtainavialcontainingthedescendantsofoneFounderfemalefromyourTA.EmptythevialintoaPetridishandseparatethemalesfromthefemales.Discardthemales.

2. Recordthephenotypesof30femalesintheF1generation.Ifyouhavefewerthan30femalesinyourvial,scoreasmanyasyouhave.ThisisyourFounderpopulation.

3. AfterTAapproval,recordthenumberoffemaleswitheachphenotypeonHomework Assignment 2attheendofthisexercise.

Data Analysis1. TheentireclassmusthavecompletedcollectingdataontheSource(S0)andS1populationsbefore

analysiscanbegin.IfyouhavegivenyourdatatotheTAandtherestoftheclassisnotready,youmaybeginin-classexercise#5onquantitativegenetics.

2. Allele frequencies in the S0 sample:

YourTAwilladdupthenumbersoffemaleswithdifferenteyecolorsfromeachpairofstudentstogetclassroomtotalsfortheS0sample.RecordtheclassroomtotalnumberoffemalesintheS0samplewith red, pink, and white eyes on Homework Assignment 2. Follow your TA as s/he converts thecounts to genotypic frequencies and then computes the allele frequencies in the S0 sample. Youlearnedhowtodo this inexercise#1, so it shouldbeveryclearnow.Record theseallelevaluesonHomework Assignment 2.

3. Expected genotypic frequencies for the source population:

Given the allele frequencies we observed for the Source population, your TA will lead you incalculatingtheexpected genotypic frequenciesfortheSourcepopulation.Follow your TA and record the method on page 145.


4. Genotype frequencies in the S1 sample:

YourTAwilladdupthenumbersoffemaleswithdifferenteyecolorsfromeachpairofstudentstogetclassroomtotalsfortheS1sample.YourTAwillwritethetotalnumberofred,pink,andwhite-eyedfemalesintheS1sampleontheboard.

RecordthesedataonHomework Assignment 2.Usethesedatatocomputethefrequencyof theC1C1,C1C2,andC2C2genotypesintheS1sampleathome.

5.Genotypic frequencies in your F1 population:

Using the data you recorded for your Founder (F1) population, calculate the observed genotypicfrequenciesforyourFounderpopulation.

Determining the Expected Hardy-Weinberg Equilibrium Genotype FrequenciesThegoalofthisexerciseistoseewhetherthegenotypefrequenciesforS1andF1areinHardy-Weinbergequilibrium.Usetheallelefrequenciesfromthecorrectpopulationtopredict the proportions of each genotype expected if the population is at equilibrium.Youshouldbeabletofigureoutwhichcomparisonstomake.Compareexpectedgenotypefrequencieswithourobservedgenotypefrequenciesandseehowtheydiffer.Perhapssomegenotypeswillbemorefrequentthanexpected—recallfromlecturethatpositiveassortativemating results in an increase in observed frequency of homozygotes as compared with the expectedfrequency.

YouwillcontinueworkonHomework Assignment 2afterclass.


In-Class Exercise 5: Quantitative Genetics in Fruit FliesEyecolorandbodycolorinfruitfliesaresinglegenetraits,butothertraits,suchasbodysizeorwinglength, result from the combined actions of many genes. We call such traits polygenic, additive, orquantitativetraits.

Bodysize infruit flies ismeasuredusingthe lengthofthethorax(themiddleportionofthefly’sbody).ThoraxlengthismeasuredfrompointA(thedorsalportionofthethoraxnearestthehead)topointB(thetipoftheprojectionwherethethoraxextendsoverthedorsalsideoftheabdomen)asshowninFigure5-8.


Figure 5-8Thereisnotenoughtimetohaveyoumeasurethefliesinlabthisweek,buttheprocedurewouldbe

similartothatusedinLab3tomeasureBlepharisma.Bodysizedatahavebeencollectedforyoufromfliesofsixgeneticlines.Yourtaskistounderstand

thegeneticbasisofthisquantitativetrait.Howcanwe identifywhichgenesarecontributingtoquantitative traits?To lookforquantitative

traitloci(QTLs),researcherscreateorganismshomozygousatalllociviainbreeding(matingamongcloserelatives).

WeillustrateachromosomefromaninbredhomozygouslinewithasinglecolorasshowninFigure5-9.

Wewillusegreento indicatechromosomesfromonehighly inbredparentandyellowto indicatechromosomesfromanotherhighlyinbredparent(Figure5-10).

Figure 5-9 ©ThienMai2008.Usedwithpermission.



If we cross two inbred organisms, the offspring will have one chromosome from each parent asshowninFigure5-11:

Whenreproductivecells intheseoffspringundergomeiosis tomakegametes,chromosomescrossover,exchangingDNA.Crossingoverresultsinrecombinationofgenes,sochromosomesingametesmayhavegeneticcombinationsnotpresentineitherparentasshowninFigure5-12.

Eachtimethatmeiosisoccurs,thepatternofrecombinationislikelytodiffer,sotherewillbeawiderangeofchromosometypesthatcanbeincorporatedintogeneticlinesforstudy.

Weknowthatallyellowchromosomesareidentical.SothetwochromosomesshowninFigure5-13differonlyinthegeneticmaterialthathasbeeninsertedfromagreenchromosomeafterrecombination.Ifwewantedtoproduceaflywiththegenotypeshownbelow,wewouldcombinegametesthathaveonesetofrecombinantchromosomeswithgametesfromaninbredorganism.

We can use recombinant inbred lines (RILs) to discover whether the inserted region of DNAcontributestoaphenotypictraitofinterest.WedothisbycomparingthephenotypeoffliesthathavethatoneparticularregionofDNAwiththephenotypeoffliesthatdonothavethatregion.YoucouldcomparethephenotypeoftheflywhosechromosomesareshowninFigure5-10withthephenotypeoftheflywhosechromosomesareshowninFigure5-13.Thesetwofliesdifferonly intheinsertedgreenregionofthechromosome.

EachlineRILisidentifiedbyanumber.Intheexercisebelowyouwillexamine6RILsoffruitfliesthatdifferinbodysize(lines362,080,338,210,050,and124);theselinesarearrangedinorderofbodysize,fromlargest(line362)tosmallest(line124).

WehaveidentifiednineregionsofDNAthatinfluencebodysizeinfruitflies.Yourtaskistodiscoverwhetherthegeneproductsofalltheregionsmakeequalcontributionstobodysize.

Fruit flieshaveonly four chromosomes; eachwitha left and rightarm.Thegenesof interest aredistributedacrosschromosomes2and3,aswellastheXchromosome.Theterms2Land2Rrefertotherightandleftarmsofthechromosome.Youwillexamineonechromosomefromeachofthesixstudylinesasshownonpage139.





Discovering the Value of Quantitative Trait LociFindtheninegenesoutlinedinredboxesonthechromosomemapsfromthesixstudylinesonpage139.

Usetheyellowandgreenmarkersprovidedinlabtorecreate the color pattern fortheninegenesfromeachflylineinthetableonthepage138.Forexample,thefirstcolumnofthetableisforGene1.Gene1isyellowinlines362and080butgreeninline338,soyouwouldcolorthefirsttworowsofcolumn1yellowandthethirdrowgreen.Continueinthiswayuntilthetablebelowrepresentsthegenepatternsinallsixflylines.

1. Noticethebodysizecolumnontherightsideofthetableandconfirmthatfliesarearrangedfromlargesttosmallest.Noticethatthelargestfly linehasayellowversionofthefirstgene,whereasthesmallestflyhasthegreenversionofthisgene.Forallninegenes,theversiondiffersinthelargestandsmallestflies.Let’sassumethatalltheversionsinthelargeflylinecontributetotheincreasedbodysizeoftheseflies.Wecancalltheseversions“plus”alleles.Line362has9plus-alleles,whereasline124has0plus-alleles.Line50has4plus-alleles.Fillinthecolumnmarked“#of+”withthenumberofplus-allelesineachline.

2. Findthedatacolumnforflybodysize.ComparethebodysizeofeachRILlinetothatofthesmallest fly(line124)andfind the difference in body size.Forexample,sizedifference1isbetweenline50and124,sowetake0.780–0.756=0.024mm.Sizedifference#2isbetweenline210andline124.Maketheremaining4comparisonsandrecordthe5differencesnexttolines50to362inTable1.

3. Youknowthatlines124and50DIFFERat4alleles(the4plus-alleles),soyouknowthatsomeorallofthese4allelesproducethe0.024mmdifferenceinbodysize.Outlinethecoloredsquaresforeachofthese4alleles(genes1,3,4,and8).

4. Next,discoverwhatcontributiontosizeismadebyeachallele.Youarelookingattheallelesthatarenot sharedbetweenthelinesbecausethesedifferencesproducethedifferenceinbodysizethatyousee.Assumethateachallelecontributesanequalamounttothesizeofthefly.Thesizedifferencebetweenfliesofline50andline124is0.024mm(seestep2ifyouareunsurewherethisvaluecamefrom).Tofigureoutthecontributionofeachallele,dividethedifferenceinflysize(0.024)bythenumberofallelesthatdiffer(4)betweenthetwolines:0.024/4=0.006.Wethereforeassumethateachalleleofthese4plus-allelescontributedavalueof0.006mmtothesizeofthefliesinline50.

Fromthispointonward,assumethateachofthese4plus-alleleshasaknownvalueof0.006mmofbodysize.

Nowexpandtheprocesstoconsiderline210,usingwhatyoujustlearnedabouttheallelesinRIL50.Somepartsoftheprocessareverysimilar,butthereareafewdifferences.

1. Ifyouhavenotdonesoalready,calculatethesizedifferencebetweenRILs210and124.Didyougetadifferenceof0.045mm?

2. Findtheplus-allelessharedbetweenRILs210and50.Thesesharedalleleswillcontribute,inpart,tothesizeofthefliesofline210.(Youshouldsee2sharedallelesforgenes1and3).

3. Fromyourpreviouscomparison,youknowthatthesethreesharedallelesareeachworth0.006mmofbodysize.Sumthecontributionsofeachoftheseplus-allelestothebodysizeofthefliesinline210:(0.006+0.006=0.012).

4. The sizedifferencebetween210and124 is0.045mm.Youhaveaccounted for0.012mmof thisdifferenceusingthecontributionsofallelesfromgenes1and3.Tofindtheamountofbodysizeforwhichyouhavenot yet accounted, subtract0.012 (the contributionof the2knownalleles) from0.045.Thereis0.033mmofadditionalbodysizetobeexplained.

5. Toexplainthisportionofbodysize,findtheplus-allelesthatRIL210DOESNOTsharewithRIL124.Thereshouldbetwo,oneforgene2andoneforgene9.


6. Tofigureoutthecontributionofeachofthesealleles,dividetheremainingsizedifference(0.033)by2(becausethisisthenumberofplus-allelesNOTsharedbetweenlines210and124).Wecannowassumethattheallelesongenes2and9eachcontribute0.014mmtobodysize.

7. Usingthesamelogicalprocess,determinethevalueoftheallelescontributingtobodysizeforlines338and80.Whenthisprocess is complete,youshouldhaveestimatesofcontribution forall thegenes.

8. Sumthevalueofalltheplus-genestoseehowmuchofbodysizedifference#5canbeexplainedusingthismethod.

TroubleshootingMakesureyourdecimalsarecorrect.

Make sure you include all the known values for each plus allele from all the previous RILs youexamined.Itiseasytogowrongifyouforgettoincludealltheknownvalues.


Gen

e:Li

ne:

1

2

3

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5

6

7

8

9

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Size

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362

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75.

800.

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338

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Figure 5-14

141

Homework AssignmentsDUE at the Beginning of the Next Laboratory

Assignment 1: Paternity Assessment in Fruit FliesAfemalefruitflyhasanebonybodyandsepiaeyes.Youobservehermatingwithamalewithanebonybodyandsepiaeyes(MaleA)butsuspectthatshemighthavealsomatedwithanothermale,whohasabrownbodyandredeyes(MaleB).

The female lays 80 eggs. You allow the offspring from these eggs to mature and then score theirphenotypes.Youfindthat50ofheroffspringhaveebonybodiesandsepiaeyes,10havebrownbodiesandredeyes,10havebrownbodiesandsepiaeyes,and10haveebonybodiesandredeyes.

1. Examine thegenotypesandphenotypes in theoffspring.What evidence indicates that the femalematedwithbothMaleAandMaleB? [Hint:What is the female’sgenotype,andwhat isMaleA’sgenotype?]

2. Listall the possiblegenotypesforMaleB:

3. AssumingthatMaleB’sgenotypewasEeSS,useaPunnettsquaretopredictthefrequenciesofthegenotypesandphenotypesoftheoffspringexpected if the female had mated with Male B.(¾ point)

4. AssumingthatMaleB’sgenotypewasEeSs,useaPunnettsquaretopredictthefrequenciesofthegenotypes and phenotypes of the offspring expected if the female had mated only with Male B.(¾ point)

5. Assumethat50%ofthefemale’soffspringwerefatheredbyMaleA,andthat50%werefatheredbyMaleB.Basedontheinformationprovidedabove,whichgenotypeismostlikelyforMaleB:

(Chooseone)EeSS or EeSs.

Explainyouranswer.(¼ point)

Name: _______________________________ TA: __________________________ Date: ___________

Assignment 2: Population Genetics of Fruit FliesRecordclassroomdatainsectionsA,B,andCbelow.

A. Source Population at Time 0 (S0)Phenotype Genotype Number of Females Genotype Frequency Redeyes C1C1 _______________ _______________ Pinkeyes C1C2 _______________ _______________ Whiteeyes C2C2 _______________ _______________ Totalnumberoffemalesscored:_______________

Record the counting methodtodetermineallelefrequenciesasdemonstratedbyyourTA:

AllelefrequenciesforS0:C1=_______________ C2=_______________

B. Source Population at Time 1 (S1)Phenotype Genotype Number of Females Genotype Frequency Redeyes C1C1 _______________ _______________ Pinkeyes C1C2 _______________ _______________ Whiteeyes C2C2 _______________ _______________ Totalnumberoffemalesscored:_______________

CalculatethegenotypicfrequenciesandrecordtheminpartD.(½ point)

142

143

C. Founder Population at Time 1 (F1)Phenotype Genotype Number of Females Genotype Frequency Redeyes C1C1 _______________ _______________ Pinkeyes C1C2 _______________ _______________ Whiteeyes C2C2 _______________ _______________ Totalnumberoffemalesscored:_______________

CalculatethegenotypicfrequenciesandrecordtheminpartD.(½ point)Fill in summary table D below.

D. Population Genetics Summary Table

Population

Genotype Frequencies S0 S1 F1

FrequencyofC1C1

FrequencyofC1C2

FrequencyofC2C2

Allele Frequencies S0

FrequencyofC1

FrequencyofC2

Describing Expectations Based on Hardy-Weinberg EquilibriumIfapopulationmeetsthefiveconditionsforHardy-Weinbergequilibrium,wedonotexpecttoobserveevolutioninthepopulation.Examinetheexperimentalprotocolonpages131toanswerthequestionsbelow.

6. DoestheSourcepopulationappeartomeettheseconditions?___________Brieflyexplain.(¼ point)

7. Does the Founder population appear to meet these conditions? ________________ Briefly explain.(¼ point)

8. ShortlyyouwillcalculateexpectedgenotypicfrequenciesbasedonallelefrequenciesinS0.YouwillcomparetheexpectedgenotypicfrequencieswiththeobservedgenotypicfrequenciesinS1andinF1.Whichcomparisonhasagreaterlikelihoodofobservingevolution,andwhy?(½ point)

9. Totakeafirstlookatyourdata,calculatethedifferencesbetweenthegenotypefrequenciesinS0andS1asChange(Δ)inFrequencies=|S1-S0|.

Change(Δ)inGenotypeFrequencies(¼ point)

ΔC1C1_____________ ΔC1C2______________ ΔC2C2______________

10.Whatdothedifferencesmean?Circle the correct termineachsentencebelowtodescribewhatyousee.For now, we define a substantial difference as a difference greater than 0.05. ¼ point each (1.5 total)In the Source population from generation S0 to generation S1:

TheC1C1frequencies:increasedsubstantially/decreasedsubstantially/didnotchange.



Theproportionofredeyedflies:increasedsubstantially/decreasedsubstantially/didnotchange.

Theproportionofpinkeyedflies:increasedsubstantially/decreasedsubstantially/didnotchange.

Theproportionofwhiteeyedflies:increasedsubstantially/decreasedsubstantially/didnotchange.

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145

Determining the Expected Hardy-Weinberg Equilibrium Genotype FrequenciesReturntoTableDonpage143tofindthefrequenciesneededbelow:

FollowyourTAass/heusestheProutSquaretocalculatetheexpectedgenotypicfrequenciesunderHardy-Weinberg equilibrium. Notice that you must sum two terms to get the frequency of theheterozygotes.

f(C1) = 0.___ f(C2) = 0.___

f(C1) = 0.___ f(C1C1)=0.____ f(C2C1)=0.____

f(C2) = 0.___ f(C1C2)=0.____ f(C2C2)=0.____

Summarizeyourresultsinthetablebelow.

f(C1C1) f(C1C2) f(C2C2)

ExpectedfrequencyforS1

ObservedfrequencyforS1(fromTableD)


11.Were the observed genotype frequencies in the Source population close to those expected under

Hardy-Weinberg equilibrium? _______________________________________ Succinctly describe the

differences.______________________________________________(½ point)

12. HowwillyoudeterminetheexpectedgenotypefrequenciesforthefounderpopulationF1atT1?(1 point)

146 Lab1: DiscoveringDiversity

Makethecorrectcomparisontoseeiftherehasbeenevolutioninthefounderpopulation.

f(C1C1) f(C1C2) f(C2C2)

ExpectedfrequencyforF1

ObservedfrequencyforF1(fromTableD)


13.Weretheobservedgenotypefrequencies intheFounderpopulationclosetothoseexpectedunderHardy-Weinbergequilibrium?(¼ point)

14.What do these results imply about differences in the evolution of eye color between the SourcepopulationandtheFounderpopulation?Bespecific.(1 point)

15.AssumethatyourFounderpopulationhasdifferentallelefrequenciesandgenotypefrequenciesthantheSourcepopulation.Whatisthemostlikelyexplanationforthischange?(½ point)

147

Assignment 3: Understanding a Quantitative Trait DistributionManyphenotypictraitsaredeterminedbyacombinationofallelesataseveraldifferentgeneticloci.Insuchacase,eachallelehasarelativelysmalleffectonthetraitandthecombinationofalleleshasalargeeffect.Forinstance,heightinhumansisdeterminedbytheactionofmanydifferentgenes.Theproductsofthesegenescausegrowthindifferentpartsofthebody(e.g.,legs,torso,neck,head)atvarioustimesduringdevelopment.

Thenumberofgenesinfluencingatraitaffectsthedistributionofphenotypeswithinthepopulation.Supposeonlyonegene,withtwoalleles,determinesaparticulartrait.Letoneallelebedominantoverthe

other.Howmanydifferent phenotypescouldexistforthistrait?_______________________________

Supposeonlyonegene,withtwoalleles,determinesaparticulartrait,buttheallelesarecodominant.

Howmanydifferent phenotypescouldexistforthistrait?______________________________________

Supposetwogenes,eachhavingadominantandarecessiveallele,togetherdetermineoneparticulartrait.Wecanwritetheallelesateachlocusas“A”and“a,”and“B”and“b.”Anyrecessiveallelecontributesoneunittobodysizeoftheorganismpossessingit.Anydominantallelecontributesthreeunitstobodysizeoftheorganismpossessingit.

Howmanydifferent phenotypescouldexistforthistrait?____________________________________Listthesizesoforganismsthatcouldexistifsizewerecontrolledasjustdescribed:

Areyoustartingtoseehowthenumberofgenescontributing to a trait determines the number ofpossiblephenotypeswithinapopulationoforgan-isms?

Singlegenetraitswithonlytwoallelesproducea small number of discrete phenotypes, like blueeyes or brown eyes, or tall and short pea plants. Afrequencyhistogramforadiscrete traitsuchaseyecolor has only two categories as shown in Figure5-15.

However, there are other kinds of phenotypicdistributions.Forexample,humansdonotcomeintwodiscretesizeclasses(tallandshort);instead,thereiscontinuousvariationinheightamonghumans,fromveryshorttoverytall.

Toanalyzethesetraits,itisnecessarytomeasureindividuals.Whenweincludemeasurementsfromallindividualswithinapopulationinonehistogram,weseeadistributionwithagreatmanycategoriesforthetraitvalue.Becausemeasurementisrequiredtostudythesesortsofphenotypictraits,theyarecalledquantitativetraits.Othertermsusedaredosage-dependent,additive,ormetrictraits.

Figure 5-15

When there are a great many categories on ahistogram, scientists typically just smooth thedistribution and represent it with a curve. This isoften known as the bell curve or a normal distribution.

Whenscientistsmeasurequantitativetraitsforanumber of different individuals in the samepopulation, they frequently find that these traitshave a distribution which is similar to a normaldistribution(i.e.,manyindividualshaveintermediatetrait values, and relatively few have extreme traitvalues).

Thereasonquantitativetraitsoftenapproximateanormaldistributionisrelatedtotheideaofgeneproduct“dosage”asdescribedinthepre-lab.Ifeachlocusiscontributingacertaindosageofproducttothephenotype,andindividualsvaryinwhichallelestheyhave,therearealotofpossiblecombinationsinthepopulation.

Why are very small and very large organisms relatively rare? Theseorganismsoccuronlyatthetailsofanormaldistributionwheretheirfrequencyisquitelow.Forverysmallorganisms,all or almost all,oftheirallelescontributelowdosesofgeneproducts.Forverylargeorganisms,all or almost all,oftheirallelescontributehighdosesofgeneproducts.Thinkaboutthegenotypesofparentsthatcouldproducethesekindsofoffspringandthefrequencyofmatingsbetweensuitableparents.

Verysmallorverylargeoffspringcouldonlybeproducedbyamatingbetweentwoparentswhobothhadalmostallallelesofthistype.Matingssuchasthesewouldberareinarandomlymatingpopulation.Theyaremuchmorelikelytooccurinapopulationthatismatingpositiveassortativelyforsize(thatis,“like-with-like”matings).

Why are average-sized organisms the most common, or the most frequent, in a population?Anaverage-sizedorganismpossessesacombination of alleles for low doses of the gene product and high doses of the gene product. A great many kinds of matings could produce offspring of this type. For example, someoffspringofaveragesizewouldoccurwhenalargeorganismmatedwithasmallorganism,orwhentwoaverage-sizedorganismsmated,orinalmostalltheothermatings.Theonlymatingsinwhichtheywouldnotbeproducedwouldbethosebetweentwoextremelylargeortwoextremelysmallparents.

16.Describeinyourownwords,howyouknowthatthoraxlengthinDrosophilaisaquantitativetrait.(½ point)

17.Ofthetraitsbelow,underlinethoselikelytobequantitativetraits:eyecolorinhumans,eyecolorinfruitflies, leg lengthingrasshoppers,bodyweight indairycows,humanbloodtype(A,B,O),taillengthinmice.(½ point)

148

Figure 5-16

Succession in a Marine Fouling Community

Goals and ObjectivesAt the end of this laboratory you should be able to:

1. Estimatethedistributionandabundanceofspecieswithinacommunity.

2. Explainwhatresourcesaresharedamongcommunitymembers.

3. Predictthepossibleoutcomesofcompetitionforsharedresources.

4. Describethemechanismsbywhichresourcesarerenewed.

5. Analyzethechangeincommunityconstituentsovertime.

6. Proposemechanismstoaccountfortheobservedchanges.

7. Explaintheprocessofecologicalsuccessionandsuggestmechanismsunderlyingobservedchanges.

8. Describetheextenttowhichscientificstatementsarelimitedbydata.


Acommunityisacollectionofdifferentorganismslivinginaparticularplace.Recallthatthecommunitiesyouexaminedinthediversitylabdifferedfromeachotherinspeciesrichnessandabundance.Communityrichness and composition changes over time in a process called succession. Biologists have longwondered why communities differ in diversity and composition, so there are many hypothesizedexplanations,eachofwhichplacesdifferentamountsofemphasisonthearrivalofcolonists,availabilityofresources,andinteractionsamongresidents.Someofthesehypothesesarelistedbelow:

Community age—oldercommunitieshavehadmoretimetobecolonizedbyspeciesfromsurroundingareas(increasingdiversity)andforinteractionsamongspeciestohaveeffects(increasingordecreasingdiversity).

Energy inputs—allfoodwebsarebasedonprimaryproductivity(carbonfixation),soproductiveareas,suchasequatorialregionswithabundantlightandwarmtemperatures,shouldsupportmorespecies.

Resource availability—communitieswitharangeofresources(heterogeneoushabitats)shouldcontainmorespecies.

Lab 6

149

150 Lab6: SuccessioninaMarineFoulingCommunity

Competition—given sufficient time, a superior competitor may dominate a community, monopolizeresources,andexcludeotherspecies(reducesdiversity).

Predation—large or ecologically important predators reduce prey population sizes; this may reducecompetition,permittingmanydifferentspeciestopersist.

Facilitation or habitat modification—theactivitiesormerepresenceofsomespeciesmodifythephysicalenvironment in a way that enhances the success of other species, leading to changes in speciescomposition.

Access to colonists—isolated communities receive fewer colonists than less isolated communities, soisolatedcommunitiesmayhavelowerdiversity.

Rates of disturbance —frequentdisturbanceremovesbothindividualsandspecies,continuallyprovidingopen space and preventing dominance by superior competitors. This may lead to an increase indiversity.

The potential explanations for differences in diversity just presented are not mutually exclusive,meaningthatmorethanonecanbeacceptedastrue.Todistinguishamongpotentialexplanations,onemusthaveuniquetestablepredictionsforeach.Testsofahypothesiscanbemadeusingexperimentalorcomparativemethods.

Comparative methods are often used when experiments are very difficult to do. For example, if youwantedtoknowwhetherplantdiversitydeclinedasredwoodforestsaged,itwouldbedifficulttocreatereplicateforestsandfollowspeciesovertimebecauseredwoodtreesliveforsolong.However,itwouldbepossibletocomparethetreespeciespresent innaturalredwoodforestsofdifferentagesto lookatpatternsandcausesofsuccession.

Natural experiments are another useful tool for studies of community development and speciessuccession. As stated earlier, succession describes the pattern of change in species composition in acommunity. Succession often proceeds through “stages” characterized by different types of species.Sometimesthesestagesarequitedistinct,butmoreoftentheygradeintooneanother.Ecologistsstudysuccessionasawayofpredictingthetrajectoryofcommunityrecoveryafterahuman-causedornaturaldisturbance that creates free space. A natural experiment presents biologists with the opportunity tostudytheresultsofaneventthatcouldnotbeexperimentallyinduced.Forexample,thevolcaniceruptiononMountSt.Helenscreatedemptyhabitats,resultingintheperfectopportunityforanaturalsuccessionexperiment.Biologistswonderedwhetherplantswouldcolonizetheslopesofthemountaininaparticularorder.Theybegantostudythesiteimmediately.Lupineswereamongthefirstplantstocolonize,andsubsequentstudyrevealedthatlupinesmodifythesoilbyaddingnitrogentoit.Asymbioticassociationwithanα-proteobacterium(Rhizobium)permitslupinestofixatmosphericnitrogen;atfirstnitrogenisbound in plant tissues, but it later enters the soil when dead plant matter falls to the ground anddecomposes.Thisnitrogenadditionisvitaltolatercolonistswithoutsimilarsymbioticassociations.Bycombiningobservationsfromalargenaturalexperimentwithresultsfromdetailedexperimentalstudies,ecologists were able to show why certain plants were among the first colonists and how these plantsmodifiedtheenvironmentforspeciesthatarrivedlater.

©Stachowicz2008.Usedwithpermission.

Figure 6-1

Lab6: SuccessioninaMarineFoulingCommunity 151

In communities with long-lived organisms like trees, succession is often studied using comparativeapproaches in combination with natural experiments. For example, ecologists have developed anunderstanding of how forests regenerate by comparing the species composition of communities thatcurrentlyexistinareasthatwereoncefarmedbuthavesincebeenabandoned.Thesesitesareusuallycalled“oldfields.”Ecologistsusethetimesinceold-fieldabandonmentasasurrogatefortheageofthecommunityandarrangecommunitiesinorderofagetoinferthelikelysequenceofsuccession.AbandonedfieldsareshowninFigure6-1onthepreviouspage:movingfromlefttoright,thefirstfieldisonly2yearsold,thesecondisabout10yearsold,andthethirdhasbeenabandonedformorethan100years.Youwillnoticethatsuccessioncanbecharacterizedbydiscretestages,althoughthereis,ofcourse,considerableoverlapinspeciescompositionbetweenadjacentstagesasacommunitymovesfromonestagetoanother.

Noticetheopennessoftheyoungest“oldfield.”Thefirstcolonistsaregrassesandherbs.Thinkaboutseedproduction—dograssesmakemanyseedsorfew?Thinkaboutseedmovement—dograssesdispersewellorpoorly?Thinkaboutresourcessuchaslightornutrients—aretheylikelytobelimited,andwillresidentshavetocompeteforthem?

Noticethatthe10-year-oldfieldhasshrubs.Thinkaboutplantsize,density,andreproductiverates.Thinkaboutresourcecompetitionincomparisontothatintheyoungerfield.Isanewgrassplantlikelytoestablishitself(recruit)inthisfield?

Noticethatthe100-year-oldfieldhastreeswithlittleundergrowth.Theplantsthatarepresentintheundergrowthareferns—whatisthetypicallightregimeforafern?Noticethatthereissomeopenspacebeneaththetrees.Whyisthisspaceopen?Howhasthehabitatforpotentialcolonistschanged?Isitmorelikelythatpotentialcolonistscannotreachthehabitatorthattheycouldnotsurviveiftheydidreachit?

The rhetorical questions just posed were designed to help you think analytically about succession. Using the information and ideas outlined above, in conjunction with what you have learned in lecture, answer the numbered questions throughout the sections below:

Pre-lab Questions—Record Your Answers on the Online VersionSpeciesateachstageofsuccessionoftensharemanytraitsincommon.Recallfromearlierinthequarterourdiscussionofdifferentlife-historystrategies.

1. Wouldyouexpectearlysuccessionalspeciestober-selectedorK-selected?

2. Why?

3. Whataboutspecies thatdominate later insuccession?Wouldyouexpect themtober-selectedorK-selected?

Why do early successional species give way to later species? This is usually because they are poorcompetitors, and they either alter the environment in a way that reduces their own success oreventuallysuccumbtocompetitionfromotherspecies.Whydon’tlatesuccessionalspeciesdominatefromthestart?Thereareseveralpossibilities.Oneisthatitmaytakealongtimeforcolonizationbythesespeciestooccurbecauseofpoordispersalabilitiesandslowgrowthrates.Thisiscalledneutralor tolerance succession. In contrast, early successional species can either facilitate or inhibitsuccession.TheexampleofsuccessionofplantsonMountSt.Helensfollowingavolcaniceruptionisanexampleoffacilitativesuccessionbecauseearlycolonizersmodifytheirenvironmentbyaddingnitrogen,whichfacilitatesorenhancestheabilityofotherspeciestocolonize.Withoutlupines,latersuccessionalspecieswouldbecomeestablishedatamuchslowerrate.

Ininhibitionsuccession,earlysuccessionalspeciessuppresslatercolonistsandsuccessionproceedsonlywhensomeotheragentofmortality(suchaschangingphysicalconditions,orpredators)causesearlycoloniststodie.Underinhibitionsuccession,latesuccessionalspecieswouldbecomeabundantmore quickly in the absence of the early colonizing species. This occurs most often when earlycolonistspreemptresourcesbyoccupyingallavailablespace,light,orotherlimitingresource,thereby


inhibitingtheestablishmentorgrowthoflatersuccessionalspecies.Wewillseeexamplesofthisinlecture.

Inthefirstlabyouwereinterestedincharacterizingpatternsofspeciesdiversitywithinacommunity.Becauseofsuccession,communitycompositionchangesovertime.Howdoyouthinkthatspeciesdiversity changes over time as succession proceeds? From the list of potential causal hypothesesabove, you can see that making a prediction may not be straightforward. Early successionalcommunitiesmightbediversebecausefewspecieshavebeenexcludedandresourcesareabundant,ortheymightbespecies-poorbecauseconditionsaresufficientlystressfulandfewspeciescantoleratetheseconditions(asintheMountSt.Helensexample).Similarly,moretimemightallowmorespeciesto colonize,buton theotherhand, this alsomightallowmore time for competitive exclusion tohappen.Aresolutionofthesetwoideassuggeststhatdiversitymightbegreatestincommunitiesofintermediateagebecausethesehavesufficienttimeformanyspeciestobecomeestablishedbutnotenough for competitive exclusion to have resulted indeclinesindiversity.Lookbackatthephotographsofoldfields of different ages—can you see any evidence insupportofthishypothesis?

In the lab exercise for this week, you will look for elements of succession in a community of marine organisms that encrust hard surfaces in ocean waters.You will use the comparative method by examining thespeciespresentonhardsurfacesplaced intheoceanfordifferentlengthsoftime.Youwillseecommunitiesthatdiffer inage:approximately3months,6months,and9monthsold.

At firstglance,manyof theanimalsyouwill studymayremind you of plants. Many adult marine animals aresessile—they attach to hard surfaces and most cannotmoveoncetheyhavesettledintoaparticularplace.Somesessileanimalsareunitary—theygrowbyaddingnewcellstoanexistingindividual,butmostaremodular andgrowby copying feeding units of the body many times. Perhapsyou have seen a spider plant, Bermuda grass, or astrawberrygrowinthisway.

When the first strawberry plant is established, it sendsrunnersoutacrossthesoil(Figure6-2).

New plants develop when the runners put out roots (Figure6-3).

The new plants are attached to the first plant, but once they have roots, they can survive if therunnersbreak.

This modular growth strategy allows a single genetic individual to exploit a large area of thehabitat.

Fromlab#2,youknowsomethingaboutaplant’sphysiologicalneeds.Plantshavetocollectresourcesand compete with other plants for access to those resources. Imagine a modular plant, like astrawberry, grown in the same planting box with a unitary plant, like a daisy. Both start fromseedlings.Bothplantsneedlight,buttheirgrowthandcompetitivestrategiesdiffer.


Figure 6-2

Figure 6-3



4. Tworesourcesaregivenbelow.Addathirdofyourchoice,andbriefly discuss the relative costs and benefitsofhavingamodulargrowthstrategyforeachresource:

(a) Competitionforlight:

(b) Competitionforsoilnutrients:

(c) Competitionfor_____________________________________________.

5. Is a modular growth strategy better for capturing concentrated resources or widespread (dilute)resources?Brieflyexplainyourthinking.

Modular animals have some similarities to modular plants, but they also have some importantdifferencessuchastheabilitytofeedoncetheyareovergrown.Belowarethumbnailsketchesofafewcommontypesofencrustingmarineunitaryandmodularorganismsthatyouarelikelytoseeinlab.Youdonothavetomemorizeanytaxonomicterms,butapassingfamiliaritywithcommonnamesmaymakeiteasiertodiscusswhatyouseewithyourclassmates.

Aunitary(solitary)tunicate,alsocalledasea squirt isshowninFigure6-4.Theimportantthingtonoticeisthebloblikebodywithtwosiphonsatthetop.Theanimalfeedsbycollectingfoodparticlesfromthewateraroundit;waterentersthebodythroughonesiphon,passesthroughaninternalmeshbasketwhereparticlesarefilteredout,andexitsviatheothersiphon.Youmayseeseveralspecies.

Modular sea squirts develop as one animal buds to produce another.The buds remain attached to each other. Such animals are calledcolonial tunicates,andeachbuddedmoduleisusuallyverysmall.Thecolony has a firm, fleshy appearance and is covered with small poreswherethesiphonsofeachmoduleopentothecolonysurface (Figure6-5). The siphon openings are visible and are sometimes arranged inpathways or clusters. Many tunicate colonies have an extremely lowsurface pH; this might affect colonization or overgrowth by otherspecies?

Figure 6-4


Figure 6-5Bryozoansarestiff,somewhatcrunchy,encrustinganimals.Theyhaveamodulargrowthpattern:thefirstorganismmakesaseriesofbuds,andeachbudmakesothers,untilacolonyisformed.Theshapeofthecolonycanbetreelike,aflatcrust,orsomethinginbetween(Figure6-6).Bryozoansfeedbycollectingtinyparticlesfromthesurroundingwateronaringoftentacles.

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Some of the other unitary organisms you may encounter:

Tube-dwelling worms collect particles on a retractable crown of tentacles. The mouth is in thecenterofthetentaclecrown.Theanimal inFigure6-7hasaflexibletube(seearrow),whereastheanimalinFigure6-8hasahardtube(seearrow)madeofcalcium.

Barnaclescollectfoodbycombingafeatherystructurethroughthewaterabovetheirbodies.Thefeedingstructure,asetoflegs,canbepulledintothehardenedshell(asintheupperanimalinFigure6-9)orextendedthroughtheslitatthetopofthebody(loweranimal).

Mussels attachtohardsurfacesbyasetofstrongthreads(byssalthreads).Theyhavetwoshellswithasmallgape,oropening,betweenthem.Water,containingfoodandoxygen,entersviathegape(seearrowsinFigure6-10)betweenshellsandfoodparticlesarecollectedonaninternalgillcoveredwithmucus.


Figure 6-7 Figure 6-8©ThienMai2008.Usedwithpermission.


Figure 6-6©Santos2008.Usedwithpermission.

Youcannotseetheindividualmodulesinthesephotos.


6. Whatdomostsessileanimalseat?

7. Brieflydescribeonewayanimalscouldcompeteforthiskindoffoodsource.

Figure6-11depictsapanelsimilartothoseyouwillseeinlabthisweek.Thepanelishard,soanimalsdonotpenetrateitasrootsdowithsoil.Thispanelwasplacedintheoceanonlyashorttimeago,somuchofitisstillbare(thegrayarea).

8. Whatkindofanimalisthelargestorangecrust?

9. Forwhatresourcemightsessileanimalscompete?

ThepanelinFigure6-12hasbeenleftintheoceanformanymonths.

Barnacleswithfeedinglegsretractedandextended

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10.Nametwokindsofanimalsthatarevisible:

(a)

(b)

SpaceisclearlyatapremiumontheolderpanelshowninFigure6-12.

11.Describeonegrowthstrategyyouseeherethatwasnotvisibleontheyoungerpanel:

12.Whatcanyouseethatindicatescompetitionamongorganisms?

13.Whymightitbeadvantageoustohaveamodulargrowthpatterninthissituation?

Haveyoustartedtowonderhowanimalsgetontothepanelsinthefirstplace?Animalscolonizethepanelsasfree-swimminglarvae.Somelarvaefeedintheswimmingstageandothersdonot.Thosethatdonotfeedhaveonlyashorttimeinwhichtofindasettlementsite—iftheydelaysettlement,theywillnothavesufficientenergyreserveslefttometamorphoseintoadultsaftertheysettle.

Allnewlyexposedpanelsareempty,sothedevelopmentofthecommunityisaffectedbywhichlarvaeareinthewateratthetimethepanelsareexposed.Thepresenceoflarvaeinthewaterdependsonwhichspeciesinthesurroundingcommunityarereproductivelyactiveatthetimespaceisavailable.Reproductiveactivityinanimalsvariesseasonallyinthesamewaythatsomewildflowersbloominlatewinterandothersbloominmid-summer.However,unlikeseedsthatcanremaindormantforsometime,thelarvaeofencrustingmarineorganismsbegingrowingassoonastheysettle.Oncetheblankpanelisfilled,thereisanopportunityfornewsettlerstolandontopofexistinganimalsandforcompetitionamongorganismstoaffectcommunitycomposition.

14.Listonefactorthatyouimaginewouldlimitsettlementontopoflivingorganisms.

Thepanelshangverticallyfromthesideofadock.Animalssuchascrabsandsnailsliveonthedockandcancrawlontothepanels.Theoceancanbestormyattimes,andheavyrainsmixfreshwaterintotheoceansurfacewaters.

15.Useyour imagination todescribe twoways inwhichdisturbanceof the sort justdescribedmightaffecttheanimalsthatliveonthepanelsandamountoffreespaceavailable.

(a)

(b)

16. Inwhatwaymightgrowth form(shape)ofthepanelinhabitantsaffectthelikelihoodofsurvivingonthepanels?

17.Aremodularorganismsmoreorlesslikelytobepartofthe long-termcommunitythanareunitaryorganisms?Explainyourthinking.

In lab this week you will need to observe patterns of species distribution and to develop testablemechanisticexplanationsforthesepatterns.Youmaywanttolookbackattheorganismsjustdescribedtorefreshyourmemoryontheirnaturalhistoryandlifestyles.


157

Lab 6: Succession in a Marine Fouling CommunityInlab,youandapartnerwillcatalogthenumberandrelativeabundancesofspeciesincommunitiesofthreedifferentageslivingon10cm×10cmpanels.Findatleastthreemorepairsofstudentstopoolyourdatawith.

Select a panel and place it in a large plastic tub filled with seawater. Be sure to randomize yourselectionusingthediceprovided.Usethestringgrid,similartotheoneinthepicturebelowtoestimatepercentcoverforthedifferentspecies.Donotcountanygrowththatextendsbeyondthepanel.Your grid will divide the panel surface into 25 squares each equal to 4% of the total surface area.Usethefractionofasquarefilledbyaparticularorganismtoestimatepercentcover.Forexample,thecolonialtunicateoutlinedinredintheupperrightofthepanelinFigure6-13coversabout100%ofthetotalareain4squares,orabout16%ofthetotalsurfacearea(4squares@4%each).

Usingwhatyoujustlearnedaboutthebasickindsofencrustinganimalsandthephotographickeyprovidedinlab,countthenumberofsessilespecieson one panel of each age.Youwilldistinguishbetweenthenumberofspeciesontheprimarysubstrate(thepanel)andthoseonsecondarysubstrate(growingonotherorganisms=overgrowth)oneachpanel.

Examinethepanelsatthelabbench;youcanputthemunderthedissectingmicroscopeifyouwish.Thismaybenecessary forpanels that lookbare—theyarenot!Begentlewiththeanimalsas theyaredelicateandwillbehandledbymanystudentsthisweek.Itisnotimportantthatyoucorrectlyidentifyeach species with its proper scientific name; the keys provided are meant only as a guide. What isimportantisthatyouareconsistentforallthepanelsyoucountandclearlydelineatethecharacteristics

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ofyourspecies.Forexample,theoutlinedcolonyinthefigureaboveisBotrylloides violaceous.Youwillhaveapicturekeyinlabthatwillclearlyidentifythis.However,whetheryoucallit“Botrylloides violaceous”or“orangeencrustingblob”onyourdatasheetdoesnotmattermuch,solongasyoualwaysidentifythisorganism in the same way. Meet with the other three partners in your group to standardize youridentifications. Your TA will give you some other hints on how to identify and operationally classify“species”forthislab.Useyourexperienceindelineating“operationaltaxonomicunits”foryoursurveyinlab#1asaguide.

Record Your Data on the Grids on Pages 159 to 161(1) Estimatetherelativeabundance(aspercentcover)ofeachspeciesoneachofthepanels;beconsistent

aboutwhatyoucallaspecies.First,recordthetotalcoverofeachspeciesonthepanel.Placethesedata in columnAof thedatagrid.Then record thepercent coverof each species that isgrowingdirectlyonprimarysubstrate(thepanel),ratherthanontopofotherorganisms.RecordthesedataincolumnBofthedatagrid.Thetotalareaoccupiedbyallspeciesmaybegreaterthan100%becauseofovergrowth,butthetotalamountofprimaryspaceoccupiedshouldnotexceed100%.

(2) Countthenumberofspeciesoneachpanel.

(3) Usenumberof speciesandpercentof totalcover tocalculate thediversityofeachpanelwiththeShannon-Wienerindexyoulearnedinlab#1.

(4) Calculatethepercent overgrowthforeachspeciesoneachpanelbysubtractingthepercentonprimarysubstrate(columnB)fromthepercentoftotalarea(columnA)foreachspecies.

Instructionsforfurthercalculationsandthelabreportfollowthedatasheetpages.Instruction (5) is on the top of page 162.

Whenyouhavefinishedwithapanel,putitbackintheflumeexactlywhereyoufoundit.


Data Collection in LabPanel Age

Totalnumberofspecies=

Total%cover=

Shannon-WienerIndexvalue=

Total%onprimarysurface=

Total%overgrowth=

Commonname %Coveron % or Total% panel Overgrowth Species identifier cover(A) Pi surface(B) (=A–B)




Total%cover=



Total%overgrowth=



(5) Poolyourdatawith theother threepairsof students inyourgroup fora reasonable sample size.Calculate average values for number of species, total percent cover, the Shannon-Wiener index,percentcoveronpanel,andpercentovergrowthfor panels of each age.

Panel Numberof %coveron %coveras age species %cover H'value panelsfc overgrowth

3months

3months

3months

3months

Average:

SD


6months

6months

6months

6months

Average:

SD


9months

9months

9months

9months

Average:

SD

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Laboratory Report(6) Ploteachofthefivevalues(numberofspecies,totalpercentcover,Shannon-Wienerindex,percent

coveronpanel,andpercentovergrowth)onthey-axisagainstpanelageonthex-axis(5plotstotal).YourTAwillprovidegraphpaper.Youcanmakeonesetofplotsperpairofstudentsandphotocopythem;otherwisemakeonesetperindividual.(½ point per plot)

(7) Describetheresultsshownineachplotinafewsentences.Writethesentencesonthesideofeach graph.(½ point per description)

(8) Selectoneresultthatinterestsyouandyourpartner.Developapotential explanationforthisresultthatisconsistentwiththeobservations.

Forexample,youmightobserveadeclineinthenumberofspeciesovertimebutnoticethatallthespeciesontheoldestpanelswerecolonialtunicates.Perhapsyouwouldliketoconsiderwhatfactor(s)are responsible for the decline in species richness and why colonial tunicates dominate latesuccessionalcommunities.Youmighthypothesizethatthepreponderanceoftunicateswasduetosuperiorcompetitiveability,orbetterfoodcapturerates,oragreatabundanceoftunicatelarvae,or...Thereareseveralpotentialexplanationsonpages149–150;weave these ideas together with information gained from the pre-lab text on succession.

Results: (½ point)

Proposed hypothesis to explain this result: (½ point)

Name: _______________________________ TA: __________________________ Date: ___________


(9) Developatestablepredictionofyourhypothesis.

Our hypothesis predicts: (½ point)

Our mathematical prediction: (½ point)

Brieflyoutline a feasible experimentthatwouldtestyourhypothesis.Outlinetheessentialfeaturesofthedesignbydescribingwhatyouwoulddoandwhichdatayouwouldcollect.

We could test our hypothesis by: (½ point)

(10) Describeoneexperimentaloutcomethatwouldsupport your hypothesis andone outcome that would refute your hypothesis.Remembertorefertoyourprediction.

Our hypothesis would be supported if the experimental outcome was: (½ point)


Our hypothesis would be refuted if the experimental outcome was: (½ point)

(11) Findanotherpairofstudentsandaskthemtoexplaintheresulttheyelectedtostudy.Listentotheirproposedexplanationfortheresult.Developanalternative explanationfortheresultthispairofstudentselectedtostudy.They will do the same thing for your hypothesis.

My classmates, ______________________________________________, observed: They hypothesized:

Their prediction was:


An alternate explanation and prediction for their result is: (½ point)

(12) Examinethealternateexplanationthattheotherpairofstudentshasdevelopedforyourresult.

My classmates thought my result could be explained by:

Their prediction was:

Would your planned experiment indicate that only one hypothesis was correct? (explain—couldyouruleouttheotherhypothesis?)(½ point)

(13) Assumethattheexperimentthatyou and your partneroutlinedwentasplanned.Whatconclusion(oneortwosentences)wouldyoubeabletodraw?Theconclusionmustbebasedonthedatayouintendedtocollect. (½ point)

If the experiment went as planned, I would conclude that:

Trophic InteractionsGoals and ObjectivesAt the end of this laboratory you should be able to:

1. Explainthetermsprimaryproducer,consumer,preyitem,predator.

2. Giveexamplesofhowpreybehaviorinfluencespredationrate.

3. Giveexamplesofhowpredatorsinfluencepreybehavior.

4. Explaintheconceptofarefugeinspace,time,andsize.

5. Explainhowrelativecostsandbenefitsinfluencefeedingtime.

6. Explainwhypreymayhaveaninappropriateresponsetoanovelpredator

7. Describeafoodweb.

8. Describeatrophiccascade.

9. Analyzetherelationshipbetweennumberoftrophiclevelsinafoodwebandpopulationsizeoftheprimaryproducer.


Theenergyforalmostalllifeonearthcomesfromthesun.Photosyntheticorganismsusethisenergytocombine(fix)carbonfromtheatmosphere(CO2)intoorganiccompounds.Thisessentialprocess,calledprimary production, occurs in autotrophic primary producers such as single-celled algae ormulticellularplants.Primaryproducersarethemembersofafoodwebthatproduceanenergysupply(food)fortherestoftheweb.Primaryproducersareeatenbyprimary consumers; anherbivorousinsectisaprimaryconsumer,soisacow.Secondary consumersfeedonprimaryconsumers,soabirdthateatsinsectsisasecondaryconsumer.Weorganizethesedifferentgroupsintolevels,calledtrophiclevels,asillustratedintheFigure7-1onthenextpage.Energyflowsfromonetrophicleveltothenext,butthemovement of energy can be hard to measure. Instead, biologists often record the consumption ofbiologicaltissuevolume,calledbiomass,asorganismsareeatenbyotherorganisms.

Acollectionoftrophiclevelsconnectedbytheflowofenergyorbiomassiscalledafood web.AsimplefoodwebisshowninFigure7-1.Thewidthofeachboxrepresentstherelativebiomasspresentateachlevelofthefoodweb.Thearrowsindicatetheflowofenergyaseachlevelconsumesmembersofthelevelbelowit.

Lab 7

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168 Lab7: TrophicInteractions

Thisblandlittlediagram,whileuseful,failstodepict the dynamic processes within each box.Organisms within each level feed, reproduce,compete,anddieasthepopulationstheycompriseevolveovertime.

These organisms not only interact with eachother,but they interactwithpopulationsofotherorganismsbothwithinandbetweentrophiclevels.Asaresult,populationsofproducersandconsumersusuallyinteractformultiplegenerations.Thislong-term interaction creates an opportunity for thepopulations to co-evolve. A plant species whoseleaves are consumed by insects may evolve amorphologicaldefensesuchasincreasedtoughness,spines, or trichomes. In response, some of theherbivorespeciesmayevolvemouthpartsthatcanhandlespines,whereasothersmayconsumeotherspecies instead. In addition to morphologicaldefenses, antipredator defenses can be chemical,developmental, or behavioral. Behavioral changesincludealteringthetimeofactivitytoaperiodwherethepredator is lessactive.Forexample,aplantspeciesmaynotreplaceabove-groundvegetationwhenherbivoresarepresent.Bydyingbacktoitsrootsforsomepartoftheyear,itmaybemorelikelytocoexistwiththeherbivorepopulation.Theherbivorewillhavetodiversifyitsdietifitistosurvive.

Effects of Predators on Individual Prey BehaviorThebehaviorpatternsofpreyreflectabalancebetweentheactionsnecessaryforsurvivalandtherisksinherent in those actions. For example, an animal must feed, but searching for food will make itvulnerabletoitsownpredators.Theanimalmightbalancethisriskbyhidingduringsomeofthetimeitcouldbefeeding,thusshorteningthetotaltimeithastofeed.Decisionsaboutwhethertofeedorhidefrompotentialpredatorsareextremelyimportantformostorganisms.Starvationisafrequentsourceofmortalityinnaturalpopulationsbecausefoodsuppliesareusuallyinsufficienttosupportalltheadultsandoffspringbornineachgeneration.Juvenileorganismsusenutrientsforgrowth,butadults(sexuallymatureorganisms)directanyenergybeyondthatneededforbasicmetabolismintoreproduction.Ifanorganismcanreproducesoonerthanotherorganismsinthepopulation,orleavemoresurvivingoffspringthan the average for the population, its genes will increase in futuregenerations.Sohigherfeedingratesleadtogreaterpotentialfitness,iftheorganismsurvivestoreproduce.

Thecostsandbenefitsof feedingareeasilyoutlinedusingasimpleterrestrialfoodwebexample(Figure7-2).Insuchfoodwebs,theprimaryproducersareusuallygreenplants,theprimaryconsumersareherbivorousinsectssuchasgrasshoppersorbeetles,andthepredatorsmaybeotherinsectsorvertebrates(suchasbirds)tonamejusttwo.

From the perspective of a grasshopper, food is often abundant.However,tofeed,thegrasshoppermustclimbontotheplant’sbrancheswhere it is more visible to bird predators than it would be if it simplyshelteredatthebaseoftheplant.Thus,thegrasshopperincursapotentialcost,theriskofdeath,eachtimeitfeeds.

Howmuchtimeshouldagrasshopperspendfeeding?Ifmorefoodmeans more offspring but also greater predation risk, how might an

Figure 7-1

Figure 7-2

Lab7: TrophicInteractions 169

organism achieve a balancebetween these factors? Onepossibility is to feed for theminimum possible time and toremain out of the predator’sreachfortherestofthetime.

Ifapreyanimalmovesintoaplace where a predator cannotgo,biologistssayithasarefuge.A beetle that lives between thespines on a plant, as shown inFigure 7-3, might have a refugefromabirdpredator,aslongasthe spines are longer than thepredator’sbeak.

The term “refuge” usuallyrefers toa safeplace,but it canhave other meanings. If a preyanimalspendspartofitslifeinastageorphasewherethepredatorcannoteatit,biologistscallthisisarefuge in time.Aninsectlikeacicada,whichspendsitsentirelarvalphaseunderground,hasarefugefrombirdpredationasajuvenile.

However,whenanadultcicadaemergestomate,itisvulnerabletobirdpredators.Biologistshavealsoidentifieda refuge in body size:apreyorganismmaygrowtoolargetoconsume,orreachasizewhereitcandefenditselffromthepredator.

Refugeuseisinterestingbecausedecidingwhentoleavearefugerequiresarisk/benefitcalculation.Anorganismmustevaluatethepotentialbenefitsoffeedingincomparisontotherisk,orcost,ofbeingeaten.Anorganismdoesnothavetomakeaconsciouscalculation;ratheranyorganismwhosecompletesuiteofbehaviorsallowedittofeed,toavoidpredators,andtoreproducesuccessfullywillleaveoffspringthatmakecorrectdecisionsforthecircumstancesunderwhichtheyevolved.

Which calculations of refuge use result in highrelativefitness?Experimentsonavarietyofanimalshavebeenusedtoanswerthisquestion.Aparticularlyelegantsetofexperimentswasdoneusingtubeworms.Youmayhave seen these on the fouling plates in lab last week.Did you notice that each tubeworm has a crown oftentacles that it extends to catch tiny particles insurroundingwater?Thetentaclesare largeandsoft,sothey attract predators (see Figure 7-4 on the right).Predation can be avoided if a tubeworm stays entirelywithinitstube,butthenitcannotfeed.

Tubeworms balance predation risk by extendingtheir tentacles for short periods. They draw into theirtubesifthereismovementinthewateraroundthem,orifashadowpassesoverhead.Youmayhaveseenthislastweek.Tubewormsuseshadowsorwatermovementsasindicatorsthatfishorotherpredatorsmaybenearby.

Howlongdoesatubewormstayinitstubeafterwithdrawing?Howwouldyoudecidewhentofeedagainifyouwerehidinginarefuge?Youmightthinkthatthehungerlevelofthewormwouldinfluencethelengthoftimethatitstayedintherefuge.Biologiststhoughtthatwormsthathadeatenlessrecently

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thanotherswouldcomeoutoftheirtubessooner,butexperimentsdidnotfindaneffectofpreviousfeedinghistory.

Whattheydidfindwassomewhatsurprising.Tubewormsweighedthecostofcomingoutofthetubeagainstthepotentialbenefitoffeeding.Costsincludeanynegativeresultofanaction,sointhiscase,acostmightbebeingeaten.Fortubeworms,remaininginthetube(refuge)inafood-richenvironmenthasa greater cost than it would in a food-poor environment. This is because the worm will lose theopportunitytoconsumemorefoodwhenhidinginafood-richenvironmentthanitwouldwhenhidinginafood-poorenvironment.Researchersvariedthefoodpresentinthewatersurroundingthetubewormsand then subjected the worms to simulated attacks. Worms that had been feeding in water with arelativelyhighfoodlevelbeforetheattackreturnedtofeedingmorequicklythandidwormsfromwaterwith a relatively low food level. Worms in the high food environment were paying a higher price bystayingintheirtubesthanwormsinthelowfoodenvironment.Thesewormsabandonedtherefugemorequicklythandidwormspayingalowercost.

Thetubewormexampleindicatesthatorganismsareabletoassessthepotentialcostsandbenefitsofstaying ina refuge, a formof risk assessment. For tubeworms,movementsor shadowsoverheadareenoughtotriggeraretreatintothetube,butthesearenotalwayscausedbypredators.Therefore,manysuchretreatsarelikelytooccurwhenpredatorsarenotpresent.Isitpossiblethatsomeorganismsareabletoevaluatetheriskofpredationmorepreciselythantubewormsdo?

Itmayhaveoccurredtoyouthatanyorganismabletogainbetterinformationaboutthepresence,orlikelypresence,ofpredatorswouldhaveanadvantageintermsofcalculatingtheriskofleavingtherefuge.Informationonthefrequencyandpredictabilityofpredatorpresencecouldbeveryvaluable.

Ifpredatorsarepresentinthepreyhabitatmostofthetime,thenpreymustbeconstantlyvigilant.Vigilancereferstoanycasewhereanimalssurveytheirsurroundings,usuallybyraisingtheirheads, ifthey have heads. In most species, all members of a population share the responsibility for vigilance;however,insomespeciesonlyonegroupmemberisvigilantatatime.Thisoccursinprairiedogcolonieswhereoneanimalwatchesforpredatorsandalertsthecolonyifapredatorisspottedwhileothermembersfeed.Theseanimalshaveawell-developedsocialstructurewheregroupmembersaregeneticallyrelated.

Ananimalthatwatchesforpredatorsandwarnsoftheirapproachpaysacostintermsofreducedfeeding time because it cannot eat and watch at the same time. Additionally, by calling out to warnothers,thisanimalmayattractthepredator’sattentionandhaveagreaterriskofbeingeaten.

Pre-lab Questions—Record Your Answers on the Online Version 1. Explainwhysuchwarningbehaviorislikelytooccuronlyingroupsthataregeneticallyrelated.

2. Doyouthinkriskassessmentwouldbeabetterstrategyforapreyanimalthanfrequentvigilance?Explainyourreasoning.

Intheabsenceofwarningsfromconspecifics,preyhaveotherwaysofdeterminingthepresenceofpredators. Some prey use sound cues to detect the approach of predators. For example, mothsrespondtotheecho-locationcallsfromtheirbatpredators;mothsdroptotheground(arefuge)oncetheyhearthesoundpulsefromthebat.

Scentcanalsoprovidecuestothepresenceofpredators—youmayhavelearnedfromnatureprogramsthatmanyterrestrialpredatorsapproachpreyfromadirectionwherethewindwillnotbringintheirscenttoalerttheprey.Scentcanalsoplayaroleinaquaticsystemswherechemicalcuesfromthepredatormaybeperceivedbyprey.Preymayalsobeabletodetectthatothersoftheirspecieshavebeenwoundedorconsumedbycueingonbloodorbodyfluidsinthewater.

3. How would being able to detect damaged members of a prey species be beneficial to individualprey?


4. Whatotherkindsofcuesmightbereliableindicatorsforpredatorpresence?

5. Whymightpreysizeinfluenceresponsetoapredator?

Somepreyformgroups(aggregate) inthepresenceofapredator.Onebenefitofaggregationmayresultfromdefensiveactionbytheentiregroup,butanotherpotentialbenefitliesinreducingtheper-individual likelihood of death. Detailed studies of the behaviors of individual fish in schoolsshowthatfishareinconstantmotion.Afishontheoutsideofaschoolswimsinward,leavingotherfishontheedge.Thesenew“edge”fishthenmoveinwardandthecyclerepeats.

6. Explainhowaggregationislikelytoresultinagreaterlikelihoodofsurvivalforschoolingfishinthepresenceofapredator.

7. Explainwhypredatorsandpreywithmanygenerationsofinteractionarelikelytohavepronouncedbehavioralresponsestoeachother.

8. Howwouldyouexpectpreytorespondtoapredatortheyhaveneverencounteredbefore(anovelpredator)?Explainyourreasoning.

Integrating Individual Behavior Across Trophic Levels

In the previous section, you were asked to think about the behavior ofindividual predators and prey. How do combined behaviors of manyindividualsresultininteractionsamongtrophiclevelsofafoodweb?Recallthesimplefoodwebdescribedpreviously,wherebirdsfeedongrasshoppersandgrasshoppersfeedonplants(Figure7-5).

9. Whatisthepredictedeffectofbirdpresenceonthesurvivorshiporbiomassoftheplants?

(Hint:ifthisquestionleavesyoupuzzled,itmayhelptothinkaboutfeedingtime.Iftherewerenobirds,wouldmoreorlessplanttissuebeeatenbygrasshoppers?)

10. If youwant toprotect the plants from insectdamage, shouldyouencouragebirds to forageon theplants,orputnettingovertheplantstokeepbirdsout?Explainyourreasoning.

In answering questions 9 and 10, you likely discovered the concept of a trophic cascade. Insectsaffectplantsdirectly,whereasbirdsaffectplantsindirectly,throughtheirconsumptionoftheinsects.Whenpredatorsindirectlybenefitprimaryproducersbycontrollingherbivorepopulations,therebyreducingtherateoffeedingbyherbivores,thisiscalledatrophic cascade.

InthefoodwebinFigure7-6,herbivorousinsectsareeatenbylizardsandspiders.Bothlizardsandspiderseatinsects,sotheyarecompetitors,butlizardsalsoeatspiders.

Figure 7-5


11.Dietstudiesshowthatspidersconsumesmallinsects,whereaslizardsconsumelargerinsectsandspiders.Ifthissystemisatrophiccascade,whichpartofthefoodweb likely has the largest effect on plants? Explainyourreasoning.

Experimentshave shown that removing lizards fromthe system leads to increased plant damage due toinsects,butremovingspidershasfarlesseffectontheplants.Suchremoval experimentscanbeveryhelpfulinunderstandinghowafoodwebfunctions.

12.ThefoodwebinFigure7-7depictsafour-levelsystem.Fromyourunderstandingoftheeffectsofpredatorsonprey,whichofthefisheswouldyouremoveifyouwanted to increase the survival of the population ofsingle-celledplants?Explainyourreasoning.

Howcouldyoutestyourprediction?

Kelp are large marine algae that often form under-water “forests.” These kelp forests are complexmultilayered habitats that serve as home to a widevariety of marine invertebrates (animals withoutbackbones)andvertebrates(animalswithbackbones).Theyareespeciallyimportanttothesurvivalofmanypopulations of fishes. When a kelp forest has beenremoved, the resulting empty habitat is called an“urchin barren.”

13.ExaminethestructureofthefoodwebinFigure7-8,andexplain:

(a) whytheemptyhabitatiscalledan“urchinbarren,”and

(b)whyurchinbarrensarepositivelycorrelatedwiththepresenceofkillerwhales.

Trophic Levels in the Lab

Inthisweek’slabyouwillstudytheeffectofpredatorsonthebehaviorofpreyinatwo-tofour-levelfoodweb.Youwillalsoobservetheeffectthatpreypresencehasonthebehaviorofthepredator.

Youwillbeworkingwitha freshwater foodwebwherealgaeare theprimaryproducers.Theprimaryconsumersareaquaticmosquitolarvae(Figure7-9),andthesecondaryconsumersarefish.Youwillhavethe option later of adding tertiary consumers. You will be given background information on theseconsumersinthelab.

Figure 7-7

Figure 7-6


Background Information on MosquitoesThere are several genera and many species ofmosquitoes.Youwillstudytheaquaticlarvalformof the mosquito, Culex pipiens. Adult malemosquitoesfeedonnectarorothersubstancesbutdo not take blood meals. Female mosquitoesrequire a blood meal to gain protein for egg-production. Before egg-laying, a female collectsspermpacketsfromoneormoremalesandstoresthem. Sperm and eggs are combined inside thefemale reproductive tract, after which a femalelays a raft of fertilized eggs that float on thesurfaceoffreshwaterponds,lakes,orstreams.Theeggshatch,releasingswimmingmosquito larvae.Larvaefeedonalgaeanddetritusinthewateroronthebottomofshallowponds.Larvaegothroughseveral molts in the aquatic stage before theypupateandbecomeflyingadults.Larvaereachtheadultstagein7to17days.

Onekeyfeatureofthelarvalstageisthatitdoesnothavegills—itisair-breathingandhasaposteriorbreathingtubethatitextendsabovethewater’ssurface(seeFigure7-9).Mosquitolarvaemayfloatatthewater’ssurfacewiththebreathingtubeextendedormovebetweenthepondbottom,wherefoodcollects,and thepondsurface,where theybreathe.Feedingandbreathingarebothnecessary,buthowdoesamosquitoallocateitstime?

Figure 7-8


Figure 7-9



Figure 7-10

Mosquito PredatorsCulex pipiensisanopportunisticmosquitospeciesthatlayseggsinsmallcontainers,ponds,andshallowdrainageditches.Tocontrolmosquitopopulationsizeandtoreducethespreadofdiseasestransmittedbymosquitoes,predatoryfishareoftenintroducedtosmallbodiesofwaterwherefemalesarelikelytolayeggs.

Predatory fish in the genus Gambusia (Figure 7-10) are members of the top minnow group. Twospecies,Gambusia holbrookiandGambusia affinis,areoftenintroducedtoshallowwaterstohelpcontrolmosquito populations; they are commonly called mosquito fishes. Gambusia holbrooki is native to theeasternUnitedStatesbuthasbeenintroducedthroughoutAmerica.Sincethe1920sithasbeenaddedto drainage ditches, ponds, and even swimming pools in California. Gambusia can tolerate watertemperatures between 33°F and 104°F. The fish is a generalist feeder capable of eating 100 to 500mosquitolarvaeperday.Femalesreachlengthsofabout7cm,butmalesaresmaller,typicallylessthan4cminlength.Gambusia reproducesthreetosixtimespersummerwithbroodsizesrangingfrom40to60offspring.Theirlifespanistwotothreeyears.


175

Lab 7: Trophic InteractionsYouwillstudythetimebudgetoflarvalmosquitoes.Atimebudgetisarecordofhowananimalspendsitstime.Yourtaskistodeterminehowmosquitolarvaeallocatetheirtimebetweendifferentactivitiesinthe absence and presence of predators. You will be looking for a response to predation risk, but youshouldkeep inmindthatmosquitoeshaveanevolutionaryhistorywithfishpredatorsandthattheirbehaviors are the results of multiple generations of natural selection. They may behave as thoughpredationwereapossibility,evenifpredatorsarenotpresent.Ontheotherhand,theymaynotrecognizeanovelpotentialpredatoratall,iftheyhavenoevolutionaryhistorywithanysimilarorganism.

Inthepre-lab,youconsideredcuesthatanorganismmightusetoassesstheriskofpredation.Preymightidentifypredatorsbysight,bychemicalcue,bydetectingmovement,orbydetectingpredationonnearbyindividuals.Youwillbeabletoexperimentwithsomeofthesevariablestoday.

Theclasswillbedividedintosixgroupsoffourstudents.Eachgroupwillestimatetimebudgetsofmosquitoesinfourexperimentalconditions:(1)fishabsent,(2)fishpresentbehindanacrylicpartition,(3)fishreleasedintothetankwiththemosquitolarvae,and(4)atreatmenteachgroupofstudentswillselectfromthoseavailable.Thefirsttwotreatmentswillbeusedtodeterminebaselinebehaviorofthemosquitolarvae.Baselinebehaviorsarethosethatoccurintheabsenceofanexperimentalchange.Thethirdandfourthtreatmentswillbetheexperimentaltreatments.Availablefourthtreatmentswillincludefishbehindameshpartitioninsteadofanacrylicpartition,sothatchemicalcuesfromthefishmayreachthe mosquitoes (if such cues exist); the addition of a nonfish predator of the mosquito larvae; theadditionofacommonpredatorofGambusia;andtheadditionofanovelpredatorofGambusia.Theremaybeothertreatmentsavailabledependingontheseasoninwhichyoucompletethisexperiment.

Study SystemEach2.5gallonexperimentaltankwillcontain10mosquitolarvaeinonegallonofwater.Timebudgetswillbeestimatedusingtheinstantaneousscansamplemethod.Forthismethod,youwillkeeptrackofallmosquitoes,recordingthenumberoflarvaedoingeachbehaviorat30-secondintervals.Withthescansamplemethod,youarenotconcernedwithwhatthemosquitoesaredoingbetweeneachinterval,justthebehaviortheyareengagedinateach30-secondmark.

Therearefivepossiblelarvalbehaviors:(1)feedingondetritusthathassettledonthebottomofthetank—ifyouseealarvaonthetankbottom,youmayassumethatitisfeeding;(2)hidinginthefloatingplantthathasbeenaddedtothetankasarefuge;(3)breathing—ifthelarvaeisfloatingatthewatersurfacewiththeposteriortubeextended,itisbreathing;(4)swimming,definedasactivelymovingacrossthetankorbetweenthewater’ssurfaceandthetankbottom;and(5)being eaten,whichshouldnotrequiremoreexplanation.Pleasenotethatbeingeatenisn’treallyabehaviorbutanindicatorofantipredatorsuccess(orfailure!).

Data CollectionAgroupofstudentswillobserveeachtankfor10minutes,recordingthenumberoflarvaeengagedineachactivityat30-secondintervals.Eachgroupwillbeprovidedwithatimer.Setthetimerto10minutes


andassignonegroupmembertocallouteach30-secondinterval.Assignanotherstudenttorecordthedata.Theremainingtwostudentswillberesponsibleforobservingthemosquitoes.Usingthedatasheetsat the end of the lab (a sample has been provided), place a tick mark in the box for each activity, oractivities,thatyouobserveattheendofeach30-secondinterval.Sumthenumberoftickmarksineachactivitycolumn,sumallthecolumnstogetthetotal,andthendivideeachcolumnbythetotaltofindthe average time spent on each activity for your larva. Find the average for the four larvae in yourgroup.

You will need to practice data collection for 3 minutes before you start the 10-minute study.Usethesampledatasheetprovidedbelowtorecordyourpracticedata.Onceyouhavefinishedpracticing,yourTAwillleadaclassdiscussiononstandardizingyourdatacollection.

Being Time: Feeding Hiding Breathing Swimming Eaten

Initial

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To calculate an average time budget for the mosquitoes,firstsumthenumberoftickmarksineachactivitycolumntodeterminethenumberofintervalsspentineachbehavior.Next,addallofthesesumstogethertodeterminethetotalnumberofintervalsobserved.Enterthisinthe“totalsum”box.Finally,divideeachindividualsumbythetotalsumtodeterminetheaveragepercenttimespentineachactivity.

Baseline TreatmentsBaselinetimebudgetswillbecollectedusingthefirsttwotreatments:

(1) mosquitolarvaewithoutpredatoryfish

(2) mosquitolarvaewithpredatoryfishbehindanacrylicpartition

DataSheetsareonpages182–186.

Discussthesetwotreatmentswiththeotherstudentsinyourgroupanddecidehowtheyaredifferent.


Beforeyourecorddatafromthesecondtreatment,discussthetwoquestionsbelow:

Isthereanyevidencethatthefishperceivethemosquitolarvae?

Isthereanyevidencethatthemosquitolarvaeperceivethefish?

Experimental TreatmentsAftercollectingbaselinedata,youmaybeginworkonthetwoexperimentaltreatmentsavailabletoeachgroupusingtheemptytanksprovided.

(1) Release the fish and record the time budget of the mosquito larvae when predators are present.Removing the partition can be difficult, so make sure you practice before you add fish to the tank.

(2) Testyourownhypothesis:youwilldesignyourownexperimentaltreatmenttotestahypothesisofyourchoosing.Todevelopyourhypothesis,firstdescribeyourgeneralareaofinterest.Then,stateyourexperimentalhypothesisandtheresultyouexpectbasedonyourunderstandingofpredator-preysystems.Youmustdevelopanullandalternatehypothesisanddetermineatestablepredictionofthathypothesis.Then,youcanseehowwellyouunderstoodthesystembydoingtheexperiment.

Ourgeneralquestionofinterestis:

Ourexperimentalhypothesisis:

Ournullhypothesisis:

Wewilltestthehypothesisby:

Basedonourhypothesis,wepredict:

Experimentalresults:

179

Laboratory ReportYourTAwillpostclassdatafortheaveragemosquitotimebudget.Recordthesedataandmakefourtime-budgethistograms,oneforeachtreatment(withoutfish,fishwithpartition,fishwithoutpartition,andexperimentaltreatmentyoudevised).AsamplehistogramwithimaginarydataisshowninFigure7-11.

(1) Describethebaselinemosquitotimebudgetintheno-fishtreatment.(½ point)

(2) Describethebaselinemosquitotimebudgetsinthefish-behind-partitiontreatment.(½ point)

Name: _______________________________ TA: __________________________ Date: ___________

Figure 7-11


(3) Doyouhaveanyquantitativeevidencethatmosquitolarvaealteredtheirtimebudgetinthepresenceofthefishbehindapartition?Brieflyexplainyourreasoning.(1 point)

(4) Describethemosquitotimebudgetwhenfishwerefreetomoveaboutthetank.(½ point)

(5) Doyouhaveanyquantitativeevidencethatmosquitolarvaealteredtheirtimebudgetinthepresenceoftheloosefish?Brieflyexplainyourreasoning.(1 point)

(6) Brieflydescribethequestionyoustudiedinyourownexperimentandgiveyournullandalternatehypotheses.(1 point)

(7) Brieflydescribeyourownexperimentalpredictionanddescribetheresultofyourtest.Howwelldidyourresultsmatchyourprediction?(1 point)


(8) GambusiahasbeenintroducedthroughoutmuchoftheUnitedStates.InsomelocationsGambusiafeedsonmosquitolarvaeasyouobservedinlab.However,inotherplaces,Gambusiabehavesasatruegeneralistpredator.Itisnotalwaysclearonwhichtrophiclevelitwillstarttofeed.Itcaneatmosquitolarvaeandothertinyaquaticherbivores,aswellasinvertebratepredatorsontheherbivoresandsmallnativefishes.Usethegeneralizedfoodwebforthissystem,showninFigure7-12,toanswerthethreequestionsbelow.

(a) When the introduction of Gambusia results in the sudden abundance of algae, called an algalbloom,onwhichfoodsourceisGambusialikelytobeconcentrating?(½ point)

(b)WhentheintroductionofGambusiaresultsinasuddenincreaseintheabundanceofmosquitoes,onwhichfoodsourceisGambusialikelytobeconcentrating?(1 point)

(c) How might the introduction of Gambusia lead to the extinction of native fish populations?(1 point)

Figure 7-12


Data Sheet for

Time: Feeding Hiding Breathing Swimming BeingEaten

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Data Sheet for


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Evidence for Incipient SpeciationGoals and ObjectivesAt the end of this laboratory you should be able to:

1. Observeandmeasurevariationintraitswithinapopulation.

2. Explainthefunctionalsignificanceofparticulartraits.

3. Explaintherelationshipbetweentraitfunctionandtheenvironment.

4. Explainhowtraitvaluesinfluencerelativefitnessbetweenenvironments.

5. Predicthoworganismswillevolveinresponsetoenvironmentalchange.

6. Analyzethepotentialforspeciationgivenvariationwithinaspecies.

7. Outlinetheconditionsunderwhichgeneflowinhibitsincipientspeciation.

8. Discussmechanismsofreproductiveisolation.


Thecentralideaofspeciationorcladogenesisisthatnewspeciesformfromsubsetsofexistingspecies.Organismsliveinpopulationswithothersof“thesametype.”Onedefinitionofthesametypeisbasedontheabilitytointerbreedorexchangegenes,soapopulationisagroupoforganismsthatliveinthesameplaceatthesametimeandmatewitheachother.Allorganismspresumedabletoexchangegenes(interbreed)belongtoonespecies,eveniftheyareinwidelyseparatedpopulations.

Speciationoccursasmembersofsomepopulationsceasebreedingwithmembersofotherpopulations.Twopopulationsthatdonotexchangegenesarereproductivelyisolated.Informalterms,twopopulationsarereproductivelyisolatedwhenfertilehybridsbetweenpopulationsdonotformorpersist.Itisveryeasytoimaginethatifsomepopulationsareseparatedfromothersbylongdistancesorecologicalbarrierssuch as mountain ranges, mating between populations across the barrier is unlikely. Once gene flowbetweenpopulationsceases,isolatedpopulationsmaydivergeduetogeneticdriftoradaptationtolocalenvironmentalconditions.Alocalresponsetonaturalselectioncanbequiterapidinageneticallyisolatedpopulationbecausethereisnoinfluxofallelesthatarefavoredinotherplaces.Thistypeofspeciationiscalledallopatricspeciation.Forexample,allopatricspeciationoccurredinmarineorganisms,suchasseaurchinsandporkfishes,whentheancestralpopulationsweresubdividedabout3.5millionyearsagoby

Lab 8

187

188 Lab8: EvidenceforIncipientSpeciation

theIsthmusofPanama.Oncetheisthmusformed,geneflowoccurredonlyamongpopulationsoneithersideoftheisthmus.Overtime,thepopulationsoneithersidedivergedenoughtobeconsideredseparatespecies.

Pre-lab Questions—Record Your Answers on the Online Version 1. In your own words, explain why gene flow among populations reduces the potential for local

adaptationwithineachpopulation.

Imagine thatyoucouldwatch severalpopulationsof the samespeciesover time.Whataspectsofbehaviorandmorphologymightdifferbetweenpopulations?Theanswertothisquestionwilldependontheparticularkindoforganismunderstudy.Inthislab,youwillbelookingatinsects—soapberrybugs—soitisworththinkingmoreaboutspeciationininsects.Theprocessoflineage-splittinghasbeenstudiedinsomedetailinaflyspeciescalledtheapplemaggot(Rhagoletis pomonella).Haveyoueverseentinybrownspotsonapples?Somemightbebruises,butothersoccurwhenyouhavesharedthefruitwithatinylarvalfly(longgonebythetimeyougettotheapple).

Thelifecycleofapplemaggotsmaybenewtoyou.Therearemaleandfemaleflieslivinginorchards.Thefliesaredrawntothesightandscentofapplesandmeetonfruitstomate.Thefemalelayshereggs(oviposits)onanapple.Theeggshatchtoreleasetinylarvae,maggots,whichfeedonthefruit.Whentheapplefallstotheground,flylarvaemoveintothesoilandpupate.Youhaveprobablyseenthe pupa of a butterfly. Essentially, during pupation, the animal’s body shuts down so that thecrawling larvalbodycanbe reorganized into thatof theadult. In theapplemaggot life cycle, thepupalphaseincludesarestingperiod(diapause)followedbydevelopmentofanadultbody.Adultscomeoutofthesoilinspringtofindanewfruitcrop.Theyseekthesamefruitspeciesonwhichtheyfedaslarvaewhentheyarereadytomateandlayeggs.

Wheredidapplemaggotscomefrom?Dotheyhavecloserelativesthatliveonanotherkindoffruit?ResearcherswonderedaboutthesethingsbecauseapplesarenotanativecropintheUnitedStates.Apples were introduced in the early 1600s, and originally there were no flies feeding on them.However, by the mid-1800s, apple growers noticed the flies on apples in northern regions of thecentralandeasternUnitedStates.

OneofthenativeAmericanplantspeciescloselyrelatedtoapplesisthehawthorn.Hawthornplantshave small round fruits that ripen about three weeks later than do apples. Hawthorn fruits areconsumedbylarvalfliescalledhawflies.Thesefliesareextremelysimilartoapplemaggotflies,butthe adults mate and lay eggs on hawthorn fruits. Hawthorn plants grow naturally throughoutnorthernregionsofthecentralandeasternUnitedStateswhereappleorchardswereplanted.Thus,it seemedvery likely thatapplemaggot fliesoriginatedfromasubsetofhawflypopulationsthatshiftedhostplants.Onceasinglematedfemaleflylaidhereggsonanapple,herlarvaewouldseektheapplecuewhentheywerereadytomateasadultflies,insteadofthehawthorncue.

Howwouldyoutest thehypothesis thatapple fliesoriginated fromhawflies?Muchresearchhasbeendoneinthisarea.Oneofthefirsttestswastolookfordifferencesinallelefrequenciesbetweentheapplemaggotflyandhawflypopulations.Manyalleleswereshared,butthereweredifferencesbetweenthepopulationsat6 loci.Geneticstudieswerefollowedbymorphologicalandbehavioralwork.Biologistsstudiedflypreferenceforhostplants:Dotheychooseplantsbysight,smell,ortaste?Dotheyrejectfruitsthatarenotthesameasthelarvalfoodsource?Theystudiedfliesforthelengthofthepupalandrestingphases:Domostfliescomeoutofthegroundattherighttimetofindtherightfruit?Theystudiedfliesforevidenceofgeneflowbetweenpopulations:Dofliesevermakeamistakeandgotothewrongfruit?Theystudiedtheoffspringofflieswhomatedwiththe“wrong”fly(hybridoffspring):Dohybridoffspringhavelower fitness thanoffspringofflieswhohadmatedwiththe“right”fly(pure-typeoffspring)?Theanswertoeachoftheabovequestionswas“yes.”

Lab8: EvidenceforIncipientSpeciation 189

Thus,thereisconsiderableevidenceinsupportofthehypothesisthatapplemaggotfliesoriginatedfrom haw flies. Character variation within the ancestral haw fly population is critical for ourexplanationofevolutionasaresultofahostswitch.Thefirstapplemaggotfliesmighthavecomeoutfromtherestingphaseearlierthanusual—perhapstheyfoundnoripehawfruitsbutdidfindripeapples.Alternatively,thefirstapplemaggotfliesmighthavedescendedfromafemalewithdefectiveolfactory(smell)senses.Onecanimagineseveraldifferenteventsthatmighthaveproducedthefirstapplemaggotflies,butalleventsdependonsomevariationinkeytraitsoftheancestrallifecycle.Biologistsexaminedtheflypopulationslookingfornaturallyoccurringvariationingeneticallybasedtraitsthataffectsurvivorshipandreproduction.Basedonlecturematerial,youwillnotbesurprisedtohearthatthenaturalflypopulationshaveconsiderablevariationonwhichnaturalselectionmayoperate.

Recallfromlectureandyourtextreadingsthatthereareanumberofmechanismsofreproductiveisolation. These mechanisms fall into two broad groups: prezygotic mechanisms and postzygoticmechanisms.

2. Inyourownwords,explainwhatismeantby“prezygoticmechanismsofreproductiveisolation.”

3. Review the experiments described for haw flies and apple flies and list two kinds of prezygoticisolationmentioned.

4. Inyourownwords,explainwhatismeantby“postzygoticmechanismsofreproductiveisolation.”

5. Review the experimentsdescribed forhaw flies andapple flies andnameonekindofpostzygoticisolationthatisoperating.

Biology of Soapberry BugsInthislabyouwillstudysoapberrybugs,Jadera haematoloma.Thesecharismaticgreyandredbugs(Figure8-1)arecommononcampus.Theyare12to13mminlength.Thephotoonthefollowingpageshowsamatingpair.Thefemaleisthelargeranimalontheleft.Thewhitedotonthemaleisnotnatural—weputittheretonumberhim.

Ifyouturnoverafemalebug,youwillnoticealongblackbeakthatisheldflatagainsttheundersideofthebodymostofthetime(Figure8-2).Malesandjuvenileanimalshavemuchshorterbeaks.

SoapberrybugsusetheirbeakstofeedonseedsofplantsinthefamilySapindaceae.Tofeed,bugsbore into the hard round seeds and then inject a liquefying enzyme. They drink the liquefied seedcenter.

Maleandfemalebugsliveonthegroundbelowtheirhostplants.Generallytheyfeedonfruitsthatfallfromtheplant,buttheymayclimbupatreeorvinetoreachthefruits.


Thelifecycleofthebugsisnotcomplicated.Maturefemalesfindmates.Malesinseminatefemalesdirectlyusinganintromittentorgan.Afterspermtransfer,afemaledigsasmallholeinthegroundintowhichshedepositshundredsofeggs.

Youngbugshatchfromeggsandgothroughfivemolts(instars),becomingadultswithwingsandsexorgansafterthelastmolt.Maturitytakesabout30days.Bugsliveonetotwomonthsasadults.

Wehaveavideoofmatingandegg-layingonthecourseWebpage.Matedpairsofbugsremainjoinedtogether longafter spermtransfer,but theymust separate so that the femalecan lay theeggs. If youwatchthevideocarefully,youshouldbeabletoseewhy,fromthefemaleperspective,thepairsstayjoinedwellbeyondspermtransfer.Doyouseeevidenceofcompetitionforfemales?

6. Howmanygenerationsofsoapberrybugscouldtherebeeachyear?

What Kind of Seeds Do Bugs Eat?ThereareseveralplantsfromtheSapindaceaethatareplantedinDavis.Oneistheballoonvine(Cardiospermum halicacabum),andanotheris the golden rain tree (three species ofKoelreuteria). Both the balloon vine and thegolden rain tree have fruits with three hardroundseeds,but the fruitsof the twoplantsdifferinsizeandshape.

MembersoftheSapindaceaeproducefruitasaninflatedcapsulewithseedsinthemiddle(see Figure 8-3). There is a large air spacebetween the outside of the capsule and theseed. To feed on seeds in the capsule, bugsmustpiercethroughthecapsulewallandtheairspacewiththeirbeaks(seetheballoonvinefruitinFigure8-4).Anybugwhoreachestheseedsinsidethecapsulewillhaveearlyaccesstofood.Bugswhofeedfirstwillreproducefirst,andthiswillaffectfitness.

Femalebugshavelongbeaks,sotheyboreintotheinflatedcapsulestryingtoreachoneseed.Theyarenotalwayssuccessful.Femaleswhocannotreachtheseedsinthecenterofthecapsulemustwaituntil

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the fruit’s capsuleopens (dehiscence).Thismaytakemorethanaweek.Malesandjuvenilebugsmustwaituntilthecapsuleopenstofeed.

Comparetheshapeofthesomewhatflattenedgolden rain tree fruit, on the left in Figure 8-5,withthatoftheroundfruitoftheballoonvineinFigure8-5,ontheright.

Fruits of the balloon vine have a radius ofabout7to11mm.Fruitsofthegoldenraintreehavearadiusofabout5to9mm.

Examine the histograms of fruit capsuleradius(Figure8-6)andnoticethemeandistancebetween the edge of the capsule and the seed(datafromCarroll&Boyd1992;referenceonp.199).

Can you see why beak length is a highlyfunctionalcharacter?

Can you see that natural selection will favor beaks of certain lengths depending upon the foodsource?

Figure 8-4

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7. Whymightyouexpecttoseeadifferenceinbeaklengthbetweenmalesandfemales?

A Little HistorySoapberrybugsarenativetothesouthernUnitedStateswheretheirnativefoodsourcesaretheballoonvine (Florida) and soapberry trees, Sapindus saporaria v. drummondii (south-central United States). ThegoldenraintreeisintroducedthroughouttheUnitedStates.

8. Giventheradiusofaballoonvineseedcapsuleandfitnessofbugswithdifferentbeaklengthsundernaturalselection,whatbeaklengthwouldbeidealforfeedingonballoonvinefruits?

9. InanaturalpopulationofbugslivingonballoonvineinBatonRouge,Louisiana,femalebeaklengthranged from 6.5 to 9.5 mm with a mean at 8 mm. How well does this distribution fit yourexpectation?


10.What fractionofavailableballoonvine fruits could thispopulationofbugsconsume? (Rememberthatthemeanistheaveragesize;sometimesthemode,themostfrequenttype, isthesameasthemean.)

11. Wouldhavingabeaklongerthanthelargestradiusbemoreeffectivethanhavingabeakequaltotheaveragecapsuleradius?Yesorno?(Beforeyouanswerthisquestion,lookathowthebeakiscarriedonthebodywhenthebugwalksandwherethebeakmustbepositioned to feed.Imaginemovingthebeakintothefeedingpositionandinsertingitintoaseed.)

12.Thereisnaturalvariationinbeaklengthwithinabugpopulation.Listtwoothertraitswouldyouexpecttovarywithbeaklengthinabugpopulation?(Ifnothingoccurstoyou,returntoquestion11).

i. _________________________________________________

ii._________________________________________________

A Bug’s Life in DavisSoapberrybugsfromFloridawereintroducedtogoldenraintreesinDavisinabout1993.Bugscanbefound on all three species of golden rain trees, but the most common tree on the UCD campus isKoelreuteria paniculata.YoucanseeatreewithbugslivingbeneathitonthesouthsideofWellmanHall.Thetreeisinaconcretepotatgroundlevelnearthebikeracks;itistotherightifyoufacethebelowgroundentrancetoWellman.Thebugsyouwillbelookingatinlabwerecollectedfromlocalgoldenraintrees.Fortheseinsects,theballoonvineistheintroducedfoodsource.

13.Picturethemorphologyofanaveragesoapberrybugfromaraintreepopulationandcompareittothe morphology of its new food source, balloon vine seeds. This average bug was taken from apopulationcontainingarangeofsizes.

Are large, small, or average bugs more likely to have the highest relative fitness on the new foodsource?

Figure 8-6 Fruitcapsuleradius(mm)forgoldenraintree(left)andballoonvine(right).


14.List one change each in bug morphology and behavior that you would reasonably expect in bugpopulationsthathavebeenlivingongoldenraintrees.

Morphology:_______________________________________

Behavior: __________________________________________

15.Soapberrybugsaresmallrelativetotheirhostplants,andthebugsvaryintheirabilitiestodisperse.Althoughtheyhavewings,manysoapberrybugshavepoorlydevelopedflightmusclesandwalkmoreoften than they fly. In Davis, where both balloon vines and golden rain trees are grown, would adispersing bug likely have higher fitness than one with less developed flight muscles? Explain youranswer.

16. Ifbugsdiddisperse,whatarethecostsandbenefitsofacceptingahostplantofeitherspecies?

17.Howcouldadispersingbugknowwhatspeciesofplantitwassettlingon?

18.Verybrieflyoutlineanexperimenttodemonstratethatadispersingbugwasabletodiscriminateonespeciesofplantfromanother.

19.Whatwouldconstitutea“suitable”mateforanyparticularbug,assumingthatreproductionwithasuitablemateproduceshighfitnessoffspring?

New Host: Golden Rain Tree Balloon Vine

Cost:

Benefit:


20.Tofindasuitablemate,doesabughavetocomparethebodyofthepotentialmatetoitsownbody,orarethereothercuesthatcouldbeused?

21.Howdoesthenumberofgenerationsperyearinfluencethelikelihoodofseeingaresponsetonaturalselectiononceanewhostplanthasbeencolonized?


195

Lab 8: Evidence for Incipient SpeciationSoapberrybugsinDavisliveontwohostplantsthatdiffermarkedlyinfruitshapeandsize.Atpresentweconsiderallthebugstobethesamespeciesregardlessofhabitatorbugmorphology.However,manygenerationsoflifeonasinglehostplantwouldselectformorphologicaldifferencesbetweenlocalbugpopulations.Iflocalpopulationdifferentiationwereoccurring,itmightbeaprecursorofspeciation.Inotherwords,thedifferentiationmayeventuallybecomesubstantialenoughtoresultinspeciation.Ontheotherhand,regulardispersalofbugsbetweenhostplantspecieswouldreducethelikelihoodforanypopulationdifferentiation.

We would like you and your partner to consider where and how you would look for evidence ofincipient(beginning)speciationinlabthisweek.Thelabinstructionsarequitesimple.

A. Learn to use the caliperstomeasuresoapberrybugs.Verniercalipers(Figure8-7)allowyoutomakedirectandprecisemeasurementsofsmallobjects.Practice by measuring the screw heads and other small objects on the blocks provided. The answers are on the back of the blocks.

1. Turntheblackdial(A)toopenandclosethejaws.

2. Placetheitembetweenthejawsofthecaliper(seethereddotinthefigurebelow).Oncetheitemisheldsecurelybetweenthejaws,do notmovethedial.

3. Thecalipersyouareusinghaveametricscale(lowertickmarks)andoneininches(uppertickmarks)onthemainrule.Usethemetricunits.

4. LookattheVernier(thesmalltickmarksjustbelowthemainrule)anddeterminewherethefirst tick mark on the left(B)linesupwiththemainrule.Thistellsyouhowlongyourobjectisinmillimeters(thephotoshows15mm).Thephotoshows15mmbecause(B)linesupwiththe15millimetermarkonthemainrule(1.5cm).


Figure 8-7 5. FindtheonetickmarkontheVernierthat exactly lines up with a tick markonthemainrule(C).Inthe

photo,the5thtickmarkontheVernierlinesupwiththe2cmmarkonthemainrule.The5thtickmarkindicates0.5mm.


6. Addthisnumbertoyourfirstmeasurementof15mmtodeterminetheactuallength(inthiscase,15.5mm).Thenumberofthetickmark(1-10)thatlinesupwiththemainruledeterminesthefinaldigitofyourmeasurement.Forexample,ifitwerethe6thtickmarkthatlinedup,itwouldequal0.6mmforatotallengthof15.6mm.Ifitwerethe10thtickmark,itwouldequal1mmforatotalof16mm.Calipersareaccurateup to0.1mm.Yourgroupmembers shouldpracticemeasuring thesmallitemsonthewoodenblocksbeforeyoumeasureanybugs.

B. Develop a hypothesis based on morphological traits. YourTAwill leadtheclass indevelopingahypothesis about incipient speciation, and you will use at least one of the morphological charactersbelowtotestit.Asamplemorphologicalhypothesisis:balloonvinefruitsaremoreinflatedthangoldenraintreefruits,sobugswithlongerbeaksshouldgetmorefoodandreproducemoresuccessfullyintheballoonvinepopulation.Wepredictahigherfrequencyoflongbeaksintheballoonvinepopulationthaninthegoldenraintreepopulation.Onceyoudefine“long”beaks,youwouldhaveatestablehypothesis.Youwouldtesttoseeifyourpredictionismetusingdataonbeaksizefromtheclassdistributions(seebelow).

C. Measure bugs using calipers. Eachpairofstudentswillusecaliperstomeasurebeaklength,thoraxwidth,andthoraxlengthonfivefemalebugsfromonehostplantpopulation(balloonvineorgoldenraintree).YourTAwilldividetheclassinhalfsothateachhalfwillmeasurebugsfromjustonepopulation.Todistinguishfemalesfrommales,lookforlargerabdomensonthefemalesandsmallbrownishhairsattherearofthemaleabdomens.(Picturesofadultmaleandfemalebugswillbeprovidedinlab.Tomakesureyouarealsomeasuringmaturebugs,lookforbugswithgrayundersides.)Takethemeasurementswithout harming the bugsandgivethedatatoyourTAorputitintothespreadsheet,astheTAdirects.

Host Plant Type: _____________________________________________

YourTAwillproduceafrequencydistributionforeachcharacterusingbugsfromeachpopulation.Wemayhavesimilardataforbugsfromoneortwopreviousgenerationsaswell.

D. Select one behavior that relates to incipient speciation that you and your partner would like to explore.Inwhatremainsofthefirsthourofthelab,youandyourpartneraretoselectonebehavioralcharacter that you have reason to suspect might differ between populations living on different hostplants.Besureyoucanexplainhowthecharacterrelatestoincipientspeciation.

Develop a hypothesis for the differences you expect to see,makeapredictionbasedonthehypothesis,andcollect relevant data to examine your hypothesis. The hypothesis should be straightforward and thepredictionstestable.Recordyourhypothesisandpredictiononpage201.

Bug Thorax Width Beak Length Thorax Length

1.

2.

3.

4.

5.


Asamplebehavioralhypothesisis:long-beakedbugsaremorelikelytohavehighfitnessonballoonvinesthanongoldenraintreesbecauseofthecorrelationbetweenbeaksizeandcapsulesize.Balloonvineparentsaremore likelytoproducehigh-fitness, long-beaked,offspring ifabug livingonballoonvinemates withanotherbug from the sameplace.Wepredict that femalebugs fromballoonvinespreferballoonvinemalestogoldenraintreemales.

Design and complete a test of a behavioral prediction.Doasmanytrialsastimeallows,butrememberthatyouneedtomakeafour-minutetalkonwhatyoudid,sopleasekeepaneyeontime.Rememberthatyourgoalistolookforevidence of incipient speciation,soyourhypothesesmust make sense in light of this idea.Youcould test theprediction fromthe samplehypothesisbyallowinga “balloonvine female”achoiceofmalesfromeitherballoonvinesorgoldenraintrees.Youcouldmeasurepreferenceforamaleastimespentwithin3cmofthemale in5-minutetrials,even ifmatingdoesnotoccur.Recordyourmethodsandresultsonpage199.

E. You and your partner will present your behavioral study to the class. The presentation is worth 10 points.Beguidedbytheoutlineonpage199.

Asthisisthelastlabofthequarter,thefinal70minutesoftheclasswillbespentlisteningtoeachpairofstudentsoutlinetheirhypothesis,experimentalwork,andresults.Presentationsshouldbelimitedtofourminutesbutmayincludeaposter,diagramsontheboard,orevenaslidepresentation.Therewillbeoneminuteforquestionsaftereachpresentation,buttheTAwillenforcetimestrictlysothateachpairhasachancetopresenttheirwork.

Planthepresentationtobeconciseandinformative,beingsuretojustifythereasoningbehindyourhypothesisanditstestinrelationtoincipientspeciation.Itdoesnotmatterwhetheryourhypothesiswassupportedorrejected.However,youmustbeabletoexplainwhy what you tested makes senseandstatewhetheryoufoundevidenceofincipientspeciation.

Youwillreceive8pointsforyourtalkand1pointforeachoftwoquestionsthatyouwillposetootherstudentsaftertheirtalks.

199

Name: _______________________________ TA: __________________________ Date: ___________

Outline for Talk

Behavioral Hypothesis:Prediction(s) of Hypothesis: (1 point)

Methods for Testing the Predictions (2 points)

Results of the Test (2 points)

Notes

Discussion of Results (2 points)

Conclusion: Basedontheresultsofyourtest(s),isthereanyevidencetosuggestthatsoapberrybugswilldifferentiateintotwospecies?(1 point)

Two Questions: (1 point each)

P.S.Alotofnaturalselectionworkhasbeendoneonsoapberrybugs.Ifyouareinterested,startwiththisgeneralpaper:Carroll,S.P.,andC.Boyd.1992.Hostraceradiationinthesoapberrybug:naturalhistorywithhistory.Evolution46:1052–1069.UsethelibrarydatabasestofindmorerecentworkorfindDr.CarrollinDavis.

bis 2b lab manual

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