chapter 13 introduction how populations...

© 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko

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Campbell Biology: Concepts & Connections, Seventh EditionReece, Taylor, Simon, and Dickey

Chapter 13Chapter 13 How Populations Evolve

The blue-footed booby has adaptations that make itsuited to its environment. These include

– webbed feet,

– streamlined shape that minimizes friction when it dives,and

– a large tail that serves as a brake.

Introduction

© 2012 Pearson Education, Inc.

Figure 13.0_1 Figure 13.0_2

Chapter 13: Big Ideas

Darwin’s Theoryof Evolution

The Evolution ofPopulations

Mechanisms ofMicroevolution

Figure 13.0_3

DARWIN’S THEORYOF EVOLUTION


A five-year voyage around the world helped Darwinmake observations that would lead to his theory ofevolution, the idea that Earth’s many species aredescendants of ancestral species that weredifferent from those living today.

13.1 A sea voyage helped Darwin frame his theoryof evolution


Some early Greek philosophers suggested that lifemight change gradually over time.

– However, the Greek philosopher Aristotle viewedspecies as perfect and unchanging.

– Judeo-Christian culture reinforced this idea with a literalinterpretation of the biblical book of Genesis.

Fossils are the imprints or remains of organismsthat lived in the past.

In the century prior to Darwin, fossils suggestedthat species had indeed changed over time.




In the early 1800s, Jean Baptiste Lamarck suggestedthat life on Earth evolves, but by a differentmechanism than that proposed by Darwin.

Lamarck proposed that

– organisms evolve by the use and disuse of body parts and

– these acquired characteristics are passed on to offspring.


Video: Galápagos Marine Iguana

Video: Galápagos Island Overview

Video: Albatross Courtship Ritual

Video: Soaring Hawk

Video: Galápagos Tortoise

Video: Galápagos Sea Lion

Video: Blue-footed Boobies Courtship Ritual

During Darwin’s round-the-world voyage he wasinfluenced by Lyell’s Principles of Geology,suggesting that natural forces

– gradually changed Earth and

– are still operating today.

Darwin came to realize that

– the Earth was very old and

– over time, present day species have arisen fromancestral species by natural processes.



During his voyage, Darwin

– collected thousands of plants and animals and

– noted their characteristics that made them well suited todiverse environments.



Figure 13.1A

Figure 13.1B Figure 13.1C

Darwin in 1840

NorthAmerica

Pinta

GenovesaMarchena

Santiago Equator

Daphne Islands

SantaFe

SantaCruz

PinzónFernandina

Isabela

Florenza Española0

0 40 miles

40 km

SanCristobal

PACIFICOCEAN

GalápagosIslands

GreatBritain Europe

Asia

HMS Beagle in port

Equator

Africa

PACIFICOCEAN

ATLANTICOCEAN

SouthAmerica

Cape ofGood HopePACIFIC

OCEAN

Cape Horn

Tierra del Fuego

An

des

Australia

Tasmania

NewZealand

In 1859, Darwin published On the Origin ofSpecies by Means of Natural Selection,

– presenting a strong, logical explanation of descent withmodification, evolution by the mechanism of naturalselection, and

– noting that as organisms spread into various habitatsover millions of years, they accumulated diverseadaptations that fit them to specific ways of life in thesenew environments.



Darwin devoted much of The Origin of Species toexploring adaptations of organisms to theirenvironment.

Darwin discussed many examples of artificialselection, in which humans have modified speciesthrough selection and breeding.

13.2 Darwin proposed natural selection as themechanism of evolution


Figure 13.2

Brussels sprouts

Lateralbuds Terminal bud

Flowersand stems

Cabbage

Broccoli

Stem

KohlrabiWild mustard

Leaves

Kale

Darwin recognized the connection between

– natural selection and

– the capacity of organisms to overreproduce.

Darwin had read an essay written in 1798 by theeconomist Thomas Malthus, who argued thathuman suffering was the consequence of humanpopulations increasing faster than essentialresources.



Darwin observed that organisms

– vary in many traits and

– produce more offspring than the environment cansupport.



Darwin reasoned that

– organisms with traits that increase their chance ofsurviving and reproducing in their environment tend toleave more offspring than others and

– this unequal reproduction will lead to the accumulationof favorable traits in a population over generations.



There are three key points about evolution bynatural selection that clarify this process.

1. Individuals do not evolve: populations evolve.

2. Natural selection can amplify or diminish only heritabletraits. Acquired characteristics cannot be passed on tooffspring.

3. Evolution is not goal directed and does not lead toperfection. Favorable traits vary as environmentschange.



Camouflage adaptations in insects that evolved indifferent environments are examples of the resultsof natural selection.

13.3 Scientists can observe natural selection inaction


Video: Seahorse Camouflage

Figure 13.3A

A flowermantid inMalaysia

A leaf mantid in Costa Rica

Biologists have documented natural selection in action inthousands of scientific studies.

Rosemary and Peter Grant have worked on Darwin’sfinches in the Galápagos for over 30 years. They found that

– in wet years, small seeds are more abundant and small beaks arefavored, but

– in dry years, large strong beaks are favored because all seeds arein short supply and birds must eat more larger seeds.



Another example of natural selection in action is theevolution of pesticide resistance in insects.

– A relatively small amount of a new pesticide may kill 99%of the insect pests, but subsequent sprayings are lesseffective.

– Those insects that initially survived were fortunateenough to carry alleles that somehow enable them toresist the pesticide.

– When these resistant insects reproduce, the percentageof the population resistant to the pesticide increases.



Figure 13.3B

Pesticideapplication

Chromosome withallele conferringresistance to pesticide

Additional applications of thesame pesticide will be less effective,and the frequency of resistantinsects in the population will grow.

Survivors

These examples of evolutionary adaptationhighlight two important points about naturalselection.

1. Natural selection is more of an editing process than acreative mechanism.

2. Natural selection is contingent on time and place,favoring those characteristics in a population that fit thecurrent, local environment.



Darwin’s ideas about evolution also relied on thefossil record, the sequence in which fossilsappear within strata (layers) of sedimentary rocks.

Paleontologists, scientists who study fossils, havefound many types of fossils.

13.4 The study of fossils provides strong evidencefor evolution


Figure 13.4A

Skull ofHomo erectus

Figure 13.4B

Ammonite casts

Figure 13.4C

Dinosaur tracks

Figure 13.4D

Fossilized organic matter of a leaf

Figure 13.4E

Insect in amber

Figure 13.4F

“Ice Man”

The fossil record shows that organisms haveevolved in a historical sequence.

– The oldest known fossils, extending back about 3.5billion years ago, are prokaryotes.

– The oldest eukaryotic fossils are about a billion yearsyounger.

– Another billion years passed before we find fossils ofmulticellular eukaryotic life.



Video: Grand Canyon

Figure 13.4G

Many fossils link early extinct species with speciesliving today.

– A series of fossils traces the gradual modification ofjaws and teeth in the evolution of mammals from areptilian ancestor.

– A series of fossils documents the evolution of whalesfrom a group of land mammals.



Figure 13.4H

Pakicetus (terrestrial)

Rodhocetus (predominantly aquatic)

Dorudon (fully aquatic)

Pelvis andhind limb

Pelvis andhind limb

Balaena (recent whale ancestor)

13.5 Many types of scientific evidence supportthe evolutionary view of life

Biogeography, the geographic distribution ofspecies, suggested to Darwin that organismsevolve from common ancestors.

Darwin noted that Galápagos animals resembledspecies on the South American mainland morethan they resembled animals on islands that weresimilar but much more distant.


13.5 Many types of scientific evidence supportthe evolutionary view of life

Comparative anatomy

– is the comparison of body structures in different species,

– was extensively cited by Darwin, and

– illustrates that evolution is a remodeling process.

– Homology is the similarity in characteristics that resultfrom common ancestry.

– Homologous structures have different functions butare structurally similar because of common ancestry.


Figure 13.5A

Humerus

RadiusUlna

Carpals

Metacarpals

Phalanges

Human Cat Whale Bat

Comparative embryology

– is the comparison of early stages of development amongdifferent organisms and

– reveals homologies not visible in adult organisms.

– For example, all vertebrate embryos have, at some pointin their development,

– a tail posterior to the anus and

– pharyngeal throat pouches.

– Vestigial structures are remnants of features thatserved important functions in an organism’s ancestors.

13.5 Many types of scientific evidence support theevolutionary view of life


Figure 13.5B

Pharyngealpouches

Post-analtail

Chickembryo

Humanembryo

Figure 13.4H_2

Pelvis andhind limb

Balaena (recent whale ancestor)

Advances in molecular biology reveal evolutionaryrelationships by comparing DNA and amino acidsequences between different organisms. Thesestudies indicate that

– all life-forms are related,

– all life shares a common DNA code for the proteins foundin living cells, and

– humans and bacteria share homologous genes that havebeen inherited from a very distant common ancestor.

13.5 Many types of scientific evidence support theevolutionary view of life


Darwin was the first to represent the history of lifeas a tree,

– with multiple branchings from a common ancestral trunk

– to the descendant species at the tips of the twigs.

Today, biologists

– represent these patterns of descent with anevolutionary tree, but

– often turn the trees sideways.

13.6 Homologies indicate patterns of descent thatcan be shown on an evolutionary tree


Homologous structures can be used to determinethe branching sequence of an evolutionary tree.These homologies can include

– anatomical structure and/or

– molecular structure.

– Figure 13.6 illustrates an example of an evolutionarytree.

13.6 Homologies indicate patterns of descent thatcan be shown on an evolutionary tree


Figure 13.6

Tetrapodlimbs

Amnion

Lungfishes

Amphibians

Mammals

Lizardsand snakes

Crocodiles

Ostriches

Hawks andother birds

Feathers

Te

trap

od

s

Am

nio

tes

Bird

s

1

2

3

4

5

6

THE EVOLUTION OFPOPULATIONS


13.7 Evolution occurs within populations

A population is

– a group of individuals of the same species and

– living in the same place at the same time.

Populations may be isolated from one another(with little interbreeding).

Individuals within populations may interbreed.

We can measure evolution as a change inheritable traits in a population over generations.


Figure 13.7

A gene pool is the total collection of genes in apopulation at any one time.

Microevolution is a change in the relativefrequencies of alleles in a gene pool over time.



Population genetics studies how populationschange genetically over time.

The modern synthesis connects Darwin’s theorywith population genetics.



Organisms typically show individual variation.

However, in The Origin of Species, Darwin couldnot explain

– the cause of variation among individuals or

– how variations were passed from parents to offspring.

13.8 Mutation and sexual reproduction producethe genetic variation that makes evolutionpossible


Figure 13.8

Mutations are

– changes in the nucleotide sequence of DNA and

– the ultimate source of new alleles.



On rare occasions, mutant alleles improve theadaptation of an individual to its environment.

– This kind of effect is more likely when the environment ischanging such that mutations that were oncedisadvantageous are favorable under new conditions.

– The evolution of DDT-resistant houseflies is such anexample.



Chromosomal duplication is an important source ofgenetic variation.

– If a gene is duplicated, the new copy can undergomutation without affecting the function of the originalcopy.

– For example, an early ancestor of mammals had asingle gene for an olfactory receptor. That gene hasbeen duplicated many times, and mice now have 1,300different olfactory receptor genes.



Sexual reproduction shuffles alleles to producenew combinations in three ways.

1. Homologous chromosomes sort independently as theyseparate during anaphase I of meiosis.

2. During prophase I of meiosis, pairs of homologouschromosomes cross over and exchange genes.

3. Further variation arises when sperm randomly unite witheggs in fertilization.



Animation: Genetic Variation from Sexual Recombination

13.9 The Hardy-Weinberg equation can testwhether a population is evolving

Sexual reproduction alone does not lead toevolutionary change in a population.

– Although alleles are shuffled, the frequency of allelesand genotypes in the population does not change.

– Similarly, if you shuffle a deck of cards, you will deal outdifferent hands, but the cards and suits in the deck donot change.


The Hardy-Weinberg principle states that

– within a sexually reproducing, diploid population,

– allele and genotype frequencies will remain inequilibrium,

– unless outside forces act to change those frequencies.



For a population to remain in Hardy-Weinbergequilibrium for a specific trait, it must satisfy fiveconditions. There must be

1. a very large population,

2. no gene flow between populations,

3. no mutations,

4. random mating, and

5. no natural selection.



Imagine that there are two alleles in a blue-footedbooby population, W and w.

– Uppercase W is a dominant allele for a nonwebbedbooby foot.

– Lowercase w is a recessive allele for a webbed boobyfoot.



Figure 13.9A

Webbing No webbing

Consider the gene pool of a population of 500boobies.

– 320 (64%) are homozygous dominant (WW).

– 160 (32%) are heterozygous (Ww).

– 20 (4%) are homozygous recessive (ww).

– p = 80% of alleles in the booby population are W.

– q = 20% of alleles in the booby population are w.



Figure 13.9B

Phenotypes

Genotypes

Number of animals(total 500)

Genotype frequencies

Allele frequencies

Number of allelesin gene pool(total 1,000)

0.8 W 0.2 w

40 w640 W 160 W 160 w

WwWW ww

20160320

0.64 0.32 0.04

320500 500

160 20500

800 2001,0001,000

The frequency of all three genotypes must be100% or 1.0.

– p2 + 2pq + q2 = 100% = 1.0

– homozygous dominant (p2) + heterozygous (2pq) +homozygous recessive (q2) = 100%



What about the next generation of boobies?

– The probability that a booby sperm or egg carriesW = 0.8 or 80%.

– The probability that a sperm or egg carries w = 0.2or 20%.

– The genotype frequencies will remain constantgeneration after generation unless something actsto change the gene pool.



Figure 13.9C

Gametes reflect allelefrequencies of parentalgene pool.

Sperm

Eggs

WW Ww

wwwW

W

W w

w

Next generation:

Genotype frequencies

Allele frequencies 0.2 w0.8 W

0.64 WW 0.32 Ww 0.04 ww

q 0.2

p 0.8

p2 0.64 pq 0.16

qp 0.16 q2 0.04

p 0.8 q 0.2 How could the Hardy-Weinberg equilibrium bedisrupted?

– Small populations could increase the chances that allelefrequencies will fluctuate by chance.

– Individuals moving in or out of populations add or removealleles.

– Mutations can change or delete alleles.

– Preferential mating can change the frequencies ofhomozygous and heterozygous genotypes.

– Unequal survival and reproductive success of individuals(natural selection) can alter allele frequencies.



Public health scientists use the Hardy-Weinbergequation to estimate frequencies of disease-causing alleles in the human population.

One out of 10,000 babies born in the United Stateshas phenylketonuria (PKU), an inherited inability tobreak down the amino acid phenylalanine.

Individuals with PKU must strictly limit the intake offoods with phenylalanine.

13.10 CONNECTION: The Hardy-Weinbergequation is useful in public health science


Figure 13.10

INGREDIENTS: SORBITOL,

MAGNESIUM STEARATE,

ARTIFICIAL FLAVOR,ASPARTAME† (SWEETENER),

ARTIFICIAL COLOR

(YELLOW 5 LAKE, BLUE 1

LAKE), ZINC GLUCONATE.†PHENYLKETONURICS:

CONTAINS PHENYLALANINE

PKU is a recessive allele.

The frequency of individuals born with PKUcorresponds to the q2 term in the Hardy-Weinbergequation and would equal 0.0001.

– The value of q is 0.01.

– The frequency of the dominant allele would equal 1 – q,or 0.99.

– The frequency of carriers

= 2pq

= 2 0.99 0.01 = 0.0198 = 1.98% of the U.S. population.

13.10 CONNECTION: The Hardy-Weinbergequation is useful in public health science


MECHANISMSOF MICROEVOLUTION


13.11 Natural selection, genetic drift, and gene flowcan cause microevolution

If the five conditions for the Hardy-Weinbergequilibrium are not met in a population, thepopulation’s gene pool may change. However,

– mutations are rare and random and have little effect onthe gene pool, and

– nonrandom mating may change genotype frequenciesbut usually has little impact on allele frequencies.



The three main causes of evolutionary change are

1. natural selection,

2. genetic drift, and

3. gene flow.



1. Natural selection

– If individuals differ in their survival and reproductivesuccess, natural selection will alter allele frequencies.

– Consider the imaginary booby population. Webbedboobies (ww) might

– be more successful at swimming,

– capture more fish,

– produce more offspring, and

– increase the frequency of the w allele in the gene pool.



2. Genetic drift

– Genetic drift is a change in the gene pool of apopulation due to chance.

– In a small population, chance events may lead to theloss of genetic diversity.



2. Genetic drift, continued

– The bottleneck effect leads to a loss of genetic diversitywhen a population is greatly reduced.

– For example, the greater prairie chicken once numbered in themillions, but was reduced to about 50 birds in Illinois by 1993.

– A survey comparing the DNA of the surviving chickens withDNA extracted from museum specimens dating back to the1930s showed a loss of 30% of the alleles.


Animation: Causes of Evolutionary Change

Figure 13.11A_s3

Originalpopulation

Bottleneckingevent

Survivingpopulation

Figure 13.11B

2. Genetic drift, continued

– Genetic drift also results from the founder effect, whena few individuals colonize a new habitat.

– A small group cannot adequately represent the geneticdiversity in the ancestral population.

– The frequency of alleles will therefore be different between theold and new populations.



3. Gene flow

– is the movement of individuals or gametes/sporesbetween populations and

– can alter allele frequencies in a population.

– To counteract the lack of genetic diversity in theremaining Illinois greater prairie chickens,

– researchers added 271 birds from neighboring states to theIllinois populations, which

– successfully introduced new alleles.



13.12 Natural selection is the only mechanism thatconsistently leads to adaptive evolution

Genetic drift, gene flow, and mutations could eachresult in microevolution, but only by chance couldthese events improve a population’s fit to itsenvironment.

Natural selection is a blend of

– chance and

– sorting.

Because of this sorting, only natural selectionconsistently leads to adaptive evolution.


13.12 Natural selection is the only mechanism thatconsistently leads to adaptive evolution

An individual’s relative fitness is the contribution itmakes to the gene pool of the next generationrelative to the contribution of other individuals.

The fittest individuals are those that

– produce the largest number of viable, fertile offspring and

– pass on the most genes to the next generation.


Figure 13.12

Natural selection can affect the distribution ofphenotypes in a population.

– Stabilizing selection favors intermediate phenotypes,acting against extreme phenotypes.

– Directional selection acts against individuals at one ofthe phenotypic extremes.

– Disruptive selection favors individuals at both extremesof the phenotypic range.

13.13 Natural selection can alter variation in apopulation in three ways


Figure 13.13

Originalpopulation

Evolvedpopulation

Phenotypes(fur color)

Fre

qu

en

cy

of

ind

ivid

ua

ls

Originalpopulation

Stabilizing selection Directional selection Disruptive selection

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