chapter 13 the theory · points of darwin’s theory of evolution by natural selection as it is...

Chapter 13 • The Theory of Evolution 275

Opening ActivityLiving Things Change Ask stu-dents what the word evolutionmeans. Students should recall thatevolution means “change overtime.” Ask students to brainstormtypes of animals or plants that havechanged over time.

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Answers1. Proteins are made up of chains

of amino acids linked togetherlike pearls on a necklace.

2. Each triplet of nucleotides ofDNA codes for a particularamino acid or instruction. Aspecific sequence of nucleotidesof DNA codes for only oneamino acid sequence.

3. A change in DNA is a muta-tion. Mutations in gametes canbe passed on to the next gener-ation and so are called geneticmutations.

4. Gene sequencing is the deter-mination of the nucleotidesequence of DNA and mappingof the location of all of thegenes carried by this sequenceof DNA.

5. Radiometric dating is the cal-culation of the age of an objectby measuring the proportionsof the radioactive isotopes ofcertain elements in the object.

AnswersEncourage students to maintaintheir Reader Response Logs asthey read the entire chapter, andwork together in pairs or smallgroups to answer any questionsthat individual students could not answer.

Quick Review

Reading Activity

• Vocabulary Worksheets• Concept Mapping

Chapter Resource File

Looking AheadQuick ReviewAnswer the following without referring toearlier sections of your book. 1. Describe the structure of proteins. (Chapter 2,

Section 3)2. Relate the sequence of nucleotides in DNA

to the amino acid sequence in proteins.(Chapter 10, Section 2)

3. Define genetic mutations. (Chapter 10, Section 2)

4. Describe gene sequencing. (Chapter 11,Section 1)

5. Summarize the concept of radiometric dating.(Chapter 12, Section 1)

Did you have difficulty? For help, review thesections indicated.

Section 1The Theory of Evolution by Natural Selection

Darwin Proposed a Mechanism for EvolutionEvolution by Natural SelectionDarwin’s Ideas Updated

Section 2Evidence of Evolution

The Fossil RecordAnatomy and DevelopmentBiological Molecules

Section 3Examples of Evolution

Natural Selection at WorkFormation of New Species

www.scilinks.orgNational Science Teachers Association sciLINKS Internet resources are located throughout this chapter.

Reading ActivityCreate a Reader Response Log to record yourpersonal responses to the concepts presented in this chapter. Divide your paper in half. On theleft side of the paper, copy a word, phrase, orpassage from the text. On the right side, writeyour reactions, thoughts, or questions aboutyour entries from the text.

The body of the beautiful moving leaf insect closelyresembles the leaves on which it lives. Camouflagesuch as this helps protect animals from predators.

The Theoryof Evolution

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Copyright © by Holt, Rinehart and Winston. All rights reserved.

OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. This lesson describesDarwin’s experiences as a natural-ist, how his curiosity about thethings he saw prompted him toconclude that evolution occurs, andhow he developed an explanationfor the mechanism of evolution.Students will also learn howDarwin’s theory of evolution bynatural selection has been modifiedby other scientists with regard toformation of new species.

Write the following headings on theboard: Long life span; Short lifespan. Have students write a fewsentences in which they relate anorganism’s life span to the potentialrate of evolution of a species.(Species with shorter life spans canpotentially undergo evolutionarychange much faster than species withlonger life spans.)

DemonstrationShow students pictures of severaldifferent varieties (breeds) of a com-mercially important plant or animal.Ask students how these varietiesoriginated. (All originated throughselective breeding by humans.) If pos-sible, show students pictures of thewild ancestors of the organisms. Tellstudents that observations of changein domesticated animals and plantshelped people recognize that speciescan change over time.

MotivateMotivate

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Section 1

276 Chapter 13 • The Theory of Evolution

• Directed Reading• Active Reading• Data Sheet for Quick Lab GENERAL

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TR BellringerTR D10 Darwin’s Finches

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Darwin Proposed a Mechanism for EvolutionThe idea that life evolves may have been first proposed by Lucretius,a Roman philosopher who lived about 2,000 years ago before themodern theory of evolution was proposed. Then, in 1859, the Eng-lish naturalist Charles Darwin, shown in Figure 1, published con-vincing evidence that species evolve, and he proposed a reasonablemechanism explaining how evolution occurs.

Like all scientific theories, the theory of evolution has developedthrough decades of scientific observation and experimentation. Themodern theory of evolution began to take shape as a result of Dar-win’s work. Today almost all scientists accept that evolution is thebasis for the diversity of life on Earth.

As a youth, Darwin struggled in school. His father was a wealthydoctor who wanted him to become either a doctor or a minister.Not interested in the subjects his father urged him to study, Darwinfrequently spent more time outdoors than in class. At the age of 16,Darwin was sent to Edinburgh, Scotland, to study medicine.Repelled by surgery, which at the time was done without anesthet-ics, Darwin repeatedly skipped lectures to collect biological speci-mens. In 1827, Darwin’s father sent him to Cambridge University, inEngland, to become a minister. Although he completed a degree intheology, Darwin spent much of his time with friends who were alsointerested in natural science.

In 1831, one of Darwin’sprofessors at Cambridge rec-ommended him for a positionas a naturalist on a voyage ofHMS Beagle. Although theship had an official natural-ist, the Beagle’s captain pre-ferred to have someoneaboard who was of his ownsocial class. At the age of 22,Darwin set off on a journeythat would both change hislife and forever change howwe think of ourselves. Theship and its route are shownin Figure 2.

Section 1 The Theory of Evolutionby Natural Selection

Objectives● Identify several observations

that led Darwin to concludethat species evolve.

● Relate the process of naturalselection to its outcome.

● Summarize the main points of Darwin’s theory of evolution by natural selection as it is stated today.

● Contrast the gradualism and punctuated equilibriummodels of evolution.

Key Terms

populationnatural selectionadaptationreproductive isolation gradualismpunctuatedequilibrium

Figure 1 Charles Darwin.Darwin was born in England in1809 and died in 1882.

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Teaching TipGalapagos Giant TortoisesTell students that the GalapagosIslands were named for the largetortoises that inhabit this archipel-ago. When Spanish explorersarrived, they found so many gianttortoises they called the islands “Galapagos,” Spanish for tortoise.The scientific name for the tortoisesis Geochelone elephantopus.Galapagos tortoises can weigh upto 227 kg (499 lb) and measure150 cm (5 ft) across the carapace(the upper shell). Galapagos tor-toises metabolize fat stored in theirtissues and, therefore, can survivewithout food or water for longperiods of time. Because of thischaracteristic, they became a con-venient source of fresh meat forearly explorers. The explorerscaught the tortoises on land andstored them live in ship holds forup to a year. When the Spanisharrived, there were an estimated250,000 tortoises. Today, approxi-mately 15,000 remain.

Teaching TipLaw of Use and Disuse Point out to students that Lamarck recognized that the environmentplayed an important role in evolu-tion. He theorized that when anorganism uses a part of its body,that part becomes more developedas a result of its use. He reasonedthat the modified part is thenpassed on to the organism’s off-spring. For example, he might haveargued that a bird that eats toughseeds will develop a thicker beakand will in turn have offspring with thicker beaks. Tell studentsthat if Lamarck were correct, theoffspring of bodybuilders would be born with enormously developed muscles.

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Science Before Darwin’s VoyageIn Darwin’s time, most people—including scientists—held the viewthat each species is a divine creation that exists, unchanging, as itwas originally created. But scientists had begun to seek to explainthe origins of fossils. Some scientists tried to explain their observa-tions by altering traditional explanations of creation. Others(including Darwin’s own grandfather) proposed various mecha-nisms to explain how living things change over time.

In 1809, the French scientist Jean Baptiste Lamarck (1744–1829)proposed a hypothesis for how organisms change over generations.Lamarck believed that over the lifetime of an individual, physicalfeatures increase in size because of use or reduce in size because ofdisuse. Further, according to Lamarck, these changes are thenpassed on to offspring. This part of Lamarck’s hypothesis is nowknown to be incorrect. However, Lamarck correctly pointed outthat change in species is linked to the “physical conditions of life,”referring to an organism’s environmental conditions.

Darwin’s ObservationsDuring his voyage on the Beagle, Darwin found evidence that chal-lenged the traditional belief that species are unchanging. Duringthe voyage, Darwin read Charles Lyell’s book Principles of Geology.Lyell proposed that the surface of Earth changed slowly over manyyears. As Darwin visited different places, he also saw things that hethought could be explained only by a process of gradual change.For example, in South America, Darwin found fossils of extinctarmadillos. These fossilized animals closely resembled, but werenot identical to, the armadillos living in the area.

Atlantic Ocean

Europe Asia

Australia

South America

Africa

Indian Ocean

North America

Pacific Ocean

Galápagos Islands

HMS Beagle

Figure 2 The route of HMS Beagle. HMS Beaglesailed around the world alongthe route shown on this map.The purpose of the ship’s 5-year voyage was to survey thecoast of South America.

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Teaching TipAcquired Traits Have students design a simple and hypothetical experiment in whichLamarck’s theory of evolution—the inheritanceof acquired characteristics—could be tested.They should ask questions, formulate atestable hypothesis, and suggest how theywould conduct the experiment. If time permitsand the experiment is doable, have studentscarry out their investigations.


Using the FigureHave students examine Figure 3and describe the features of thefinches’ bills. Have studentsdescribe how each finch’s bill isadapted to the bird’s diet. (Forexample, the large ground finch’s billis thick and heavy, which enablesthese birds to crack open large seeds.)

Visual

Interactive Reading AssignChapter 13 of the Holt BiologyGuided Audio CD Program to helpstudents achieve greater success inreading the chapter. Auditory

Group Activity Influences on Darwin Assign stu-dents to work in small groups toresearch what scientific data andtheories may have influencedDarwin as he began conducting hisinvestigations into the mechanism ofevolution. Have one group collectinformation on George Cuvier, who developed the theory known ascatastrophism from his work on fos-sils. Have another group collectinformation on James Hutton, whodeveloped the theory known asgradualism from his work on geo-logical formations. Have a finalgroup collect information on CharlesLyell, who developed the theoryknown as uniformitarianism fromhis work on geological processes.Have each group prepare a presen-tation on its findings, includinginformation on each man’s workand how Darwin’s ideas were influ-enced by the work. Co-op Learning

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Trends in Field BiologyRole of Naturalist Most scientists today mustspecialize in one field. However, many of thegreat scientists of the past were called natural-ists—people who studied nature from a varietyof different perspectives. Darwin’s role on theBeagle was not just to study the native plantsand animals encountered at the ship’s manystops but to study the geology, climate, andpeople of those areas as well. In fact, his inter-est in geology led him to collect many fossils,some high in the Andes Mountains of SouthAmerica. Such findings helped to stimulate histhinking about how environments and organ-isms might have changed over time.


Survival of the Fittest The most famousstatement describing Darwin’s theory, “survivalof the fittest,” did not appear in Darwin’s orig-inal work. The phrase was coined by anotherbiologist, Herbert Spencer, upon learning aboutDarwin’s theory. Darwin liked the phrase andused it to summarize his theory’s implications.Unfortunately, others misunderstood and mis-interpreted the phrase. Soon the meaning offittest was distorted to mean “most powerfulor worthy.” Political leaders and industrialistsused it to justify conquest, colonialism, andoppression under the guise of “natural law.”

Darwin visited the Galápagos Islands, located about 1,000 km (620mi) off the coast of Ecuador. Darwin was struck by the fact that manyof the plants and animals of the Galápagos Islands resembled thoseof the nearby coast of South America. Darwin later suggested thatthe simplest explanation for this was that the ancestors of Galápagosspecies such as those shown in Figure 3, migrated to the islands fromSouth America long ago and changed after they arrived. Darwin latercalled such a change “descent with modification”—evolution.

When Darwin returned from his voyage at the age of 27, he con-tinued his lifelong study of plants, animals, and geology. However,he did not report his ideas about evolution until many years later.During those years, Darwin studied the data from his voyage. AsDarwin studied his data, his confidence that organisms had evolvedgrew ever stronger. But he was still deeply puzzled about how evo-lution occurs.

Growth of PopulationsThe key that unlocked Darwin’s thinking about how evolution takesplace was an essay written in 1798 by the English economist ThomasMalthus. Malthus wrote that human populations are able to increase

faster than the food supply can. Malthus pointedout that unchecked populations grow by geometricprogression, as shown in Figure 4. Food supplies,however, increase by an arithmetic progression atbest, also shown in Figure 4. He suggested thathuman populations do not grow uncheckedbecause death caused by disease, war, and famineslows population growth.

The term population, as it is used in biology,does not only refer to the human population. Inthe study of biology, a consists of allthe individuals of a species that live in a specificgeographical area and that can interbreed.

population

Fruit eater

Insecteaters

Insect eater

Seedeater

Vegetariantree finch

Small insectivorous

tree finch

Cactusground finch

Largegroundfinch

South Americanwarbler finch

Figure 3 Darwin’s finches.Darwin discovered that thesefinches closely resembledSouth American finches.

Time

Geometric progressionArithmetic progression

Two Rates of Progression

Figure 4 Geometric andarithmetic progressions.The blue graph line showsuncontrolled populationgrowth, in which the numbersincrease by a multiplied con-stant. The red graph lineshows increased food supply,in which the numbers increaseby an added constant.

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to cure patients. Antibiotic resistance hasbecome a problem because of the widespreaduse of antibiotics, not only for treatment ofhuman diseases, but also for prevention of diseases in humans and domestic animals. With such widespread use, a mutation in a bacterium’s DNA that allows it to survive treat-ment will be perpetuated. Soon, bacteria withthese mutations may constitute the majority ofthe species alive.

Discussion Tell students that theterm “teleology” is the doctrine ofdesign which holds that the phe-nomena of life can be explained byconscious or purposeful causesdirected to definite ends. Tell themthat many people refer to adapta-tions in living things as havingevolved “in order to” accomplishsome goal. For example, someonemight say that “birds evolvedwings in order to fly.” Lead a class discussion of teleology andwhy this concept differs from evo-lution by natural selection. (Naturalselection acts on variation alreadypresent in a population — it does notinduce variation.)

Graphing Have students draw agraph with numerical data toreinforce the difference betweengeometric progression and arith-metic progression. Have studentsdraw a graph that depicts the pop-ulation size of both an animal andits primary food source over a 30-year time period. Have studentsconnect the points on their graphswith a line. Assume that at a partic-ular point in time (time “zero” onthe graph), there are 10 animals inthe population. The populationdoubles in size every 5 years. Forthe graph depicting the food source,tell students that at time “zero”there are 10 units of food in theanimal’s environment and that thefood supply increases by 10 unitsevery 5 years. Ask them which linedepicts a geometric progression.(the line depicting population size)Ask them which line depicts anarithmetic progression. (the linedepicting units of food) LogicalLS

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Antibiotics are powerful drugs that have savedmany lives. Penicillin, the first antibiotic discov-ered, was observed to inhibit bacterial growthon Petri dishes by the scientist AlexanderFleming in 1928. Many other antibiotics havebeen discovered or synthesized since then. Inthe last several decades, the effectiveness ofantibiotics has become compromised by theevolution of antibiotic-resistant strains of manyspecies. Some diseases must now be treatedwith a “cocktail” of several different antibiotics

REAL WORLDREAL WORLDCONNECTIONCONNECTION

Evolution by Natural SelectionDarwin realized that Malthus’s hypotheses about human populationsapply to all species. Every organism has the potential to producemany offspring during its lifetime. In most cases, however, only a lim-ited number of those offspring survive to reproduce. ConsideringMalthus’s view and his own observations and experience in breedingdomestic animals, Darwin made a key association. Individuals thathave physical or behavioral traits that better suit their environment aremore likely to survive and will reproduce more successfully than thosethat do not have such traits. Darwin called this differential rate ofreproduction . In time, the number of individualsthat carry favorable characteristics that are also inherited willincrease in a population. And thus the nature of the population willchange—a process called evolution.

Darwin further suggested that organisms differ from place to placebecause their habitats present different challenges to, and opportu-nities for, survival and reproduction. Each species has evolved andhas accumulated adaptations in response to its particular environ-ment. An is an inherited trait that has become commonin a population because the trait provides a selective advantage.

Publication of Darwin’s WorkIn 1844, Darwin finally wrote down his ideas about evolution and nat-ural selection in an early outline that he showed to only a fewscientists he knew and trusted. At about this time, both a newlypublished book that claimed thatevolution occurred, and Lamarck’shypotheses about evolution wereharshly criticized. Shrinking fromsuch controversy, Darwin put asidehis manuscript.

Darwin decided to publish after hereceived a letter and essay in June1858 from the young English natu-ralist Alfred Russel Wallace(1823–1913), who was in Malaysia atthe time. Wallace’s essay described ahypothesis of evolution by naturalselection! In his letter, he asked ifDarwin would help him get the essaypublished. Darwin’s friends arrangedfor a summary of Darwin’s manu-script to be presented with Wallace’spaper at a public scientific meeting.

adaptation

natural selection

www.scilinks.orgTopic: Natural SelectionKeyword: HX4128

Figure 5 Political cartoon of CharlesDarwin. This 1874 cartoon of Darwin witha monkeylike “ancestor” is an example ofhow some people ridiculed Darwinbecause of his work.

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Modeling NaturalSelectionSkills AcquiredCollecting data, inter-preting, summarizing,evaluating

Teacher’s NotesRemind students that individu-als have two copies of eachgene, and most mutations codefor recessive traits, meaning thatthe genes are expressed only ifan individual has two copies.

Answers to Analysis1. the different things that can

happen to an organism if it is exposed to change in itsenvironment

2. Most mutations will be passed on because they areharmful only if an individualhas two copies.

3. The survivors avoided chancedeath (a “die” card) and twocopies of the mutation, whichis “lethal” when expressed.

4. sample answer: does not dis-tinguish between beneficialand harmful mutations; doesnot distinguish between livingand reproducing

did you know?Erasmus Darwin Charles Darwin’s grand-father, Erasmus Darwin, wrote about evolutionmore than 60 years before his grandsons’ theorywas presented. Erasmus Darwin cited thingssuch as the metamorphosis of insects, the newvarieties produced by selective breeding, thevariations among similar organisms in differentclimates, and the similarities of vertebrate struc-ture as evidence that all life was “produced froma similar living filament.”

StrategiesStrategiesINCLUSIONINCLUSION

Using a box of animal crackers, have thestudents select several different “animals.”For each of the “animals” selected, have thestudent describe and draw changes thatwould have to take place for each of these“animals” to evolve to live in a water envi-ronment. Also have the student describe anddraw changes if these “animals” had toevolve to an environment like that at theSouth Pole.

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Darwin’s TheoryDarwin’s book On the Origin of Species by Means of Natural Selectionappeared in November of 1859. Many people were deeply disturbedby Darwin’s theory, including the suggestion, made in a later work,that humans are related to apes, as Figure 5 on the previous page sug-gests. But Darwin’s arguments and evidence that evolution occursslowly convinced biologists around the world. Darwin’s theory of evo-lution by natural selection is supported by four major points:

Inherited variation exists within the genes of every population orspecies (the result of random mutation and translation errors).

In a particular environment, some individuals of a populationor species are better suited to survive (as a result of variation)and have more offspring (natural selection).

Over time, the traits that make certain individuals of a populationable to survive and reproduce tend to spread in that population.

There is overwhelming evidence from fossils and many othersources that living species evolved from organisms that are extinct.

Modeling Natural SelectionBy making a simple model of natural selection you canbegin to understand how natural selection changes apopulation.

Materials

paper, pencil, watch or stopwatch

Student name Trial 1 Trial 2 Trial 3

DDAATTAA TTAABBLLEE

Procedure

1. On a chalkboard or overheadprojector, make a data tablelike the one shown below.

2. Write each of the followingwords on separate pieces ofpaper: live, die, reproduce,mutate. Fold each piece ofpaper in half twice so that youcannot see the words. Shuffleyour folded pieces of paper.

3. Exchange two of your piecesof paper with those of aclassmate. Make as manyexchanges with additionalclassmates as you can in 30seconds. Mix your pieces ofpaper between eachexchange you make.

4. Look at your pieces of paper.If you have two pieces thatsay “die” or two pieces thatsay “mutate,” then sit down.If you do not, then you are a“survivor.” Record yourresults in your class table.

5. If you are a “survivor,” recordthe words you are holding inthe data table. Then refoldyour pieces of paper andrepeat steps 2 and 3 two moretimes with other “survivors.”

Analysis

1. Identify what the four slipsof paper represent.

2. Describe what happens tomost mutations in this model.

3. Identify what factor(s)determined who “survived.”Explain.

4. Evaluate the shortcomingsof this model of naturalselection.

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Math Skills Provide students withthe information contained in theMath Connection: Hardy WeinbergEquilibrium at the bottom of thispage. Tell them to consider theexample of a flowering plant inwhich red color is dominant (“A”allele) and white color is recessive(“a” allele). Have them consider apopulation of 100 of these plants,of which 16 are white-flowered(“aa” genotype). Have them calcu-late the frequencies of the A allele(frequency ! p) and the a allele(frequency ! q). (Since q2 is the frequency of the aa genotype, and q2 ! 0.16 [16/100], then q ! 0.4 andp ! 0.6.) Next, tell students thatafter a period of time there aren’tas many white-flowered plantsaround—only 4 out of 100 plants.Ask them what the new frequenciesof the A allele and a allele are. (q2 ! 0.04, so q ! 0.2 and p ! 0.8.)

Logical

Demonstration Bring to class as many of the fol-lowing food crops as possible:cabbage, Brussels sprouts, broccoli,cauliflower, kale, and kohlrabi. Tryto bring a wild mustard plant or apicture of one. (This can be foundin many biology and horticulturetexts.) Point out the parts of theplants that are eaten. Tell studentsthat all of these different varietieshave been bred from the samespecies, Brassica oleracea. Ask stu-dents how these varieties of thesame species could look so differ-ent. (There is so much variation inthe genes of this species.) Is it possi-ble for these varieties to becomeseparate species? (yes) How?(Through isolation, their geneticmake-ups may become so differentthat they can no longer interbreed.)

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The frequency of one plus the frequency of theother must equal 100%, or p " q ! 1. Thechances of all possible combinations of allelesoccurring randomly is therefore (p " q)2 ! 1,or more simply, p2 " 2pq " q2 ! 1. In thisequation, p2 is the frequency of homozygousdominant (AA) individuals in a population,2pq is the frequency of heterozygous (Aa) indi-viduals, and q2 is the frequency of homozygousrecessive (aa) ones.

In 1908, two scientists named Hardy andWeinberg developed a simple equation that canbe used to determine the genotype frequenciesin a population and to track their changes fromone generation to another. This is known as the“Hardy-Weinberg equilibrium equation.” Inthis equation (p2 " 2pq " q2 ! 1), p is definedas the frequency of the dominant allele and qas the frequency of the recessive allele for atrait controlled by a pair of alleles (A and a).

MATHMATHCONNECTIONCONNECTION

Darwin’s Ideas UpdatedSince the time Darwin’s work was published, his hypothesis—thatnatural selection explains how evolution happens—has been care-fully examined by biologists. New discoveries, especially in the areaof genetics, have given scientists new insight into how natural selec-tion brings about the evolution of species.

Change Within PopulationsDarwin’s key inference was based on the idea that in any popula-tion, individuals that are best suited to survive and do well in theirenvironment will produce the most offspring. So, the traits of thoseindividuals will become more common in each new generation.

Scientists now know that genes are responsible for inheritedtraits. Therefore, certain forms of a trait become more common ina population because more individuals in the population carry thealleles for those forms. In other words, natural selection causes thefrequency of certain alleles in a population to increase or decreaseover time. Mutations and the recombination of alleles that occursduring sexual reproduction provide endless sources of new varia-tions for natural selection to act upon.

Species FormationThe environment differs from place to place. Thus, populations of thesame species living in different locations tend to evolve in differentdirections. is the condition in which two pop-ulations of the same species do not breed with one another becauseof geographic separation, a difference in mating periods, or other bar-rier to reproduction. As two isolated populations of the same speciesbecome more different over time, they may eventually become unableto breed with one another. Generally,when the individuals of two relatedpopulations can no longer breed withone another, the two populations aredifferent species. As shown in Figure6, the Kaibab squirrel, which lives onthe North Rim of the Grand Canyon inArizona, has a black belly and othercharacteristics that distinguish it fromthe Abert squirrel. The Abert squirrel,which has a white belly, lives on theSouth Rim of the Grand Canyon.Because they have been so isolatedfrom one another, they have becomedifferent enough that some biologistsconsider them separate species.

Reproductive isolation

www.scilinks.orgTopic: Theory of EvolutionKeyword: HX4175

Kaibab squirrel Abert squirrel

These two squirrel populations became isolated from each otherabout 10,000 years ago, thus preventing their interbreeding.

Figure 6 Reproductive isolation in action

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ReteachingRemind students that while Wallacewas in Indonesia, he wrote a paperdescribing his idea about howevolution occurred and sent it toDarwin. Ask students to write a let-ter that Darwin could have writtento Wallace in response. Verbal

Quiz1. What is a feature called that

provides a selective advantage toa population? (An adaptation)

2.At what level of grouping oforganisms does evolution occur? (the population level)

AlternativeAssessmentHave students make visual representations of the four majorpoints that support Darwin’s theory.For example, for point 1 studentscould cut out pictures of differentindividuals of the same species andpaste them onto the board. Belowthe pictures, they could list possiblevariations in the traits of the species. KinestheticLS

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Answers to Section Review1. Darwin found fossils of armadillos that closely

resembled living armadillos. He also observedthe resemblance between organisms on theGalapagos Islands and those on the nearestcoast.

2. Individuals with traits well-suited to theirenvironment are more likely to survive andreproduce than individuals without such traits.

3. Genetic variations in a population enable someorganisms to produce more offspring than oth-ers. Over time, populations evolve and reflectthe survival of organisms with the most advan-tageous heritable traits.

4. According to the punctuated equilibriummodel, evolution occurs in spurts in responseto strong environmental pressures. Accordingto gradualism, species evolve gradually overlong periods of time.

5. A. Incorrect. Extinct populations cannot beacted upon by natural selection. B. Correct.Isolation prevents interbreeding. C. Incorrect.If two populations are interbreeding, they willnot diverge into different species. D. Incorrect.A single population of a species will remain asingle species unless isolation occurs.


Punctuated EquilibriumTeaching StrategiesExplain that these models arejust that: abstractions thatdescribe and help us understandthe mechanics of evolutionarychange. Emphasize that a goodscientist tries to make a modelfit his or her research results,not the other way around.

The Tempo of EvolutionFor decades, most biologists have understood evolution as a gradu-al process that occurs continuously. The model of evolution inwhich gradual change over a long period of time leads to speciesformation is called . But American biologists StephenJay Gould and Niles Eldredge have suggested that successfulspecies may stay unchanged for long periods of time. Gould andEldredge have hypothesized that major environmental changes inthe past have caused evolution to occur in spurts. This model ofevolution, in which periods of rapid change in species are separatedby periods of little or no change, is called .punctuated equilibrium

gradualism

Punctuated EquilibriumHow could major environmental changes lead tospurts in evolution? The fossil record shows thatdrastic environmental changes have occurred veryinfrequently, separated by periods of time thatoften last tens of millions of years. Events such asvolcanic eruptions, asteroid impacts, and ice ageshave been linked to sudden and drastic changesin climate. Such changes have also been linked tothe extinction of many groups of organisms. As aresult, environments that were once inhabitedbecame empty. This provided opportunities forcolonization by species that could quickly adaptto the new conditions through natural selection.

What Fossils RevealDespite large gaps, due most likely to poor condi-tions for fossilization, there is some evidence ofboth gradualism and punctuated equilibrium in thefossil record. Many groups of organisms appearsuddenly in the fossil record. Some of these groupsremain virtually unchanged for millions of years,while other groups disappear as suddenly as theyappear. Still other groups of organisms appear to

change slowly through time, as predicted by thegradualism model of evolution. More study of thefossil record may reveal additional examples of oneor both types of evolution.

FurtherExploring Further

Gradualism Punctuated equilibrium

List two observations made by Charles Darwinduring his 5-year voyage that led him to concludethat living species evolved from extinct species.

Describe how natural selection occurs.

Summarize the modern theory of evolution bynatural selection.

Compare the punctuated equilibrium model ofevolution with the gradualism model.

Speciation can result whentwo populations have become A extinct. C interbred.B reproductively isolated. D one population.

Standardized Test PrepStandardized Test Prep

Section 1 Review

www.scilinks.orgTopic: Fossil RecordKeyword: HX4088

www.scilinks.orgTopic: EvolutionKeyword: HX4074

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Section 2

OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. Tell students that the pur-pose of this lesson is to teach themabout the different types of infor-mation scientists have found thatprovide evidence of evolution.These include the fossil record;molecular similarities in proteinsand nucleic acids; anatomical simi-larities among organisms; andsimilarities among developmentalpatterns of organisms.

Bring in several fossils and passthem around to student groups. Ask students to brainstorm whatorganisms they think the fossilsrepresent. Examples of fossilsinclude fossilized bones, shells,footprints, or leaf prints. You canthen start the lesson by discussingwhat fossils are and how they formed.

Discussion/QuestionRead the following passage fromDarwin’s On the Origin of Species:“Of this history [of the world], wepossess the last volume alone… Ofthis volume, only here and there ashort chapter has been preserved;and of each page, only here andthere a few lines.” Ask students todescribe what Darwin meant bythis passage. (Darwin was referringto the incompleteness of the fossilrecord unearthed so far.) VerbalLS

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Transparencies

TR BellringerTR D13 Evidence of Whale EvolutionTR D14 Forelimbs of VertebratesTR D16 Hemoglobin Comparison

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The Origin of Species

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The Fossil Record Have you ever looked at a series of maps that show how a city hasgrown? Buildings and streets are added, changed, or destroyed asthe years pass by. In the same way, fossils of animals show a patternof development from early ancestors to modern descendants. Fos-sils offer the most direct evidence that evolution takes place. Recallthat a fossil is the preserved or mineralized remains or imprint ofan organism that lived long ago. Fossils, therefore, provide anactual record of Earth’s past life-forms. Change over time (evolu-tion) can be seen in the fossil record. Fossilized species found inolder rocks are different from those found in newer rocks, as youcan see in Figure 7.

After observing such differences, Darwin predicted that interme-diate forms between the great groups of organisms would eventuallybe found. Since Darwin’s time, some of these intermediates havebeen found, while others have not. For example, fossil intermedi-aries have been found between fishes and amphibians, between rep-tiles and birds, and between reptiles and mammals, adding valuableevidence about the fossil history of the vertebrates.

Today, Darwin’s theory is almost universally accepted by scien-tists as the best available explanation for the biological diversity onEarth. Based on a large body of supporting evidence, most scien-tists agree on the following three major points:

1. Earth is about 4.5 billion years old.

2. Organisms have inhabited Earth for most of its history.

3. All organisms living today share common ancestry with earlier, simpler life-forms.

Evidence of Evolution Section 2

Objectives● Describe how the fossil

record supports evolution.

● Summarize how biologicalmolecules such as proteinsand DNA are used as evidence of evolution.

● Infer how comparing theanatomy and developmentof living species providesevidence of evolution.

Key Terms

paleontologistvestigial structurehomologousstructure

Figure 7 Fossils. Fossilsof early multicellular life-forms,such as the crinoid, occur in 800-million-year-old rocksfound in Indiana. Fossils of the pterodactyl, an extinctreptile, occur in 140- to 210-million-year-old rocks.

Crinoid Pterodactyl

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Teaching TipExtinct Organisms Tell studentsthat scientists estimate that 99 per-cent of all animal and plant speciesthat ever existed are now extinct.Also, a large number of extinctspecies have been described bypaleontologists. For example, inone small area in Wyoming, earlyEocene rocks have yielded fossils ofmore than 50 species of animals.

Using the Figure Tell students that the first three ani-mals in Figure 8 are known onlyfrom their fossil remains. Thefourth animal is a living species.Tell students that it is rare to find acomplete skeleton in any one fossil.As more fossils of a species arefound, all species’ bones mayeventually be found. Using theirknowledge of anatomy, paleontolo-gists project what these bones willlook like. Ask students how thebackbone changed relative to thetime these animals spent in water.(The backbone became heavier.)What is the advantage of thischange? (Whales use up-and-downmotions of their bodies to swim. Aheavier backbone better supports themuscles used for this motion.)

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it decays, causing the ratio of carbon-14 to carbon-12 to decrease over time. If a fossilizedorganism has one-fourth the carbon-14 to carbon-12 ratio of a living organism, the fossilis 11,460 years old. (This is two half-lives ofcarbon-14.) Because the half-life of carbon-14is relatively short, this radioisotope is only used for dating fossils and artifacts less thanabout 50,000 years old. To date older fossils,scientists use radioactive isotopes with longerhalf-lives. For example, uranium-235 has ahalf-life of 704 million years.


Scientists can determine the ages of fossils andother artifacts by measuring the amount ofdecay of radioactive atoms in the specimen orin the surrounding sediment. Since a radioac-tive atom is unstable, it will eventually changeinto a more stable atom. The term half-lifedescribes how long it takes for one-half of theradioactive atoms in a sample to decay. Forexample, the half-life of carbon-14 is 5,730years. (The most common form of carbon inliving things is carbon-12.) When an organismdies, the carbon-14 it has steadily decreases as

CHEMISTRYCHEMISTRYCONNECTIONCONNECTION

Formation of FossilsThe fossil record, and thus the record of the evolution of life, is notcomplete. Many species have lived in environments where fossils donot form. Most fossils form when organisms and traces of organ-isms are rapidly buried in fine sediments deposited by water, wind,or volcanic eruptions. The environments that are most likely tocause fossil formation are wet lowlands, slow-moving streams,lakes, shallow seas, and areas near volcanoes that spew out volcanicash. The chances that organisms living in upland forests, moun-tains, grasslands, or deserts will die in just the right place to beburied in sediments and fossilized are very low. Even if an organismlives in an environment where fossils can form, the chances are slimthat its dead body will be buried in sediment before it decays. Forexample, it is very likely to be eaten and scattered by scavengers.

Ambulocetus natans apparently walked on landlike modern sea lions and swam by flexing itsbackbone and paddling with its hind limbs (as domodern otters). They were about 3 m (10 ft) long.They existed about 50 million years ago.

Mesonychids are one hypothesized link betweenmodern whales and certain hoofed mammals.They were about 2 m (6 ft) long. They are thoughtto have lived about 60 million years ago. Somescientists favor an alternative hypothesis linkingwhales to other ancestral hooved mammals.These hooved mammals are also ancestral tohippopotamuses or pigs.

Whales are thought to have evolved from an ancestral line of four-legged mammals, which are represented here by their fossils and artistic reconstructions showing what scientists think they may have looked like.

Figure 8 Evidence of whale evolution

Reading EffectivelyRead the heading “Forma-tion of Fossils,” and ask oneor more Who, What, Where,When, Why, or How ques-tions. For example, How arefossils formed? As you read,answer your questions.

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Teaching TipPenguins “Fly” UnderwaterTell students that, although pen-guins are not able to fly in the air,the movement of their wings underwater resembles the motion of thewings of birds that do fly. However,penguins have much heavier bonesthan birds that fly in the air. Askstudents what adaptive advantagethis might give penguins. (The heftybone structure of a penguin’s wings isan advantage for moving throughwater, which is far denser than air.Also, heavier bones are not a disad-vantage because of the buoyancy provided by water.)

Demonstration Have students study samples orpictures of different types of fossils.Ask how these organisms are simi-lar to modern organisms. (Answerswill vary. Students should note thesimilarities in bone and shell struc-ture.) Diatomaceous earth used inaquarium filters provides excellentexamples of microfossils forexamination. VisualLS

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MISCONCEPTION ALERT

Vestigial Structures A discussion of vestig-ial structures begins on the next page. Manypeople think that structures are labeled asvestigial because they are useless. Many suchstructures do, in fact, have functions.Structures are labeled vestigial if they aresmaller, less functional, or perform a different

function from that of homologous structuresin ancestral life forms. For example, the yolksac of a mammal is homologous to the yolksac of birds and reptiles, but it is consideredvestigial because it does not provide nutrientsfor the growing embryo. However, the mam-malian yolk sac does produce blood cells.

Furthermore, the bodies of some organisms decay faster than oth-ers do. For example, an animal with a hard exoskeleton (such as acrab) would have a better chance of becoming fossilized than woulda soft-bodied organism, such as an earthworm.

Although the fossil record will never be complete, it presentsstrong evidence that evolution has taken place. When a fossil is dis-covered, (scientists who study fossils) analyze thesediments around it. By radiometric dating certain types of rocksand minerals in those sediments, paleontologists can arrange thefossils in order from oldest to youngest. When this is done, orderlypatterns of evolution can be seen. Based on existing fossils,Figure 8 shows an artist’s idea of the appearance of three extinctspecies that might have been ancestral to modern whales. They arearranged in the order that they evolved, based on their fossil’s ageas determined by radiometric dating.

paleontologists

Modern whales have forelimbs that are flippersand hind limbs that have been reduced to only afew internal functionless hind-limb bones.

Rodhocetus kasrani, a more recent ancestor ofmodern whales, probably spent little time on land.Its reduced hind limbs could not have aided inwalking or swimming. It is thought to have existedabout 40 million years ago.

www.scilinks.orgTopic: PaleontologyKeyword: HX4134

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Teaching TipMaking Mutations To show stu-dents that the amino acid sequenceof a protein is determined by thenucleotide sequence of a gene, put the following sequence ofnucleotide bases on the board oroverhead: CUU, GUU, CCU, GGC,AGG. Have students look up theamino acids encoded by thesetriplets. (leucine, valine, proline,glycine, and arginine) Have studentstake turns substituting one of theother three nitrogen bases for abase in one of the triplets. Havestudents look up the name of theamino acid encoded by the newtriplet. Tell students that the substitu-tions they made represent mutations.Ask how mutations affect the pro-teins encoded by DNA. (Mutationschange the genetic blueprints forprotein production.)

Using the Figure Direct students’ attention toFigure 10. Ask them which specieslisted in this table shares the mostrecent common ancestor withhumans. (gorilla) Ask them if afamily tree produced with theamino acid data in Figure 10 wouldshow the same relationships as afamily tree based on nucleotidesubstitutions. (yes) Why? (Anucleotide sequence determines anamino acid sequence.)

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Graphic Organizer

Use this graphic organizer withReteaching on the next page.

Homologousstructures

Birds HumansDolphins

Whales

Molecules

Vertebrates

Mice

Chickens Frogs

LampreysRhesus monkeys

EvolutionVestigial

structures

Proteins

Pelvis

Forelimbs

Anatomy and Development Comparisons of the anatomy of different types of organisms oftenreveal basic similarities in body structures even though the structure’sfunctions may differ between organisms. For example, sometimesbones are present in an organism but are reduced in size and eitherhave no use or have a less important function than they do in other,related organisms. Such structures, which are considered to beevidence of an organism’s evolutionary past, are called (vehsTIJ ee uhl) . For example, the hind limbs of whales are ves-tigial structures.

As different groups of vertebrates evolved, their bodies evolveddifferently. But similarities in bone structure can still be seen,suggesting that all vertebrates share a relatively recent commonancestor. As you can see in Figure 9, the forelimbs of the vertebratesshown are composed of the same basic groups of bones. Such struc-tures are referred to as homologous (hoh MAHL uh guhs).

are structures that share a common ances-try. That is, a similar structure in two organisms can be found in thecommon ancestor of the organisms.

Most scientists believe that the evolutionary history of organismsis also seen in the development of embryos. At some time in theirdevelopment, all vertebrate embryos have a tail, buds that becomelimbs, and pharyngeal (fuh RIN jee uhl) pouches. The tail remainsin most adult vertebrates. Only adult fish and immature amphib-ians retain pharyngeal pouches (which contain their gills). Inhumans, the tail disappears during fetal development, and pharyn-geal pouches develop into structures in the throat.

Homologous structures

structuresvestigial

The word vestigial comesfrom the Latin word ves-tigium, meaning “footprint.”Homologous is from theGreek word homologos,meaning “agreeing.”

The forelimbs of vertebrates contain the same kinds of bones, which form in thesame way during embryological development.

Figure 9 Homologous structures

Humerus

Radius

Ulna

Carpals

Metacarpals

Phalanges

Penguin Alligator

HumanBat

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Answers to Section Review1. Answers will vary. The fossil record shows

changes in organismal forms that can be tracedforward and backward in time along differentancestral lines.

2. Similarities in amino acid sequences suggestthat organisms are related, and may indicatethe degree of relatedness. Differences in aminoacid sequences suggest that mutations haveintroduced genetic variation into populations oforganisms that eventually resulted in evolution.

3. Anatomical similarities between living species,such as similar bones used for similar func-tions, suggest that these species may haveevolved from a common ancestor.

4. A. Incorrect. These two animals should haverelatively similar nucleotide sequences becausethey are both primates and thus closely related.B. Incorrect. These two animals should havesomewhat similar nucleotide sequences becausethey are both mammals. C. Incorrect. Thesetwo animals are both vertebrates and shouldhave nucleotide sequences that are more simi-lar than those of a vertebrate (shark) and inver-tebrate (butterfly). D. Correct. The shark is avertebrate, and the butterfly is an invertebrate.They are the least related and would have theleast similar nucleotide sequences.

ReteachingHave students design a graphicorganizer that describes at leasttwo kinds of physical traits thatcan be used to support the theoryof evolution by natural selection.

Logical

Quiz1. What does the presence of hind

limb remnants in the whale indi-cate? (The presence of these bonesindicates that whales evolved froma land-dwelling ancestor.)

2.What is a vestigial structure?(bones or other structures that arepresent in an organism but arereduced in size and either have no use or have a less importantfunction than they do in other,related organisms)

AlternativeAssessmentTell students that the earliest phylo-genetic diagrams were constructedusing only evidence of morphologi-cal characteristics. Ask students whysuch diagrams might not reflect trueevolutionary relationships. (Similarmorphological characteristics mightreflect adaptation to a similar environ-ment, rather than a common ancestry.)Have students use library resourcesto construct phylogenetic diagrams of an organism or organisms oftheir choosing. Remind them toconsider more than morphologicalcharacteristics.

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Biological MoleculesThe picture of successive change seen in the fossil record allows sci-entists to make a prediction that can be tested. If species havechanged over time as the fossil record indicates, then the genes thatdetermine the species’ characteristics should also have changed bymutation and selection. As species evolved, one change after anothershould have become part of their genetic instructions. Therefore,changes in a gene’s nucleotide sequence should build up over time.

ProteinsThis prediction was first tested by analyzing the amino acidsequences of similar proteins found in several species. If evolutionhas taken place, then, in general, species descended from a recentcommon ancestor should have fewer amino acid differencesbetween their proteins than do species that share a common ances-tor in the more distant past.

Comparing the same hemoglobin protein in several speciesreveals the pattern shown in Figure 10. Species that are thought tohave shared a common ancestor more recently (for example,humans and gorillas) have few amino acid sequence differences.However, those species that are thought to have shared a commonancestor in the more distant past (such as humans and mice) havemany amino acid sequence differences.

DNA SequencesThis pattern, however, does not hold true for all proteins. A certainprotein may evolve more rapidly in some groups than others. Com-parisons of proteins, therefore, may not reflect evolutionary rela-tionships supported by the fossil record and other evidence.Evolutionary histories, however, are generally not inferred from anysingle protein’s amino acid sequences. More accurate hypothesesabout evolutionary histories are based on large numbers of genesequences. These evolutionary histories based on DNA sequencestend be very similar to evolutionary histories inferred by biologistsbased comparative anatomy and evidence from the fossil record.

Hemoglobin Comparison

Species Amino AcidDifferencesfrom HumanHemoglobinProtein

Gorilla 1

Rhesus monkey 8

Mouse 27

Chicken 45

Frog 67

Lamprey 125

Figure 10 Hemoglobindifferences. The more similar organisms’ hemoglobinproteins are, the more recentthe organisms’ commonancestor is likely to have been.

Section 2 Review

Relate how the fossil record provides evidencethat evolution has occurred.

State how comparing the amino acid sequenceof a protein can provide evidence that evolutionhas taken place.

Describe how comparing the anatomy of livingspecies provides evidence of evolution.

Which two organismswould likely have the least-similar nucleotidesequences in a given gene? A chimpanzee and gorillaB gorilla and dogC dog and sharkD shark and butterfly


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OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. This lesson provides exam-ples of population changes that haveresulted from natural selection. Italso describes how the process ofevolution may result in the forma-tion of new species.

Have student groups brainstorm toidentify characteristics of birds thatprovide information about theirdiets. (This information could beinferred from beak characteristics. A large, anvil-like beak suggests a dietof large seeds. A long, narrow beaksuggests a diet of insects or possiblynectar. A very large claw-like beaksuggests a diet of the flesh of animals.)

Discussion/QuestionPoint out that within the phylumArthropoda, the class Insecta hasmore species than any other class.In fact, about one-third of all ani-mals are beetles, which constitutejust one order of insects. To date,more than 1 million species ofinsects have been classified, andmany scientists estimate that thereare probably several million more.Ask students how they think somany different kinds of species ofinsects could have evolved. (Insectsare adapted to many different kindsof environments and rely on manydifferent kinds of food sources.)

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Section 3 Examples of Evolution

Natural Selection at WorkHow does evolution occur? The heart of Darwin’s theory of evolu-tion is that natural selection is the mechanism that drives evolution.Darwin wrote: “Can we doubt . . . that individuals having any advan-tage, however slight, over others, would have the best chance ofsurviving and of procreating their kind? On the other hand, we mayfeel sure that any variation in the least degree injurious would berigidly destroyed. This preservation of favorable variations, I callNatural Selection.” In his writings, Darwin offered examples of hownatural selection has shaped life on Earth. There are now manywell-known examples of natural selection in action.

The key lesson scientists have learned about evolution by nat-ural selection is that the environment dictates the direction andamount of change. If the environment changes in the future, theset of characteristics that most help an individual reproduce suc-cessfully may change. For example, the polar bear’s white fur,shown in Figure 11, enables it to hunt successfully in its snowyenvironment. In a warmer environment, having white fur wouldno longer be an advantage.

Factors in Natural SelectionThe process of natural selection is driven by four important pointsthat are true for all real populations:

All populations have genetic variation. That is, in any popu-lation there is an array of individuals that differ slightly fromeach other in genetic makeup. While this may be obvious inhumans, it is also true in species whose members may appearidentical, such as a species of bacteria.

The environment presents challenges to suc-cessful reproduction. Naturally, an organismthat does not survive to reproduce or whose off-spring die before the offspring can reproducedoes not pass its genes on to future generations.

Individuals tend to produce more offspringthan the environment can support. Thus indi-viduals of a population often compete with oneanother to survive.

Individuals that are better able to cope withthe challenges presented by their environmenttend to leave more offspring than those indi-viduals less suited to the environment do.

Objectives● Identify four elements in the

process of natural selection.

● Describe how natural selection has affected thebacteria that causetuberculosis.

● Relate natural selection tothe beak size of finches.

● Summarize the process ofspecies formation.

Key Terms

divergencespeciationsubspecies

Figure 11 Polar bear.Camouflage benefits predatorsand prey alike.

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Teaching TipSelection Pressure and Rates ofEvolution Tell students that theterm selection pressure refers to theimpact of natural selection on aparticular trait. If individualsexpressing a particular trait all diebefore they reach reproductive age,then we say that there is verystrong selection pressure againstthat trait. An example is the humandisease cystic fibrosis, whose vic-tims have until only recently rarelysurvived to reproductive age. Anallele that has strong selection pres-sure against it is not likely to persistin a species. However, if the trait isrecessive, as it is for cystic fibrosis,individuals who are heterozygouswill not be subjected to selectionpressure themselves but will passthe allele on to future generations.In this way, harmful alleles can per-sist for a long time in a species.

Group ActivityExamples of Evolution Havestudents work in small groups toresearch examples of evolutionusing library, Internet, or othersources. Their examples shouldshow evolutionary change within ahuman lifespan. Assign one groupto work on examples of artificialselection by humans. This groupcould select examples amongdomestic and farm animals, or cropplants. Assign a second group towork on examples of selection inbacteria resulting from human useof antibiotics. This group couldselect examples among pathogenicbacteria. Assign a third group towork on examples of selection inlaboratory organisms. This groupcould select examples of organismssuch as Neurospora, Drosophilamelanogaster, or small rodents.Have each group prepare an illus-trated poster displaying their find-ings. Display the posters for thewhole class to examine.

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Natural selection has made insect pestsharder to fight. When DDT was first intro-duced, for example, it was a highly effectiveinsecticide. Over time, DDT became less andless effective; individuals that were resistantto the insecticide survived and produced thenext generation. In fact, many populations ofinsects are now resistant to DDT. AlthoughDDT is now banned in the United Statesbecause of its persistent toxicity, farmers haverepeatedly had to deal with insect popula-tions that develop resistance to insecticides.

TECHNOLOGYTECHNOLOGYCONNECTIONCONNECTIONStrategiesStrategies

INCLUSIONINCLUSION

Using the examples of a polar bear and abrown bear, have the students make a chartof the differences between how the twobears live and survive in their environments.The chart should include the habitats of thebears, what kind of food they eat, how theircamouflage helps them survive in theirhabitat, and why each might not survivein the other’s environment.

• Developmental Delay • Learning Disability

Example of Natural SelectionThe lung disease tuberculosis (TB) is usually caused by the bac-terium Mycobacterium tuberculosis, shown in Figure 12, and it killsmore adults than any other infectious disease in the world. In the1950s, two effective antibiotics, isoniazid and rifampin, becameavailable, and they have saved millions of lives. In the late 1980s,however, new strains of M. tuberculosis that are largely or com-pletely resistant to isoniazid and rifampin appeared. Rates of TBinfection began to skyrocket in many countries, and in 1993 theWorld Health Organization declared a global TB health emergency.

How did antibiotic-resistant strains of M. tuberculosis evolve? Adetailed look at a single typical case reveals how: through naturalselection. This case is of a 35-year-old man living in Baltimore whowas treated with rifampin for an active TB infection. After 10 months,the antibiotics cleared up the infection. Two months later, however,the man was readmitted to the hospital with a severe TB infection,and despite rifampin treatment, he died 10 days later. The strain of M.tuberculosis isolated from his body was totally resistant to rifampin.

How had TB bacteria within his body become resistant torifampin? Doctors compared DNA of the rifampin-resistant bacteriato DNA from samples of normal, rifampin-sensitive M. tuberculosis.There seemed to be only one difference: a single base change fromcytosine to thymine in a gene called rpoB.

Evolution of Antibiotic Resistance Rifampin acts by binding to M. tuberculosis RNA polymerase,preventing transcription and so killing the bacterial cell. Themutation in the polymerase’s rpoB gene prevents rifampinfrom binding to the polymerase. The mutation, however, doesnot destroy the polymerase’s ability to transcribe mRNA. Themutation likely occurred in a single M. tuberculosis bacterialcell sometime during the first infection. Because its poly-merase function was no longer normal, the mutant bacteriumcould not divide as rapidly as normal bacteria can, but it stillcould divide. The antibiotic caused the normal bacterial cellsto eventually die. The mutant bacteria continued to grow andreproduce in the antibiotic-containing environment.

Because the total number of M. tuberculosis bacteria wasreduced drastically by the first antibiotic treatment, thepatient’s infection had seemed to clear. However, mutant,antibiotic-resistant bacteria survived and continued to growin his body. The mutant bacteria could reproduce moreeffectively in the presence of the antibiotic than the normalbacteria could. Therefore, the mutant bacteria became morecommon in the bacterial population, and they eventuallybecame the predominant type. When the patient becameacutely ill again with TB, the M. tuberculosis bacterial cellsin his lungs were the rifampin-resistant cells. In this way,natural selection led to the evolution of rifampin resistancein M. tuberculosis.

Figure 12 Tuberculosis.TB may be diagnosed from anX-ray of the lungs. TB iscaused by Mycobacteriumtuberculosis.

Mycobacterium tuberculosis

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Using the FigureDirect students’ attention to Figure 13. Ask students to suggest a reason why beak size returns tonormal following each dry year. (Theenvironmental conditions that selectedfor large beak size were removed.) Askstudents what would have to happenfor beak size to remain large in thepopulation over many years. (Thearea would have to experience contin-ually dry weather.) Visual

Group Activity Modeling Natural SelectionDivide students into small groupsand provide each group with one30-cm (1-ft) square of aluminumfoil and 10 each of 2.5-cm (1-in.)squares of aluminum foil and whitepaper. (Crinkle the foil and flattenit out before cutting the squares.)Have one student in each groupspread out the 20 small squares onthe larger square of aluminum.Give another student in each group10 seconds to pick up and removeas many squares as possible, one ata time, from the big sheet. Haveeach group count and report thenumber of aluminum squares andpaper squares “captured.” Keep atally on the board or overhead.More white paper squares than aluminum squares should be picked up. Thus, the camouflagedaluminum squares have an adaptivesurvival advantage. Ask: If the littlesquares could reproduce, whichtype would be more numerous inthe next generation? (small alu-minum squares) Why? (More ofthem were left to reproduce.)

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MISCONCEPTION ALERT

Selection, Not Induction Many people thinkthat selection pressure induces changes. Forexample, people think that antibiotics inducemutations in bacteria that make them antiobioticresistant. Remember, natural selection does notinduce changes in the genes of organisms; rather itselects for those genes that are most adaptive.Genes for antibiotic resistance are already presentin many bacteria, but organisms expressing thesegenes do not predominate until they are grown inthe presence of antibiotics and the resistanceprovides a survival advantage.

Evolution in Darwin’s FinchesDarwin collected 31 specimens of finches from three islands whenhe visited the Galápagos Islands. In all, he collected 9 distinctspecies, all very similar to one another except for their bills. Twoground finches with large bills feed on seeds that they crush in theirbeaks, while two with narrower bills eat insects. One finch is a fruiteater, one picks insects out of cactuses, and yet another creeps upon sea birds and uses its sharp beak to drink their blood.

Darwin suggested that the nine species of Galápagos finches evolvedfrom an original ancestral species. Changes occurred as different pop-ulations accumulated adaptations to different food sources. This ideawas first tested in 1938 by the naturalist David Lack. He watched thebirds closely for five months and found little evidence to support Dar-win’s hypothesis. Stout-beaked finches and slender-beaked fincheswere feeding on the same sorts of seeds. A second, far more thoroughstudy was carried out over 25 years beginning in 1973 by Peter andRosemary Grant of Princeton University. The Grants’ study presents amuch clearer picture that supports Darwin’s interpretation.

It was Lack’s misfortune to study the birds during a wet year,when food was plentiful. The size of the beak of the finch is of lit-tle importance in such times. Slender and stout beaks both workwell to gather the small, soft seeds which were plentiful.

During dry years, however, plants produce few seeds, large orsmall. During these leaner years, few small, tender seeds were avail-able. The difference between survival and starvation is the ability toeat the larger, tougher seeds that most birds usually pass by. TheGrants measured the beaks of many birds every year. They found thatafter several dry years, the birds that had longer, more-massive beakshad better feeding success and produced more offspring.

When wet seasons returned, birds tended to have smaller beaksagain, as shown in Figure 13. The numbers of birds with differentbeak shapes are changed by natural selection in response to theavailable food supply, just as Darwin had suggested.

By relating the environment to beak size, the Grants showed thatnatural selection influences evolution.

Figure 13 Natural selection in finches

Beak-Size Variation

9.0

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Teaching TipSequential Diagram ofSpeciation Have students make aGraphic Organizer that shows eachstep of speciation in the propersequence. Have students use thefollowing terms: divergence, isola-tion, natural selection, new species,and variation.

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Answers will vary. Sampleanswers might include thehypothesis that body form, especially the degree of left-rightsymmetry, correlates with geneticfitness. Studies have confirmedthat people find symmetry anattractive physical quality.

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Use this graphic organizer withTeaching Tip on this page.

Graphic Organizer

Variation New speciesIsolationNatural

selection Divergence

Analyzing Changein LizardPopulationsSkills AcquiredInterpreting, analyzing,concluding, predicting

Teacher’s NotesPoint out to students that thegraph shown here is a scatterplot. The data points on such a graph are not connected bylines. Rather, these graphs illustrate the distribution andpatterns of the data.

Answers to Analysis1. The average hind limb length

of each population changed inresponse to differences in theaverage perch diameter ofplants on the different islands.

2. The population could evolveand have longer average hindlimbs, or it could go extinct.

3. The experiment illustrates thatcharacteristics of populationscan change over time inresponse to environmentalpressures.

<x + 6x - 7 - 02

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Formation of New Species Species formation occurs in stages. Recall that natural selectionfavors changes that increase reproductive success. Therefore, aspecies molded by natural selection has an improved “fit” to itsenvironment. The accumulation of differences between groups iscalled (die VUHR jehns). Divergence leads to the for-mation of new species. Biologists call the process by which newspecies form (spee see AY shun).

Forming SubspeciesSeparate populations of a single species often live in several differ-ent kinds of environments. In each environment, natural selectionacts on the population. Natural selection results in the evolution ofoffspring that are better adapted to that environment. If their envi-ronments differ enough, separate populations of the same speciescan become very dissimilar. Over time, populations of the samespecies that differ genetically because of adaptations to differentliving conditions become what biologists call . The mem-bers of newly formed subspecies have taken the first step towardspeciation. Eventually, the subspecies may become so different thatthey can no longer interbreed successfully. Biologists then considerthem separate species.

Maintaining New SpeciesWhat keeps new species separate? Why are even closely relatedspecies usually unable to interbreed? Once subspecies becomedifferent enough, a barrier to reproduction, like the one shown inFigure 14, usually prevents different groups from breeding witheach other.

subspecies

speciation

divergence

Real LifeWhy do we find certainpeople pretty orhandsome? Some evolutionarybiologists think that manytraits that contribute to aperson’s attractivenessactually reveal the per-son’s fitness as a mate.Finding Information Research and comparehypotheses concerningbiological and culturalreasons that people judgeothers as attractive.

Figure 14 Mating activityin various frogs. Thoughthey appear to be similar, pick-erel frogs (Rana palustris) andleopard frogs (Rana pipiens)are different species. Thegraph shows that the time ofpeak mating activity variesbetween four species of frogs.

Leopard frogTree frog

Pickerel frogBullfrog

March 1 April 1 May 1 June 1 July 1

Mat

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Mating Activity in FrogsPickerel frog

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ReteachingHave students suggest why sharksand alligators, which are consid-ered “living fossils,” have changedlittle over millions of years. (Theirenvironments, to which they are welladapted, have remained fairly con-stant over this time period.)

Quiz1. Divergence can lead to the for-

mation of new ________ and new________. (subspecies, species)

2. If two similar species of flower-ing plants bloom at differenttimes, what is the name of theprocess that keeps these speciesseparate? (reproductive isolation)

AlternativeAssessmentHave students make a labeleddiagram showing the process ofnatural selection using species oftheir choice. The chosen speciescould be one the student makes up.In their diagrams, students shouldinclude the four important pointsabout natural selection discussed inthis section.

GENERAL

GENERAL

CloseClose

Answers to Section Review1. (1) genetic variation, (2) environmental chal-

lenges to reproduction, (3) overproduction ofoffspring and a struggle for survival and (4) anincrease in the number of individuals withcharacteristics suited to the environment

2. Bacteria have genetic variations that enablesome to survive and reproduce in the presenceof antibiotics. The non-resistant bacteria die,while the resistant bacteria reproduce.

3. The changes in the finches’ beaks were aresponse to changes in their food sources thatwere caused by climate changes.

4. As populations of a species spread throughoutan environment, they are exposed to varying

conditions (environmental pressures). Overtime, the separate populations become distinctand split into subspecies, and eventually, sepa-rate species.

5. It would take at least several generations andwould depend on how long it takes an organismto reach reproductive maturity.

6. A. Incorrect. The populations of finches werenot isolated from one another. B. Incorrect.There was no evidence of pollution. C. Incorrect.Rain did not cause beak enlargement (thoughlack of rain, indirectly, was involved). D. Correct. Finches with larger beaks were able to eat the larger, tougher seeds.


There are several types of barriers that may isolate two or moreclosely related groups. For example, groups may be geographicallyisolated or may reproduce at different times. Physical differencesmay also prevent mating, or they may not be attracted to one anotherfor mating. The hybrid offspring may not be fertile or suited to theenvironment of either parent.

Biologists have seen the stages of speciation in many differentorganisms. Thus, the way that natural selection leads to the formationof new species has been thoroughly documented. As changes continueto build up over time, living species may become very different fromtheir ancestors and from other species that evolved from the samerecent common ancestor, leading to the appearance of new species.

Analysis

1. Interpreting GraphicsHow did the average hind-limblength of each island’s lizardpopulation change from that ofthe original population?

2. Predict what would happento a population of lizards withshort hind limbs if they wereplaced on an island with alarger average perch diameterthan from where they came.

3. Justify the argument that this experiment supports thetheory of evolution by naturalselection.

Analyzing Change inLizard PopulationsBackground

In 1991, Jonathan Losos, an American sci-entist, measured hind-limb length of lizardsfrom several islands and the average perchdiameter of the island plants. The lizardswere descended from a common popula-tion 20 years earlier, and the islands haddifferent kinds of plants on which thelizards perched. Examine the graph at rightand answer the following questions:

<x + 6x - 7 - 02

8

493 0

52

Incr

easi

ng

per

ch d

iam

eter

Increasing hind-limb length

Each island's lizard population

Original lizard population

Hind-Limb Length Variation

List four elements of natural selection.

Describe the mechanism that causes population changes in antibiotic-resistant bacteria.

Identify what caused the change in the finch’sbeaks as seen in the Grants’ study.

Describe how speciation takes place.

Critical Thinking Evaluating Results Basedon the results of David Lack’s study and theGrants’ study of finches, what conclusion canyou make about the length of time required forevolution of a new species to take place?

The beaks of finches onthe Galápagos Islands enlarged over generationsin response to A isolation. C rain.B pollution. D limited food supply.


Section 3 Review

www.scilinks.orgTopic: Species FormationKeyword: HX4167

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AlternativeAssessmentHave students list the key termsused in this chapter and a brief def-inition of each term. Have themconstruct crossword puzzles usingthe definitions as the clues.

GENERAL


• Science Skills Worksheet• Critical Thinking Worksheet• Test Prep Pretest• Chapter Test GENERAL

GENERAL

GENERAL


Key Concepts

Study CHAPTER HIGHLIGHTS

ZONEKey Terms

Section 1population (278)natural selection (279)adaptation (279)reproductive isolation (281)gradualism (282)punctuated equilibrium (282)

Section 2paleontologist (285)vestigial structure (286)homologous structure (286)

Section 3divergence (291)speciation (291)subspecies (291)

The Theory of Evolution by Natural Selection● Charles Darwin concluded that animals on the coast of South

America that resembled those on the nearby islands evolveddifferences after separating from a common ancestor.

● Darwin was influenced by Thomas Malthus, who wrote thatpopulations tend to grow as much as the environment allows.

● Darwin proposed that natural selection favors individualsthat are best able to survive and reproduce.

● Under certain conditions, change within a species can leadto new species.

● Gradualism is a process of evolution in which speciationoccurs gradually, and punctuated equilibrium is a processin which speciation occurs rapidly between periods of littleor no change.

Evidence of Evolution● Evidence of orderly change can be seen when fossils are

arranged according to their age.● Differences in amino acid sequences and DNA sequences

are greater between species that are more distantly relatedthan between species that are more closely related.

● Similarities of structures in different vertebrates provideevidence that all vertebrates share a common ancestor.

Examples of Evolution● Individuals that have traits that enable them to survive in a

given environment can reproduce and pass those traits totheir offspring.

● Experiments show that evolution through natural selectionhas occurred within populations of antibiotic-resistantbacteria and in Darwin’s finches.

● Speciation begins as a population adapts to its environment. ● Reproductive isolation keeps newly forming species from

breeding with one another.

3

2

1

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Answer to Concept MapThe following is one possible answer to Performance Zone item 15 on the next page.

is driven by

which requires

which lead to

which results in

Evolution

natural selection

extinction

genetic variationpopulations

divergence

speciation

environment

occurs in

does not occur ifspecies cannot adapt

conditionsin the

which can result in


ANSWERS

Understanding Key Ideas1. b 3. a2. b 4. a5. a. An adaptation enables an

organism to better survive in itsenvironment; natural selection isthe process by which populationschange in response to their envi-ronment. b. Extinction is thedeath of all members of a species;isolation is the separation of pop-ulations of the same species. c. Populations are groups of indi-viduals of the same species livingin the same area; subspecies differgenetically because of adaptationsto different conditions. d. Homologous structures share acommon ancestry. Vestigial struc-tures are reduced in size or func-tion. e. Divergence is theaccumulation of differencesbetween groups; speciation is theprocess by which species form.

6. It may indicate that these animalsshare a common ancestor.

7. No; natural selection operates ongenetic variation.

8. The greater the number ofnucleotide differences betweentwo species, the more distant theirmost recent common ancestormay be.

9. Subspecies are populations of thesame species that differ geneticallybecause of adaptation. This is thefirst step toward speciation.

10. Meiosis provides a source ofgenetic variation upon which natural selection can act.

11. erosion12. The answer to the concept map is

found at the bottom of the StudyZone page.

Critical Thinking13. No14. Darwin’s theory of evolution by natural selec-

tion is supported by the observation of evolu-tion in real time, evidence of change oforganisms over time found in the fossil record,and so on. Weaknesses of Darwin’s theory mayinclude the gaps in the fossil record, the lack ofreal-time confirmation of speciation events,and so on.

15. This understanding of genetics supportsDarwin’s theory by explaining how offspringcan have characteristics found in their parents,whether or not such characteristics areexpressed.

Alternative Assessment16. Paleontologists study fossils and other

remains of past life. Paleontologists usuallyhave at least an undergraduate degree in zoology and/or geology, including training inchemistry and physics but may require aPh.D. Most paleontologists are employed byuniversities, museums, or large oil and con-struction companies.

Section Questions1 1, 3, 10, 11, 13, 142 2, 6, 8, 15, 163 4, 5, 7, 9, 12, 15, 16

Assignment Guide


CHAPTER 13

Understanding Key Ideas1. According to the modern theory of evolution,

a. Lamarck was completely wrong.b. random gene mutation is a part

of evolution.c. punctuated equilibrium has replaced

natural selection.d. the diversity of life-forms resulted

from the inheritance of acquiredcharacteristics.

2. With respect to the problem of antibiotic-resistant tuberculosis, which entity evolves?a. the patientb. the bacteriumc. the antibioticd. None of the above.

3. What is true about gradualism with respectto punctuated equilibrium?a. Each is a model of evolution.b. Neither is a model of evolution.c. Only gradualism portrays true evolution. d. Only punctuated equilibrium portrays

true evolution.

4. The process by which isolated populationsof the same species become new species iscalleda. speciation.b. reproductive isolation.c. genetic variation.d. natural seclection.

5. For each pair of terms, explain thedifferences in their meanings.a. adaptation, natural selectionb. extinction, reproductive isolationc. population, subspeciesd. homologous, vestigiale. divergence, speciation

6. Adult lobsters and barnacles look verydifferent. The larvae of barnacles andlobsters, however, are practically identical.What does this indicate about the evolu-tionary history of these organisms?

7. Could a population of identical organismsundergo natural selection? Why or why not?

8. Explain the relationship between thenumber of nucleotide differences betweentwo species and the time since the speciesshared a common ancestor.

9. What is a subspecies, and how is forma-tion of a subspecies related to the processof speciation?

10. How is meiosis beneficial to the evolutionof a species by natural selection? (Hint:See Chapter 7, Section 1.)

11. Other than punctuatedequilibrium, what naturally occurringphenomena might explain large gaps in the fossil record?

12. Concept Mapping Make a conceptmap that shows how natural selection leadsto speciation. Try to include the followingterms in your map: evolution, naturalselection, genetic variation, environment,speciation, and divergence.

Critical Thinking13. Applying Information If a favorable trait

increases the life span of an organismwithout affecting reproductive success,does evolution occur?

14. Evaluating Analyze Darwin’s theory ofevolution by natural selection and describeone strength and one weakness.

15. Justifying Conclusions About 40 years afterthe publication of On the Origin of Species,genetics was recognized as a science.Explain how information about geneticsmight support Darwin’s theory of evolution.

Alternative Assessment16. Career Connection Paleontologist Research

the field of paleontology, and write a reporton your findings. Your report shouldinclude a job description, trainingrequired, kinds of employers, growthprospects, and starting salary.

PerformanceZONE

CHAPTER REVIEW

294



Standardized Test Prep

Understanding ConceptsDirections (1–4): For each question, write ona separate sheet of paper the letter of thecorrect answer.

1 What term describes the process by whicha species becomes better suited to itsenvironment?A. adaptation C. gradualismB. equilibrium D. natural selection

2 What are anatomical structures that sharea common ancestry?F. analogous structuresG. evolutionary structuresH. homologous structuresI. vestigial structures

3 In the Grants’ study, the effect of climateon the size of the finch’s beak provides anexample of which of the following?A. fossilizationB. natural selectionC. speciationD. reproductive isolation

4 The woodpecker finch and the warblerfinch are different species. Which of thefollowing can you conclude about thesetwo birds?F. They cannot interbreed.G. The lack a common ancestor.H. They lack homologous structures.I. They have very different embryos.

Directions (5): For the following question,write a short response.

5 Generation time is the time from thebeginning of an organism’s life to the pointof reproduction. What effect would thetime from the beginning of an organism’slife to the point of reproduction have onthe rate of evolution of a species?

Reading SkillsDirections (6): Read the passage below.Then answer the question.

Alfred Russel Wallace was a biologist whocollected insects on an 1848 expedition tothe Amazon. He also made observations inthe Malay Archipelago between 1854 and1862. Wallace discovered that animals on the western islands of the Malay Archipelagodiffered sharply from those on the easternislands.

6 What condition might have caused theseanimals to evolve into different species?A. fossilizationB. population growthC. punctuated equilibriumD. reproductive isolation

Interpreting GraphicsDirections (7): Base your answer to question7 on the diagram below.

Vertebrate Evolution

7 Which organism has DNA that is probablymost similar to the glyptodont’s DNA?F. armadillo H. warbler finchG. finch-like bird I. woodpecker finch

TestFor a question about a structure or phenomenon thathas a complex name, write down the name andreview its meaning before answering the question.

Woodpeckerfinch

Warblerfinch

Glyptodont

Armadillo

Recent commonancestor

(armadillo-likemammal)

Recent commonancestor

(finch-like bird)

Remote commonancestor

(early vertebrate)

295

Question 3 Answer B is the cor-rect choice. Answer A is incorrectbecause fossilization occurs afteran organism’s death and does notaffect natural selection. Answer Cis incorrect because the Grantsstudied differences in populationsfrom one year to the next, tooshort a time to show speciation.Answer D is incorrect because theclimate was not preventing twopopulations from interbreeding.

Question 5 A shorter periodbetween one generation and thenext means that more generationscan occur in a specific period oftime, and thus evolution can occurfaster.

Question 6 Answer D is the cor-rect choice. Reproductive isolationcan lead to formation of separatespecies. Answer A is incorrectbecause fossilization occurs afteran organism’s death and does notaffect adaptation. Answer B isincorrect because populationgrowth describes how populationschange in size. Answer C is incor-rect because punctuated equilib-rium is a model of evolution andwould not explain the mechanismthat caused specific species todiverge from one another.

Question 7 Answer F is the cor-rect choice. The DNA of theglyptodont and the armadillo ismost similar. Answer G is incorrectbecause the common ancestor ofthe finch-like bird and theglyptodont predates the armadillo-like ancestor. Answer H is incor-rect because the common ancestorof the warbler finch and theglyptodont predates the armadillo-like ancestor. Answer I is incorrectbecause the common ancestor ofthe woodpecker finch and theglyptodont predates the armadillo-like ancestor.

Answers1. A2. H3. B4. F5. A shorter generation time causes a faster rate

of evolution for an organism.6. D7. F

Standardized Test Prep


MODELING NATURALSELECTION

Teacher’s Notes

Time RequiredOne 50-minute class period

Ratings

TEACHER PREPARATION

STUDENT SETUP

CONCEPT LEVEL

CLEANUP

Skills Acquired• Collecting Data• Constructing Models• Experimenting• Organizing and Analyzing Data

Scientific MethodsIn this lab, students will:• Make Observations• Ask Questions• Test the Hypothesis• Draw Conclusions

Safety CautionsRemind students to be careful withscissors and always cut in a direc-tion away from the face and body.

E A S Y H A R D

Tips and TricksStudents will simulate the breeding of severalgenerations of the birds and observe the effectof various phenotypes on the evolutionarysuccess of these animals. The random natureof mutations is demonstrated by randomlychanging the anterior and posterior wing posi-tion and wing circumference of the birds. Eachstudent group will need one sheet of construc-tion paper, a meter stick or tape measure, apair of scissors, a straw, cellophane tape, a

coin, and a die. To save time, have studentscreate their table before beginning the lab, andhave students work in pairs or small groups.You may wish to review and clarify the princi-ples of selection. This investigation reinforcesthe concept of natural selection as differentialreproduction rather than merely differentialsurvival. Have students dispose of their paperscraps and paper birds in the trash.


Exploration Lab

Before You Beginoccurs when organisms

that have certain survive to reproducemore than organisms that lack those traitsdo. A population evolves when individualswith different survive or repro-duce at different rates. In this lab, you willmodel the selection of favorable traits in anew generation by using a paper model of abird—the fictitious Egyptian origami bird(Avis papyrus), which lives in dry regions ofNorth Africa. Assume that only birds thatcan successfully fly the long distancesbetween water sources will live long enough to breed successfully.

1. Write a definition for each boldface term inthe preceding paragraph.

2. Make a data table similar to the one shownbelow.

3. Based on the objectives for this lab, write aquestion you would like to explore aboutthe process of selection.

ProcedurePART A: Parental Generation1. Cut two strips of paper, 2 ! 20 cm each.

Make a loop with one strip of paper, lettingthe paper overlap by 1 cm, and tape theloop closed. Repeat for the other strip.

genotypes

traitsNatural selection

Exploration Lab

SKILLS• Modeling a process

• Inferring relationships

OBJECTIVES• Model the process of

selection.

• Relate favorable mutations toselection and evolution. 3 cm

3 cm2 cm

2 cm

Egyptianorigami bird

Modeling Natural Selection

Bird Coin flip(H or T)

Die throw(1–6)

Anterior wing (cm) Posterior wing (cm)Average distanceflown (m)Width

Parent

Generation 1

Chick 1

Chick 2

Chick 3

Generation 2

Chick 1

Chick 2

Chick 3

Circum. Distancefrom front

Width Circum. Distancefrom back

NA NA 2 19 3 2 19 3

MATERIALS• scissors

• construction paper

• cellophane tape

• soda straws

• felt-tip marker

• meterstick or tapemeasure

• penny or other coin

• six-sided die

296


Answers to Before You Begin1. natural selection—process by

which populations change inresponse to their environment asindividuals better adapted to theenvironment leave more offspring;traits—distinguishing characteris-tics; genotype—the genetic consti-tution of an organism as indicatedby its set of alleles

3. Answers will vary. For example:Are the effects of natural selectionobvious in only a few generations?

Answers to Procedure3. Answers will vary.6. Answers will vary.7. Answers will vary.8. Answers will vary.9. Answers will vary.

Answers to Analyze andConclude1. Most students should answer

“yes” as the best-flying birds areselected as the sole parents of thenext generation.

2. This lab demonstrates that organ-isms can change significantly overonly a few generations. The labtherefore shows how isolated pop-ulations could diverge to the pointthat they constitute differentspecies, as happened to finches onthe Galápagos Islands.

3. Answers will vary. For example:Does natural selection act on onetrait at a time, or can selectionpressure affect the evolution of several traits at once?

Answers to On the JobPopulation biologists study evolutionby examining changes in populationsize over time. Techniques populationbiologists use to study evolutioninclude population sampling, computersimulations, and various statisticaltechniques for analyzing data.


2. Tape one loop 3 cm from each end of thestraw, as shown above. Mark the frontend of the bird with a felt-tip marker. Thisbird represents the parental generation.

3. Test how far your parent bird can fly byreleasing it with a gentle overhand pitch.Test the bird twice. Record the bird’s aver-age flight distance in your data table.

PART B: First (F1) Generation4. Each origami bird lays a clutch of three

eggs. Assume that one of the chicks is aclone of the parent. Use the parent to rep-resent this chick in step 6.

5. Make two more chicks. Assume that thesechicks have mutations. Follow Steps A–Cbelow for each chick to determine theeffects of its mutation.

Step A Flip a coin to determine whichend is affected by a mutation.

Heads = anterior (front)

Tails = posterior (back)

Step B Throw a die to determine how themutation affects the wing.

Step C A mutation is lethal if it causes awing to fall off the straw or a wing with acircumference smaller than that of thestraw. If you get a lethal mutation, disre-gard it and produce another chick.

6. Record the mutations and the wingdimensions of each offspring.

7. Test each bird twice by releasing it with agentle overhand pitch. Release the birdsas uniformly as possible. Record the dis-tance each bird flies. The most successfulbird is the one that flies the farthest.

PART C: Subsequent Generations8. Assume that the most successful bird in

the previous generation is the sole parentof the next generation. Repeat steps 4–7using this bird.

9. Continue to breed, test, and record datafor eight more generations.

PART D: Cleanup and Disposal10. Dispose of paper scraps in the

designated waste container.

11. Clean up your work area and all lab equipment. Return lab equip-

ment to its proper place. Wash yourhands thoroughly before you leave the laband after you finish all work.

Analyze and Conclude1. Analyzing Results Did the birds you

made by modeling natural selection flyfarther than the first bird you made?

2. Inferring Conclusions How might thislab help explain the variety of species ofGalápagos finches?

3. Further Inquiry Write another questionabout natural selection that could beexplored with another investigation.

1 = Wing position moves 1 cm toward the endof the straw.

2 = Wing position moves 1 cm toward the mid-dle of the straw.

3 = Wing circumferenceincreases by 2 cm.

4 = Wing circumferencedecreases by 2 cm.

5 = Wing width increasesby 1 cm.

6 = Wing width decreasesby 1 cm.

On the JobPopulation biology is the study ofpopulations. Do research to discoverhow population biologists study evolu-tion. For more about careers, visitgo.hrw.com and type in the keyword HX4 Careers.

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chapter 13 the theory · points of darwin’s theory of evolution by natural selection as it is...

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