chapter 15: the theory of evolution - polson...

400 THE THEORY OF EVOLUTION

The Theory of Evolution

What You’ll Learn■ You will analyze the theory

of evolution.■ You will compare and

contrast the processes of evolution.

Why It’s ImportantEvolution is a key concept for understanding biology.Evolution explains the diversityof species and predicts changes.

Look closely at the figuresthat appear in the chapter.For several of the drawings,make notes on how the fig-ure shows an aspect of evo-lution. As you read thechapter, review the figuresand add new information toyour notes.

To find out more about evolu-tion, visit the Glencoe ScienceWeb site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

15

This crayfish (above) and cricket(inset) live in dark caves and areblind. They have sighted rela-tives that live where there islight. Both the cave-dwelling species and theirrelatives are adapted todifferent environments. Aspopulations adapt to newor changing environments,individuals in the popula-tion that are adapted success-fully survive.

ChapterChapter

BIOLOGY

http://science.glencoe.com

Section

Charles Darwin andNatural Selection

The modern theory of evolution isa fundamental concept in biology.Recall that evolution is the change inpopulations over time. Learning theprinciples of evolution makes it easierto understand modern biology. Oneplace to start is by learning about theideas of English scientist CharlesDarwin (1809–1882)—ideas sup-ported by fossil evidence.

Fossils shape ideas about evolution

Biologists have used fossils in theirwork since the eighteenth century. Infact, fossil evidence formed the basisof the early evolutionary concepts.

Scientists wondered how fossilsformed, why many fossil species wereextinct, and what kinds of relation-ships might exist between the extinctand the modern species.

When geologists provided evi-dence indicating that Earth wasmuch older than many people hadoriginally thought, biologists beganto suspect that life slowly changesover time, or evolves. Many explana-tions about how species evolve havebeen proposed, but the ideas firstpublished by Charles Darwin are thebasis of modern evolutionary theory.

Darwin on HMS BeagleIt took Darwin years to develop his

theory of evolution. He began in1831 at age 21 when he took a job as

15.1 NATURAL SELECTION AND THE EVIDENCE FOR EVOLUTION 401

You need only to look aroundyou to see the diversity oforganisms on Earth. About

150 years ago, Charles Darwin,who had studied an enormousvariety of life forms, proposed anidea to explain how organismsprobably change over time.Biologists still base theirwork on this idea becauseit explains the livingworld they study.

SECTION PREVIEW

ObjectivesSummarize Darwin’stheory of natural selection.Explain how the structural and physio-logical adaptations of organisms relate tonatural selection.Distinguish among the types of evidencefor evolution.

Vocabularyartificial selectionnatural selectionmimicrycamouflagehomologous structureanalogous structurevestigial structureembryo

15.1 Natural Selection and theEvidence for Evolution

An Asian leopard and a cheetah (inset)

a naturalist on the English ship HMSBeagle, which sailed to South Americaand the South Pacific on a five-yearscientific journey.

As the ship’s naturalist, Darwinstudied and collected biological speci-mens at every port along the route.As you might imagine, these speci-mens were quite diverse. Studyingthe specimens made Darwin curiousabout possible relationships amongspecies. His studies provided thefoundation for his theory of evolu-tion by natural selection.

Darwin in the GalapagosThe Galapagos (guh LAHP uh gus)

Islands are a group of small islandsnear the equator, about 1000 km offthe west coast of South America. The

observations that Darwin made andthe specimens that he collected therewere especially important to him.

On the Galapagos Islands, Darwinstudied many species of animals andplants, Figure 15.1, that are uniqueto the islands, but similar to specieselsewhere. These observations ledDarwin to consider the possibilitythat species can change over time.However, after returning to England,he could not at first explain how suchchanges occur.

Darwin continues his studiesFor the next 22 years, Darwin

worked to find an explanation forhow species change over time. Heread, studied, collected specimens,and conducted experiments.

AUSTRALIA

NORTHAMERICA

ASIA

AFRICA

EUROPE

SOUTHAMERICA

ATLANTICOCEAN

PACIFICOCEAN

PACIFICOCEAN

INDIAN OCEAN

ARCTIC OCEAN

Lima

Tahiti

GalapagosIslands

Valparaiso

Rio de Janeiro

Montevideo

Falkland IslandsCape Horn

Sydney

KingGeorgeSound

CocosIslands

Mauritius

St. HelenaBahia

AzoresCanaryIslands

Cape VerdeIslands

Hobart NewZealand

Figure 15.1The five-year voyageof HMS Beagle tookDarwin around theworld. Animal speciesin the GalapagosIslands have uniqueadaptations.

Galapagos marine iguanas eat algae on the ocean’s bottom, an unusual food source for reptiles. Large clawshelp them cling to slippery rocks.

CC

The beak of thisGalapagos finch isadapted to feedon cacti.

AA

Galapagos tortoises arethe largest on Earth, dif-fering from other tortoisesin body size and shape.

BB

402

Finally, English economist ThomasMalthus proposed an idea that Darwinmodified and used in his explanation.Malthus’s idea was that the humanpopulation grows faster than Earth’sfood supply. How did this helpDarwin? He knew that many speciesproduce large numbers of offspring.He also knew that such species hadnot overrun Earth. He realized thatindividuals struggle to survive. Thereare many kinds of struggles, such ascompeting for food and space, escap-ing from predators, finding mates, andlocating shelter. Only some individu-als survive the struggle and produceoffspring. Which individuals survive?

Darwin gained insight into themechanism that determined whichorganisms survive in nature from his pigeon-breeding experiments.Darwin observed that the traits ofindividuals vary in populations—even in a population of pigeons. Sometimesvariations are inherited. By breedingpigeons with desirable variations,Darwin produced offspring with thesevariations. Breeding organisms withspecific traits in order to produce off-spring with identical traits is calledartificial selection. Darwin hypoth-esized that there was a force in naturethat worked like artificial selection.

Darwin explains natural selectionUsing his collections and observa-

tions, Darwin identified the process ofnatural selection, the steps of which youcan see summarized in Figure 15.2.Natural selection is a mechanismfor change in populations. It occurswhen organisms with certain varia-tions survive, reproduce, and passtheir variations to the next generation.Organisms without these variationsare less likely to survive and repro-duce. As a result, each generation con-sists largely of offspring from parentswith these variations that aid survival.


Figure 15.2Darwin proposed the idea of natural selectionto explain how species change over time.

In nature, organ-isms produce moreoffspring than cansurvive. Fishes, forexample, can some-times lay millionsof eggs.

AA

In any population,individuals havevariations. Fishes,for example, maydiffer in color, size,and speed.

BB

Individuals with certain useful varia-tions, such asspeed, survive intheir environment, passing those variations to thenext generation.

CC

Over time, off-spring with certainvariations make up most of thepopulation andmay look entirelydifferent fromtheir ancestors.

DD

Darwin was not the only one torecognize the significance of naturalselection for populations. As a resultof his studies on islands nearIndonesia in the Pacific Ocean,Alfred Russell Wallace, anotherBritish naturalist, had reached a simi-lar conclusion. After Wallace wroteDarwin to share his ideas about nat-ural selection, Darwin and Wallacehad their similar ideas jointly pre-sented to the scientific community.However, it was Darwin who pub-lished the first book about evolutioncalled On the Origin of Species byNatural Selection in 1859. The ideasdetailed in Darwin’s book are today abasic unifying theme of biology.

Interpreting evidence after Darwin

Volumes of scientific data havebeen gathered as evidence for evolu-tion since Darwin’s time. Much ofthis evidence is subject to interpreta-tion by different scientists. One of

the problems is that evolutionaryprocesses are difficult for humans toobserve directly. The short scale ofhuman life spans makes it difficult tocomprehend evolutionary processesthat occur over millions of years. Forsome people the theory of evolutionis contradictory to their faith, andthey offer other interpretations of thedata. Many biologists, however, havesuggested that the amount of scien-tific evidence supporting the theoryof evolution is overwhelming. Almostall of today’s biologists accept the the-ory of evolution by natural selection.However, biologists are also nowmore aware of genetics. Evolution ismore commonly defined by modernbiologists as any change in the genepool of a population.

Adaptations: Evidencefor Evolution

Have you noticed that some plantshave thorns and some plants don’t?


Some ancestral rats may haveavoided predators better than others because of variations suchas the size of teeth and claws.

BBThe ancestors of today’scommon mole-rats prob-ably resembled Africanrock rats.

AA

Have you noticed that some animalshave distinctive coloring but othersdon’t? Have you ever wondered howsuch variations arose? Recall that anadaptation is any variation that aidsan organism’s chances of survival inits environment. Thorns are an adap-tation of some plants and distinctivecolorings are an adaptation of someanimals. Darwin’s theory of evolutionexplains how adaptations may developin species.

Structural adaptations arise over time

According to Darwin’s theory,adaptations in species develop overmany generations. Learning aboutadaptations in mole-rats can helpyou understand how natural selec-tion has affected them. Mole-ratsthat live underground in darkness areblind. These blind mole-rats havemany adaptations that enable themto live successfully underground.Look at Figure 15.3 to see how

these modern mole-rat adaptationsmight have evolved over millions ofyears from characteristics of theirancestors.

The structural adaptations of com-mon mole-rats include large teethand claws. These are body parts thathelp mole-rats survive in their envi-ronment by, for example, enablingthem to dig better tunnels. Structuraladaptations such as the teeth andclaws of mole-rats are often used todefend against predators. Some adap-tations of other organisms that keeppredators from approaching include arose’s thorns or a porcupine’s quills.

Some other structural adaptationsare subtle. Mimicry is a structuraladaptation that enables one species toresemble another species. In oneform of mimicry, a harmless specieshas adaptations that result in a physi-cal resemblance to a harmful species.Predators that avoid the harmfulspecies also avoid the similar-looking,harmless species. See if you can tell


Figure 15.3Darwin’s ideas aboutnatural selection canexplain some adapta-tions of mole-rats.

Ancestral rats that survived passed theirvariations to offspring. After many generations, most of the population’sindividuals would have these adaptations.

CC Over time, natural selection producedmodern mole-rats. Their blindness mayhave evolved because vision had nosurvival advantage for them.

DD

the difference between a harmless flyand the wasp it mimics when youlook at Figure 15.4.

In another form of mimicry, two ormore harmful species resemble eachother. For example, yellow jacket hor-nets, honeybees, and many other

species of wasps all have harmfulstings and similar coloration andbehavior. Predators may learnquickly to avoid any organismwith their general appearance.

Another subtle adaptationis camouflage (KAM uh flahj),an adaptation that enables

species to blend with their surround-ings, as shown in Figure 15.4.Because well-camouflaged organismsare not easily found by predators,they survive to reproduce. Try theMiniLab to experience how camou-flage can help an organism survive.Then use the Problem-Solving Lab onthe next page to analyze data from anEnglish study of camouflaged pep-pered moths.

406

Camouflage Provides an Adaptive AdvantageCamouflage is a structural adaptation that allows organismsto blend with their surroundings. In this activity, you’ll dis-cover how natural selection can result in camouflage adapta-tions in organisms.

Procedure! Working with a partner, punch 100

dots from a sheet of white paper with a paper hole punch. Repeat with a sheet of black paper. These dots will represent black and white insects.

@ Scatter both white and black dots on asheet of black paper.

# Decide whether you or your partner will role-play a bird.$ The “bird” looks away from the paper, then turns back,

and immediately picks up the first dot he or she sees.% Repeat step 4 for one minute.

Analysis1. What color dots were most often collected?2. How does color affect the survival rate of insects?3. What might happen over many generations to a similar

population in nature?

MiniLab 15-1MiniLab 15-1 Formulating Models

Figure 15.4Mimicry and camouflage are protective adaptations of organ-isms. The colors and body shape of a yellow jacket wasp (a)and a harmless syrphid fly (b) are similar. Predators avoidboth insects. Camouflage enables organisms, such as this leaf frog (c), to blend with their surroundings.

a

b

c

Physiological adaptations candevelop rapidly

In general, most structural adapta-tions develop over millions of years.However, there are some adaptationsthat evolve much more rapidly. Forexample, do you know that some ofthe medicines developed during thetwentieth century to fight bacterialdiseases are no longer effective?When the antibiotic drug penicillinwas discovered about 50 years ago, itwas called a wonder drug because itkilled many types of disease-causingbacteria and saved many lives. Today,penicillin no longer affects as manyspecies of bacteria because somespecies have evolved physiological(fihz ee uh LAHJ ih kul) adaptations toprevent being killed by penicillin.Look at Figure 15.5 to see howresistance develops in bacteria.

Physiological adaptations arechanges in an organism’s metabolicprocesses. In addition to species ofbacteria, scientists have observedthese adaptations in species of insectsand weeds that are pests. After yearsof exposure to specific pesticides,many species of insects and weedshave become resistant to these chem-icals that used to kill them.

How can natural selection beobserved? In some organisms thathave a short life cycle, biologists haveobserved the evolution of adaptationsto rapid environmental changes. In theearly 1950s, English biologist H. B.Kettlewell studied camouflage adapta-tions in a population of light- and dark-colored peppered moths, Biston betularia.The moths rested on the trunks of treesthat grew in both the country and the city.Moths are usually speckled gray-brown, and dark moths, whichoccur occasionally, are black. Birds pluck the moths from thetrees for food. Urban industrial pollution had blackened thebark of city trees with soot. In the photo, you see a city treewith dark bark similar to the color of one of the moths.

AnalysisKettlewell raised more than 3000 caterpillars to provide

adult moths. He marked the wings of the moths these cater-pillars produced so he would recapture only his moths. In aseries of trials in the country and the city, he released andrecaptured the moths. The number of moths recaptured in atrial indicates how well the moths survived in the environ-ment. Examine the table below.

Thinking CriticallyCalculate the percentage of moths recaptured in each

experiment and explain any differences in survival rates in thecountry and the city moths in terms of natural selection.

Problem-Solving Lab 15-1Problem-Solving Lab 15-1

LocationNumbers of light moths

Numbers of dark moths

Country Released 496 488

Recaptured 62 34

City Released 137 493

Recaptured 18 136

Table 15.1 Comparison of country and city moths

Biston betularia

Figure 15.5The development of bacterialresistance to antibiotics isdirect evidence for evolution.

The bacteria in a populationvary in their ability to resistantibiotics.

AA When the population isexposed to an antibiotic, onlythe resistant bacteria survive.

BB The resistant bacteria liveand produce more resistantbacteria.

CC

Interpreting Data

Non-resistantbacterium

Resistantbacterium

Antibiotic

Other Evidence for Evolution

The development of physiologicalresistance in species of bacteria,insects, and plants is direct evidenceof evolution. However, most of theevidence for evolution is indirect,coming from sources such as fossilsand studies of anatomy, embryology,and biochemistry.

FossilsFossils are an important source of

evolutionary evidence because theyprovide a record of early life and evo-lutionary history. For example, pale-ontologists conclude from fossils thatthe ancestors of whales were proba-bly land-dwelling, doglike animals.

Although the fossil record providesevidence that evolution occurred, therecord is incomplete. Working withan incomplete fossil record is some-thing like trying to put together a jig-saw puzzle with missing pieces. But,after the puzzle is together, even withmissing pieces, you will probably stillunderstand the overall picture. It’s

the same with fossils. Although pale-ontologists do not have intermediateforms of most species, they can oftenstill understand the overall picture ofhow a species evolved.

Fossils are found throughout theworld. As the fossil record becomesmore complete, the sequences ofevolution become more clear. Forexample, in Figure 15.6 you can seehow paleontologists sequenced thepossible forms that led to today’scamel after piecing together fossilskulls, teeth, and limb bones.

AnatomyStructural features with a common

evolutionary origin are called homologous structures. Homolo-gous structures can be similar inarrangement, in function, or in both.For example, look at the forelimb bonesof the animals shown in Figure 15.7.Although the bones of each forelimbare modified for their function, thebasic arrangement of the bones ineach limb is similar. Evolutionarybiologists view homologous structuresas evidence that organisms evolved

Figure 15.6Paleontologists haveused fossils to tracethe evolution of themodern camel. About66 million years agothe ancestors ofcamels were as smallas rabbits.

Paleocene66 millions of

years ago

Eocene54 millions of

years ago

Oligocene37 millions of

years ago

Miocene26 millions of

years agoPresent

Whale forelimb

from a common ancestor. It would beunlikely for so many animals to havesimilar structures if each speciesarose separately.

The structural or functional simi-larity of a body feature doesn’t alwaysmean that two species are closelyrelated. In Figure 15.8, you cancompare the wing of a butterfly withthe wing of a bird. Bird and butterflywings are not similar in structure, butthey are similar in function. Thewings of birds and insects evolvedindependently of each other in twodistantly related groups of ancestors.The body parts of organisms that donot have a common evolutionary ori-gin but are similar in function arecalled analogous structures.

Although analogous structuresdon’t shed light on evolutionary rela-tionships, they do provide evidenceof evolution. For example, insect andbird wings probably evolved sepa-rately when their different ancestorsadapted independently to similarways of life.


Figure 15.7The forelimbs of crocodiles,whales, and birdsare homologous structures. Thebones of each are modified fortheir function.

Crocodile forelimb

Bird wing

Figure 15.8Insect and bird wingsare similar in functionbut not in structure.Bones are the frame-work of bird wings,whereas a toughmaterial called chitincomposes insectwings.

Another type of body feature thatsuggests evolutionary relationship is avestigial structure (veh STIHJ ee ul)—a body structure that has no functionin a present-day organism but wasprobably useful to an ancestor. Astructure becomes vestigial when thespecies no longer needs the feature.Although the structure has no func-tion, it is still inherited as part of thebody plan for the species.

Many organisms have structureswith no apparent function. The eyesof blind mole-rats and cave fish arevestigial structures because they haveno function. In Figure 15.9, you seetwo flightless birds—an extinct ele-phant bird and an African ostrich—with extremely reduced forelimbs.Their ancestors probably foraged onland for food and nested on theground. As a result, over time, the

ancestral birds probably became quitelarge and unable to fly, features evi-dent in fossils of the elephant birdand present in the African ostrich.

EmbryologyIt’s very easy to see the difference

between an adult bird and an adultmammal, but can you distinguishbetween them by looking at theirembryos? An embryo is the earlieststage of growth and development ofboth plants and animals. The embryosof a fish, a reptile, a bird, and a mam-mal are shown in Figure 15.10. Youcan see a tail and gill slits in all theembryos. You know that reptiles,birds, and mammals do not have gillswhen they are mature. As develop-ment continues, the differences in theembryos will increase until you candistinguish among them. However,the similarities among the youngembryos suggest evolution from a dis-tant, common ancestor.

BiochemistryBiochemistry also provides evi-

dence for evolution. It reveals infor-mation about relationships between


Figure 15.10Comparing embryoscan reveal their evo-lutionary relation-ships. The presenceof gill slits and tails in early vertebrateembryos shows thatthey may share acommon ancestor.

Figure 15.9Vestigial structures, such asthe forelimbs of the extinctelephant bird (a) and thoseof the present-day ostrich(b), are evidence of evolution because theyshow structural change over time.

OriginWORDWORD

vestigialFrom the Latinword vestigium,meaning “sign.”The forelimbs of ostriches are vestigial structures.

Fish Reptile Bird Mammal

Gill slits

Tail

Gill slits

Tail

a

b

Archaea EukaryotaEubacteria

Purplebacteria Gram

positivebacteria

Animals Ciliates Fungi

Plants

Flagellates

Sporozoans

Extremehalophiles

Extremethermophiles

Methanogens

Cyanobacteria

Flavobacteria

Green non-sulfur

bacteria

Thermotoga

Ancestral prokaryotes

individuals and species. Comparisonsof the DNA or RNA of differentspecies produce biochemical evidencefor evolution. Today, many scientistsuse the results of biochemical studiesto help determine the evolutionaryrelationships of species.

Since Darwin’s time, scientistshave constructed evolutionary dia-grams that show levels of relation-ships among species. In the 1970s,some biologists began to use RNA

and DNA nucleotide sequences toconstruct evolutionary diagrams. Theevolutionary diagram you see inFigure 15.11 was constructed usingthe results of biochemical analysisand other data, including anatomicaldata. Notice that it divides all speciesinto three domains: the Archaea, theEubacteria, and the Eukaryota—anidea that underlies one of the mostrecently developed classification sys-tems for organisms.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. Briefly explain Darwin’s ideas about natural

selection.2. Some snakes have vestigial legs. Why is this con-

sidered evidence for evolution?3. Explain how mimicry and camouflage help

species survive.4. How do homologous structures provide evidence

for evolution?

Thinking Critically5. A parasite that lives in red blood cells causes the

disease called malaria. In recent years, new

strains of the parasite have appeared that areresistant to the drugs used to treat the disease.Explain how this could be an example of naturalselection occurring.

6. Sequencing Fossils indicate that whales evolvedfrom ancestors that had legs. Using your knowl-edge of natural selection, sequence the stepsthat may have occurred during the evolution ofwhales from their terrestrial, doglike ancestors.For more help, refer to Organizing Informationin the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Figure 15.11This evolutionarydiagram is based oncomparisons oforganisms’ RNA and supported by otherdata.

Section


Population Genetics and Evolution

When Charles Darwin developedhis theory of natural selection in the1800s, he did so without knowingabout genes. Since Darwin’s time,scientists have learned a great dealabout genes and modified Darwin’sideas accordingly. At first, geneticinformation was used to explain thevariation among individuals of a pop-ulation. Then, studies of the complexbehavior of genes in populations ofplants and animals developed into thefield of study called populationgenetics. The principles of today’smodern theory of evolution arerooted in population genetics andother related fields of study and areexpressed in genetic terms.

Populations, not individuals, evolve

Can individuals evolve? That is,can an organism respond to naturalselection by acquiring or losing char-acteristics? Recall that genes deter-mine most of an individual’s features,such as tooth shape or flower color. Ifan organism has a feature—a varia-tion called a phenotype in geneticterms—that is poorly adapted to itsenvironment, the organism may beunable to survive and reproduce.However, within its lifetime, it can-not evolve a new phenotype inresponse to its environment.

Rather, natural selection acts onthe range of phenotypes in a popula-tion. Recall that a population consistsof all the members of a species thatlive in an area. Each member has the

You may recognize the birds shown here as meadowlarks. These birds range throughout

much of the United States. Meadowlarkslook so similar that it’s often difficult totell them apart. But if you listen, you’llhear a melodious bubbling sound from theWestern meadowlark, whereas theEastern meadowlark produces a clearwhistle. Although they are closely relatedand occupy the same ranges in parts ofthe central United States, these differentmeadowlarks do not normally interbreedand are classified as distinct species.

SECTION PREVIEW

ObjectivesSummarize the effectsof the different types of natural selection ongene pools.Relate changes ingenetic equilibrium to mechanisms of speciation.Explain the role of natural selection in convergent and divergent evolution.

Vocabularygene poolallelic frequencygenetic equilibriumgenetic driftstabilizing selectiondirectional selectiondisruptive selectionspeciationgeographic isolationreproductive isolationpolyploidgradualismpunctuated equilibriumadaptive radiationdivergent evolutionconvergent evolution

15.2 Mechanisms ofEvolution

Eastern meadowlark—

Sturnella magna

Western meadowlark—

Sturnella neglecta

genes that characterize the traits ofthe species, and these genes exist as pairs of alleles. Just as all of theindividuals make up the population,all of the genes of the population’sindividuals make up the population’sgenes. Evolution occurs as a popula-tion’s genes and their frequencieschange over time.

How can a population’s geneschange over time? Picture all of thealleles of the population’s genes asbeing together in a large pool called agene pool. The percentage of anyspecific allele in the gene pool is calledthe allelic frequency. Scientists calculate the allelic frequency of anallele in the same way that a baseballplayer calculates a batting average.They refer to a population in whichthe frequency of alleles remains thesame over generations as being ingenetic equilibrium. In the MathConnection at the end of the chapter,you can read about the mathematicaldescription of genetic equilibrium.You can study the effect of naturalselection on allelic frequencies in theBioLab at the end of the chapter.

Look at the population of snap-dragons shown in Figure 15.12. Apattern of heredity called incompletedominance, which you learned aboutearlier, governs flower color in snap-dragons. If you know the flower-colorgenotypes of the snapdragons in apopulation, you can calculate theallelic frequency for the flower-coloralleles. The population of snapdrag-ons is in genetic equilibrium whenthe frequency of its alleles for flowercolor is the same in all its generations.

Changes in genetic equilibriumA population that is in genetic

equilibrium is not evolving. Becauseallelic frequencies remain the same,phenotypes remain the same, too.Any factor that affects the genes inthe gene pool can change allelic fre-quencies, disrupting a population’sgenetic equilibrium, which results inthe process of evolution.

You have learned that one mecha-nism for genetic change is mutation.Environmental factors, such as radia-tion or chemicals, cause many muta-tions, but other mutations occur by

15.2 MECHANISMS OF EVOLUTION 413

Figure 15.12Incomplete domi-nance produces three phenotypes:red flowers (RR),white flowers (R'R'),and pink flowers(RR'). Although the phenotype frequencies of the generations vary, the allelic frequencies for the R and R' alleles donot vary.

R = 0.75

R' = 0.25

White = 0

Pink = 0.5

Red = 0.5

First generation Phenotype frequency

Allele frequency

RR RR RR' RR' RR RR' RR RR'

R = 0.75

R' = 0.25

White = 0.125

Pink = 0.25

Red = 0.625

Second generation Phenotype frequency

Allele frequency

RR RR' RR RR' RR R'R' RR RR

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chance. Of the mutations that affectorganisms, many are lethal, and theorganisms do not survive. Thus,lethal mutations are quickly elimi-nated. However, occasionally, amutation results in a useful variation,and the new gene becomes part ofthe population’s gene pool by theprocess of natural selection.

Another mechanism that disrupts apopulation’s genetic equilibrium isgenetic drift—the alteration ofallelic frequencies by chance events.Genetic drift can greatly affect smallpopulations that include the descen-dants of a small number of organ-isms. This is because the genes of the

original ancestors represent only asmall fraction of the gene pool of theentire species and are the only genesavailable to pass on to offspring. Thedistinctive forms of life that Darwinfound in the Galapagos Islands mayhave resulted from genetic drift.

Genetic drift has been observed insome small human populations thathave become isolated due to reasonssuch as religious practices and beliefsystems. For example, in LancasterCounty, Pennsylvania, there is anAmish population of about 12 000people who have a unique lifestyleand marry other members of theircommunity. By chance, at least oneof the original 30 Amish settlers inthis community carried a recessiveallele that results in short arms andlegs and extra fingers and toes in off-spring, Figure 15.13. Because of thesmall gene pool, many individualsinherited the recessive allele overtime. Today, the frequency of thisallele among the Amish is high—1 in14 rather than 1 in 1000 in the largerpopulation of the United States.

Genetic equilibrium is also dis-rupted by the movement of individu-als in and out of a population. Thetransport of genes by migrating indi-viduals is called gene flow. When anindividual leaves a population, itsgenes are lost from the gene pool.


Figure 15.13Genetic drift canresult in an increaseof rare alleles in asmall populationsuch as in the Amishcommunity ofLancaster County,Pennsylvania.


Detecting a Variation Pick almost any trait—height, eye color, leaf width, or seed size—and you can observe how the trait varies in a population. Some variations are an advantage to an organism and some are not.

Procedure! Copy the data table shown here, but include

the lengths in millimeters (numbers 25 through 45) that are missing from this table.

@ Use a millimeter ruler to measure a peanut shell’s length.In the Checks row, check the length you measured.

# Repeat step 2 for 29 more shells.$ Count the checks under each length and enter the total in

the row marked My Data.% Use class totals to complete the row marked Class Data.

Analysis1. Was there variation among the lengths of peanut shells?

Use specific class totals to support your answer.2. If larger peanut shells were a selective advantage, would

this be stabilizing, directional, or disruptive selection? Explain your answer.

MiniLab 15-2MiniLab 15-2 Collecting Data

Length in mm 20 21 22 23 24 — 46 47 48 49 50

Checks

My Data—Number of shells

Class Data—Number of shells

Data Table

When individuals enter a population,their genes are added to the pool.

Mutation, genetic drift, and geneflow may significantly affect the evo-lution of small and isolated genepools, such as those on islands.However, their effect is ofteninsignificant in larger, less isolatedgene pools. Natural selection is usu-ally the most significant factor thatcauses changes in established genepools—small or large.

Natural selection acts on variations

As you’ve learned, traits have vari-ation, as shown in the butterflies pic-tured in Figure 15.14. If you mea-sured the thumb lengths of everyonein your class, you’d find average,long, and short thumbs. Try measur-ing the variations in peanut shells inthe MiniLab on this page.

Recall that some variationsincrease or decrease an organism’schance of survival in an environment.These variations can be inherited andare controlled by alleles. Thus, theallelic frequencies in a population’sgene pool will change over genera-tions due to the natural selection ofvariations. There are three differenttypes of natural selection that act on variation: stabilizing, directional,and disruptive.

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Figure 15.14These swallowtail butterflies live in different areas of North America.Despite their slightvariations, they can interbreed to produce fertile offspring.

Stabilizing selection is naturalselection that favors average individ-uals in a population as shown inFigure 15.15. Consider a populationof spiders in which average size is asurvival advantage. Predators in thearea might easily see and capture spi-ders that are larger than average.However, small spiders may find itdifficult to find food. Therefore, in this environment, average-sizedspiders are more likely to survive—they have a selective advantage, orare “selected for.”

Directional selection occurswhen natural selection favors one ofthe extreme variations of a trait. Forexample, imagine a population ofwoodpeckers pecking holes in treesto feed on the insects living underthe bark. Suppose that a species ofinsect that lives deep in tree tissuesinvades the trees in a woodpeckerpopulation’s territory. Only wood-peckers with long beaks could feedon that insect. Therefore, the long-beaked woodpeckers in the popula-tion would have a selective advantageover woodpeckers with very short oraverage-sized beaks.

Finally, in disruptive selection,individuals with either extreme of atrait’s variation are selected for.Consider, for example, a populationof marine organisms called limpets.The shell color of limpets rangesfrom white, to tan, to dark brown. Asadults, limpets live attached to rocks.On light-colored rocks, white-shelledlimpets have an advantage becausetheir bird predators cannot easily seethem. On dark-colored rocks, dark-colored limpets have the advantagebecause they are camouflaged. Onthe other hand, birds easily see tan-colored limpets on either the light ordark backgrounds. Disruptive selec-tion tends to eliminate the interme-diate phenotypes.


Figure 15.15Different types of natural selection act over the range ofa trait’s variation. The red, bell-shaped curve indicates atrait’s variation in a population. The blue, bell-shapedcurve indicates the effect of a natural selection.

Selection foraverage sizespiders

Normal variation

Normalvariation

Selectionfor longerbeaks

Selection forlight limpets

Selection fordark limpets

Normalvariation

Stabilizing selection favors average individuals. This typeof selection reduces variation in a population.

AA

Directional selection favors one of the extreme variationsof a trait and can lead to the rapid evolution of a population.

BB

Disruptive selection favors both extreme variations of atrait, resulting eventually in no intermediate forms of the traitand leading to the evolution of two new species.

CC

Natural selection can significantlyalter the genetic equilibrium of apopulation’s gene pool over time.Significant changes in the gene poolcan lead to the evolution of a newspecies over time.

The Evolution of Species

You’ve just read about how naturalprocesses such as mutation, geneticdrift, gene flow, and natural selectioncan change a population’s gene poolover time. But how do the changes inthe makeup of a gene pool result in theevolution of new species? Recall that aspecies is defined as a group of organ-isms that look alike and can inter-breed to produce fertile offspring innature. The evolution of new species,a process called speciation (spee sheeAY shun), occurs when members ofsimilar populations no longer inter-breed to produce fertile offspringwithin their natural environment.

Physical barriers can preventinterbreeding

In nature, physical barriers canbreak large populations into smallerones. Lava from volcanic eruptionscan isolate populations. Sea-levelchanges along continental shelves cancreate islands. The water that sur-rounds an island isolates its popula-tions. Geographic isolation occurswhenever a physical barrier divides a population.

A new species can evolve when apopulation has been geographicallyisolated. For example, imagine a pop-ulation of tree frogs living in a rainforest, Figure 15.16. If small popula-tions of tree frogs were geographi-cally isolated, they would no longerbe able to interbreed and exchangegenes. Over time, each small popula-tion might adapt to its environmentthrough natural selection and developits own gene pool. Eventually, thegene pools of each population mightbecome so different that new species


Figure 15.16When geographic isolationdivides a population of treefrogs, the individuals no longermate across populations.

OriginWORDWORD

speciationFrom the Latinword species, meaning “kind.”Speciation is aprocess that pro-duces two speciesfrom one.

View an animationof Figure 15.16 in thePresentation Builder ofthe Interactive CD-ROM.

CD-ROM

of tree frogs would evolve in thedifferent forest patches or the popu-lations might become extinct.

Reproductive isolation can result in speciation

As populations become increas-ingly distinct, reproductive isolationcan arise. Reproductive isolationoccurs when formerly interbreedingorganisms can no longer mate andproduce fertile offspring.

There are different types of repro-ductive isolation. One type occurswhen the genetic material of the pop-ulations becomes so different thatfertilization cannot occur. Some geo-graphically separated populations ofsalamanders in California have thistype of reproductive isolation.Another type of reproductive isola-tion is behavioral. For example, ifone population of tree frogs mates inthe fall, and another mates in thesummer, these two populations willnot mate with each other and arereproductively isolated.

A change in chromosome numbers and speciation

Chromosomes can also play a rolein speciation. Many new species of

plants and some species of animalshave evolved in the same geographicarea as a result of polyploidy (PAHL

ih ployd ee), which is illustrated inFigure 15.17. Any species with amultiple of the normal set of chro-mosomes is known as a polyploid.

Mistakes during mitosis or meiosiscan result in polyploid individuals.For example, if chromosomes do notseparate properly during the firstmeiotic division, diploid (2n) gametescan be produced instead of the nor-mal haploid (n) gametes. Polyploidyresults in immediate reproductiveisolation. When a polyploid mateswith an individual of the normalspecies, the resulting zygotes may notdevelop normally because of the dif-ference in chromosome numbers. Inother cases, the zygotes develop intoadults that probably cannot repro-duce. However, polyploids within apopulation may interbreed and forma separate species.

Polyploids can arise from within aspecies or from hybridizationbetween species. Many floweringplant species, as well as many impor-tant crop plants, such as wheat, cot-ton, apples, and bananas, originatedby polyploidy.


OriginWORDWORD

polyploidyFrom the Greekword polys, meaning“many.” Polyploidplants contain multiple sets ofchromosomes.

Parent plant(2n)

Meiosis beginsNormalmeiosis

Abnormalgametes (2n)

Normalgametes (n)

Zygote(3n)

Zygote(4n)

Sterile plant

Newpolyploidspecies

Nondisjunction

Fertilization

Fertilization

Figure 15.17Many floweringplants, such as thisCalifornia tarweed,are polyploids—indi-viduals that resultfrom mistakes madeduring meiosis.

Speciation can occur quickly or slowly

Although polyploid speciationtakes only one generation, mostother mechanisms of speciation donot occur as quickly. What is theusual rate of speciation?

Scientists once argued that evolu-tion occurs at a slow, steady rate, withsmall, adaptive changes graduallyaccumulating over time in popula-tions. Gradualism is the idea thatspecies originate through a gradualchange of adaptations. Some evi-dence from the fossil record supportsgradualism. For example, fossil evi-dence shows that camels evolvedslowly and steadily over time.

In 1971, Stephen J. Gould andNiles Eldridge proposed anotherhypothesis known as punctuatedequilibrium. This hypothesis argues

that speciation occurs relativelyquickly, in rapid bursts, with longperiods of genetic equilibrium inbetween. According to this hypothe-sis, environmental changes, such ashigher temperatures or the introduc-tion of a competitive species, lead torapid changes in a population’s genepool. Speciation happens quickly—inabout 10 000 years or less. Like grad-ualism, punctuated equilibrium issupported by fossil evidence as shownin Figure 15.18.

Biologists generally agree thatboth gradualism and punctuatedequilibrium can result in speciation,depending on the circumstances. Itshouldn’t surprise you to see scien-tists offer alternative hypotheses toexplain observations. The nature ofscience is such that new evidence ornew ideas can modify theories.


Millions

of

Year

s A

go

55 million years agoAncestral species

6

5

4

3

2

Loxodontaafricana Elephas

maximus

Mammuthusprimigenius

Primelephas

Mammuthus

Elephas

Loxodonta

1

0

Figure 15.18The fossil record ofelephant evolutionsupports the view ofpunctuated equilib-rium. Three elephantspecies may haveevolved from anancestral populationin a short time.

Kauai

Niihau

Oahu

Lanai

Kahoolawe

Molokai

Maui

Hawaii

Patterns of EvolutionBiologists have observed different

patterns of evolution that occurthroughout the world in differentnatural environments. These patternssupport the idea that natural selectionis an important agent for evolution.

Diversity in new environmentsAn extraordinary diversity of

unique plants and animals live orhave lived on the Hawaiian Islands,among them a group of birds calledHawaiian honeycreepers. This groupof birds is interesting because,although similar in body size andshape, they differ sharply in colorand beak shape. Different species ofhoneycreepers evolved to occupytheir own niches.

Despite their differences, scientistshypothesize that honeycreepers, as

shown in Figure 15.19, evolved froma single ancestral species that lived onthe Hawaiian Islands long ago. Whenan ancestral species evolves into anarray of species to fit a number ofdiverse habitats, the result is calledadaptive radiation.

Adaptive radiation in both plantsand animals has occurred and contin-ues to occur throughout the worldand is common on islands. For exam-ple, the many species of finches thatDarwin observed on the GalapagosIslands are a typical example of adap-tive radiation.

Adaptive radiation is a type ofdivergent evolution, the pattern ofevolution in which species that oncewere similar to an ancestral speciesdiverge, or become increasingly dis-tinct. Divergent evolution occurswhen populations adapting to differ-ent environmental conditions change,


Figure 15.19Evolutionary biolo-gists have suggestedthat the ancestors ofall Hawaiian Islandhoneycreepersmigrated from NorthAmerica about 5 mil-lion years ago. Asthis ancestral birdpopulation settled inthe diverse Hawaiianniches, adaptive radi-ation occurred.

AncestralLasan finch

Amakihi

Extinct mamo

Crestedhoneycreeper

Apapane

Grosbeakfinch

Ou

Mauiparrotbill

Liwi

PalilaAkikiki

Akiapolaau

Akepa

Akialoa

becoming less alike as they adapt,eventually resulting in new species.

Different species can look alikeA pattern of evolution in which

distantly related organisms evolvesimilar traits is called convergentevolution. Convergent evolutionoccurs when unrelated speciesoccupy similar environments in dif-ferent parts of the world. Becausethey share similar environmentalpressures, they share similar pres-sures of natural selection.

For example, in Figure 15.20, yousee a cactus of the family Cactaceae,an organ pipe cactus, that grows inthe deserts of North and SouthAmerica and a plant of the familyEuphorbiaceae that looks like thecactus and lives in African deserts.Although these plants are unrelatedspecies, their environments are simi-lar. You can see that they both havefleshy bodies and no leaves. Thatconvergent evolution has apparentlyoccurred in unrelated species, is fur-ther evidence for natural selection.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. Explain why the evolution of resistance to

antibiotics in bacteria is an example of directional selection.

2. How can geographic isolation change a population’s gene pool?

3. Why is rapid evolutionary change more likely to occur in small populations?

4. How do gradualism and punctuated equilibriumdiffer? How are they similar?

Thinking Critically5. Hummingbird moths are night-flying

insects whose behavior and appearance are

similar to those of hummingbirds. Explain howthese two organisms demonstrate the concept ofconvergent evolution.

6. Designing an Experiment Biologists have dis-covered two species of squirrels living on oppo-site sides of the Grand Canyon. They hypothesizethat they both evolved from a recent, commonancestor that lived in the area before the GrandCanyon formed. What observations or experi-ments could provide evidence for this hypoth-esis? For more help, refer to Practicing ScientificMethods in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Figure 15.20Unrelated species of plantssuch as the organ pipe cactus(a) and this Euphorbia (b)share a similar fleshy bodytype and no leaves.

a

b


Natural Selection and Allelic Frequency

E volution can be described as the change in allelic frequencies of agene pool over time. Natural selection can place pressure on specific

phenotypes and cause a change in the frequency of the alleles that producethe phenotypes. In this activity, you will simulate the effects of eagle pre-dation on a population of rabbits, where GG represents the homozygouscondition for gray fur, Gg is the heterozygous condition for gray fur, andgg represents the homozygous condition for white fur.

INTERNETINTERNET

ProblemHow does natural selection affect

allelic frequency?

ObjectivesIn this BioLab, you will:■ Simulate natural selection by using

beans of two different colors.■ Calculate allelic frequencies over

five generations.■ Demonstrate how natural selection

can affect allelic frequencies overtime.

■ Use the Internet to collect and com-pare data from other students.

Materialscolored pencils (2)paper baggraph paperpinto beanswhite navy

beans

Skill Handbook

Use the Skill Handbook if you need additional help with this lab.

PREPARATIONPREPARATION

1. Copy the data table shown on the next page.

2. Place 50 pinto beans and 50 whitenavy beans into the paper bag.

3. Shake the bag. Remove two beans.These represent one rabbit’s genotype. Set the pair aside, andcontinue to remove 49 more pairs.

4. Arrange the beans on a flat surface in two columns repre-senting the two possible rabbit

phenotypes, gray (genotypes GGor Gg) and white (genotype gg).

5. Examine your columns. Remove25 percent of the gray rabbits and100 percent of the white rabbits.These numbers represent a ran-dom selection pressure on yourrabbit population. If the numberyou calculate is a fraction, removea whole rabbit to make wholenumbers.

PROCEDUREPROCEDURE

6. Count the number of pinto and navy beans remaining. Record this number in your data table.

7. Calculate the allelic frequencies by dividing the number of beans of one type by 100. Record these numbers in your data table.

8. Begin the next generation by plac-ing 100 beans into the bag. Theproportions of pinto and navy beansshould be the same as the percent-ages you calculated in step 7.

9. Repeat steps 3 through 8, collect-ing data for five generations.

10. Go to the Glencoe Science Web Site at the address shown below to post your data.

11. Graph the frequencies of each


1. Analyzing Data Did either alleledisappear? Why or why not?

2. Thinking Critically What does your graph show about allelic frequencies and natural selection?

3. Making Inferences What wouldhappen to the allelic frequencies if the number of eagles declined?

4. Using the Internet Explain any dif-ferences in allelic frequencies youobserved between your data and thedata from the Internet. What advan-tage is there to having a large

amount of data? What problemsmight there be in using data fromthe internet?

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

Generation Number

Start

1

2

3

4

5

50 50 0.50 50 50 0.50

Percentage Frequency Number Percentage

Allele gAllele G

Frequency

Data Table

Sharing Your DataSharing Your Data

Find this BioLab on the Glencoe Science Web site

at science.glencoe.com. Post your data inthe data table provided for this BioLab. Usethe additional data from other students onthe Internet, and graph and analyze thecombined data.

allele over five generations. Plotthe frequency of the allele on thevertical axis and the number ofthe generation on the horizontalaxis. Use a different colored pen-cil for each allele.

BIOLOGY


In the early 1900s, G. H. Hardy, a British mathematician, and W. Weinberg, a German

doctor, independently discovered how the frequency of a trait’s alleles in a population could

be described mathematically.

Suppose that in a population of pea plants, 36 plants are homozygous dominant for the

tall trait (TT ), 48 plants are heterozygous tall(Tt), and 16 plants are short plants (tt). In thehomozygous tall plants, there are (36) (2), or 72,T alleles and in the heterozygous plants there are 48 T alleles, for a total of 120 T alleles in the population. There are 48 t alleles in the heterozygous plants plus (16) (2), or 32, t allelesin the short plants, for a total of 80 t alleles in thepopulation. The number of T and t alleles in thepopulation is 200. The frequency of T alleles is120/200 or 0.6, and the frequency of t alleles in80/200, or 0.4.

The Hardy-Weinberg principle The Hardy-Weinberg principle states that the frequency ofthe alleles for a trait in a stable population willnot vary. This statement is expressed as the equa-tion p + q = 1, where p is the frequency of oneallele for the trait and q is the frequency of theother allele. The sum of the frequencies of thealleles always includes 100 percent of the alleles,and is therefore stated as 1.

Squaring both sides of the equation producesthe equation p2 + 2pq + q2 = 1. You can use thisequation to determine the frequency of genotypesin a population: homozygous dominant individuals(p2), heterozygous individuals (2pq), and recessiveindividuals (q2). For example, in the pea plantpopulation described above, the frequency of thegenotypes would be

(0.6) (0.6) + 2(0.6) (0.4) + (0.4) (0.4) = 1

The frequency of the homozygous tall genotypeis 0.36, the heterozygous genotype is 0.48, andthe short genotype is 0.16.

In any sexually reproducing, large popula-tion, genotype frequencies will remain constantif no mutations occur, random mating occurs, nonatural selection occurs, and no genes enter orleave the population.

Implications of the principle The Hardy-Weinberg principle is useful for several reasons.First, it explains that the genotypes in popula-tions tend to remain the same. Second, because arecessive allele may be masked by its dominantallele, the equation is useful for determining therecessive allele’s frequency in the population.Finally, the Hardy-Weinberg principle is usefulin studying natural populations to determinehow much natural selection may be occurring inthe population.


ConnectionMathMath

Connection Mathematics and Evolution

The general population of the United States isgetting taller. Assuming that height is a genetictrait, does this observation violate the Hardy-Weinberg principle? Explain your answer.

To find out more about the Hardy-Weinberg principle,

visit the Glencoe Science Web site.science.glencoe.com

CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY

A populationof penquins

BIOLOGY


Chapter 15 AssessmentChapter 15 Assessment

SUMMARYSUMMARY

Section 15.1

Section 15.2

Main Ideas■ After many years of experimentation and

observation, Charles Darwin proposed theidea that species originated through theprocess of natural selection.

■ Natural selection is a mechanism of changein populations. In a specific environment,individuals with certain variations arelikely to survive, reproduce, and pass thesevariations to future generations.

■ Evolution has been observed in the lab andfield, but much of the evidence for evolu-tion has come from studies of fossils,anatomy, and biochemistry.

Vocabularyanalogous structure (p. 409)artificial selection (p. 403)camouflage (p. 406)embryo (p. 410)homologous structure (p. 408)mimicry (p. 405)natural selection (p. 403)vestigial structure (p. 410)

NaturalSelection andthe Evidencefor Evolution

Main Ideas■ Evolution can occur only when a popula-

tion’s genetic equilibrium changes.Mutation, genetic drift, and gene flow canchange a population’s genetic equilibrium,especially in a small, isolated population.Natural selection is usually a factor thatcauses change in established gene pools—both large and small.

■ The separation of populations by physicalbarriers can lead to speciation.

■ There are many patterns of evolution innature. These patterns support the ideathat natural selection is an importantmechanism of evolution.

Vocabularyadaptive radiation (p. 420)allelic frequency (p. 413)convergent evolution (p. 421)directional selection (p. 416)disruptive selection (p. 416)divergent evolution (p. 420)gene pool (p. 413)genetic drift (p. 414)genetic equilibrium (p. 413)geographic isolation (p. 417)gradualism (p. 419)polyploid (p. 418)punctuated equilibrium (p. 419)reproductive isolation (p. 418)speciation (p. 417)stabilizing selection (p. 416)

Mechanismsof Evolution

CHAPTER 15 ASSESSMENT 425

1. Two closely related species of squirrels liveon opposite sides of the Grand Canyon. Theancestral species probably evolved into twospecies because of ________.a. structural isolationb. punctuated isolationc. behavioral isolationd. geographic isolation

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 2. What type of evolutionary evidence do fossilsprovide?a. structural c. physiologicalb. functional d. critical

3. Which of the following is an example ofdirect evidence for evolution?a. fossilsb. embryologyc. vestigial structuresd. bacterial resistance to penicillin


9. An example of a vestigial human structure isthe ________.a. eye c. appendixb. big toe d. ribs

10. The fish and whale shown here are notclosely related. Their structural similaritiesappear to be the result of ________.

a. adaptive radiationb. convergent evolutionc. divergent evolutiond. punctuated equilibrium

11. The scientific hypothesis that explains how an ancestral population of elephants speciated quite rapidly after a long period of stability is ________.

12. Speciation due to physical barriers occurs as a result of ________.

13. An understanding of population genetics depends on an understanding of the________, which is a collection of all the alleles in a population.

14. The mechanism Darwin proposed to explainhow species adapt to their environment overmany generations is ________.

15. The differences in the size of the peanuts in a bag are called ________.

16. ________ is the structural adaptation of an organism that enables it to resembleanother harmful or distasteful species.

17. The existence of desirable characteristics inboth crops and domestic animals results fromthe process called________ selection.

18. A subtle adaptation that allows an organismto blend in with its surroundings is known as ________.

19. The wings of bats and the forelimbs of crocodiles are examples of ________structures.

20. A species may find its way to an island and then evolve into many species in a process called ________.

426 CHAPTER 15 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Wear a WatchIf you are taking a timed test, you should makesure that you pace yourself and do not spend toomuch time on any one question, but don’t spendtime staring at the clock. When the test begins,place your watch on the desk and check it aftereach section of the test.

4. Which of the structures shown below is nothomologous with the others?a. b. c. d.

5. Which type of natural selection favors theaverage individuals in a population?a. directional c. stabilizingb. disruptive d. divergent

6. Which of the following pairs of terms is notrelated?a. analogous structures—butterfly wingsb. evolution—natural selectionc. vestigial structure—appendixd. adaptive radiation—convergent evolution

7. Unlike any other birds, hummingbirds havewings that allow them to hover and to flybackwards. This is an example of a ________adaptation.a. physiological c. reproductiveb. structural d. embryological

8. Which of the following is a true statementabout evolution?a. Individuals evolve more slowly than

populations.b. Individuals evolve; populations don’t.c. Individuals evolve by changing the gene

pool.d. Populations evolve; individuals don’t.


CHAPTER 15 ASSESSMENT 427

21. The structural characteristics of manyspecies, such as sharks, have changed littleover time. What evolutionary factors mightbe affecting their stability?

22. How might the bright colors of poisonousspecies aid in their survival?

23. Why is DNA a useful tool for determiningpossible relationships among the species oforganisms?

24. Observing and Inferring Describe adaptiveradiation as a form of divergent evolution.

25. Interpreting Data In a population of clams,let two alleles, T and t, represent shell color.The population consists of ten TT clams and ten tt clams. What are the allelic frequenciesof the T and t in the population?

26. Concept Mapping Complete the conceptmap by using the following vocabulary terms:allelic frequency, geographic isolation, gradualism, natural selection, punctuatedequilibrium, reproductive isolation, speciation.

THINKING CRITICALLYTHINKING CRITICALLY

APPLYING MAIN IDEASAPPLYING MAIN IDEAS ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

The following graph shows leaf length in apopulation of maple trees.

Interpreting Data Study the graph andanswer the following questions.1. What was the range of leaf lengths?

a. 14 cm c. 20-100 cmb. 8-22 cm d. 10-14 cm

2. What was the average leaf length?a. 8 cm c. 14 cmb. 12 cm d. 6 cm

3. What type of evolutionary pattern doesthe graph most closely match?a. artificial selectionb. stabilizing selectionc. disruptive evolutiond. directional evolution

4. Interpreting Data Use the graph belowto explain what might be occurring inthis shark population.

Length of leaves in cm

Num

ber o

f lea

ves

8 10 12 14 16 18 20

20

40

60

80

100

22

Leaf Lengths in a Maple Tree Population

Shar

ks

Tooth size

Variation in Tooth Size of Sharks

to produce

4.

1. 2. 3.

occurs slowly in

occurs rapidly in

5.

6. 7.

cause changes in

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web site.science.glencoe.com

CD-ROM


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