chapter 15: the theory of...

392

Visit to• study the entire chapter

online• access Web Links for more

information and activities onevolution

• review content with theInteractive Tutor and self-check quizzes

The Theory of EvolutionThe Theory of Evolution

This crayfish lives in dark cavesand is blind. It has sighted rela-tives that live where there islight. Both the cave-dwellingspecies and their relatives areadapted to different environ-ments. As populations adapt tonew or changing environments,individuals in the populationthat are best adapted survivelong enough to reproduce.

Understandingthe Photo

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 the key concept forunderstanding biology. Evo-lution explains the diversity ofspecies and predicts changes.

ca.bdol.glencoe.com

D. Lyon/Bruce Coleman, Inc.

0392-0392 C15 CO BDOL-829900 8/4/04 2:36 AM Page 392

http://ca.bdol.glencoe.com

15.1

Charles Darwin and Natural SelectionThe modern theory of evolution is the fundamental concept in biology.

Recall that evolution is the change in populations over time. Learning theprinciples of evolution makes it easier to understand modern biology.One place to start is by learning about the ideas of English scientistCharles Darwin (1809–1882)—ideas supported by fossil evidence.

Fossils shape ideas about evolutionBiologists have used fossils in their work since the eighteenth century.

In fact, fossil evidence formed the basis of early evolutionary concepts.Scientists wondered how fossils formed, why many fossil species wereextinct, and what kinds of relationships might exist between the extinctand the modern species.

Before geologists provided evidence indicating that Earth was mucholder than many people had originally thought, biologists suspected thatspecies change over time, or evolve. Many explanations about how speciesevolve have been proposed, but the ideas first published by CharlesDarwin are the basis of modern evolutionary theory.

SECTION PREVIEWObjectivesSummarize Darwin’s the-ory of natural selection.Explain how the struc-tural and physiologicaladaptations of organismsrelate to natural selection.Distinguish among thetypes of evidence for evolution.

Review Vocabularyevolution: the changes in

populations over time(p. 10)

New Vocabularyartificial selectionnatural selectionmimicrycamouflagehomologous structureanalogous structurevestigial structureembryo

15.1 NATURAL SELECTION AND THE EVIDENCE FOR EVOLUTION 393

Direct Evidence

Indirect Evidence

Analyze and Critique As you read Chapter 15, summarize, analyze, andcritique the direct and indirect evidence used to support the theory of evolution.

Evolution Make the following Foldable to help you analyze and critique evidence supporting the theory of evolution.

Draw a mark at the midpoint of a vertical sheet of paper along the side edge.

Turn the paper horizon-tally and fold the outside edges in to touch at the midpoint mark.

STEP 1

STEP 3

STEP 2

Label the tabs as shown.

Natural Selection and theEvidence for Evolution

Standard 8a Students know how natural selection determines the differentialsurvival of groups of organisms.

California Standards

0393-0403 C15S1 BDOL-829900 8/4/04 3:02 AM Page 393

Darwin on HMS BeagleIt took Darwin years to develop

his theory of evolution. He began in1831 at age 22 when he took a job asa naturalist on the English shipHMS Beagle, which sailed aroundthe world on a five-year scientificjourney.

As the ship’s naturalist, Darwinstudied and collected biological andfossil specimens at every port alongthe route. As you might imagine,these specimens were quite diverse.Studying the specimens made Darwincurious about possible relationshipsamong species. His studies providedthe foundation for his theory of evo-lution by natural selection.

Darwin in the GalápagosThe Galápagos (guh LAH puh gus)

Islands are a group of small islandsnear the equator, about 1000 km offthe west coast of South America. Theobservations that Darwin made andthe specimens that he collected therewere especially important to him.

On the Galápagos 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.

394 THE THEORY OF EVOLUTION

AUSTRALIA

NORTHAMERICA

ASIA

AFRICA

EUROPE

SOUTHAMERICA

ATLANTICOCEAN

PACIFICOCEAN

PACIFICOCEAN

INDIAN OCEAN

ARCTIC OCEAN

Lima

Tahiti

GalápagosIslands

Valparaiso

Rio de Janeiro

Montevideo

Falkland IslandsCape Horn

Sydney

KingGeorgeSound

CocosIslands

Mauritius

St. HelenaBahia

AzoresCanaryIslands

Cape VerdeIslands

Hobart NewZealand

Figure 15.1The five-year voyage ofHMS Beagle tookDarwin around theworld. Animal speciesin the GalápagosIslands have uniqueadaptations.

Galápagos marine iguanas eat algaefrom the ocean, an unusual food sourcefor reptiles. Large claws help them clingto slippery rocks.

C

The beak of this Galápagos finchis adapted to feed on cacti.

A

Galápagos tortoises are thelargest on Earth, differing from other tortoises in body size and shape.

B

(tl)Frans Lanting/Photo Researchers (tr)Tom Brakefield/DRK Photo (b)Barbara Cushman Rowell/DRK Photo

0393-0403 C15S1 BDOL-829900 8/4/04 3:03 AM 94

Darwin continues his studiesFor the next two decades, Darwin

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

English economist Thomas Malthushad proposed an idea that Darwinmodified and used in his explanation.Malthus’s idea was that the human pop-ulation grows faster than Earth’s foodsupply. How did this help Darwin? Heknew that many species produce largenumbers of offspring. He also knewthat such species had not overrunEarth. He realized that individualsstruggle to compete in changing envi-ronmental conditions. There are manykinds of competition, such as compet-ing for food and space, escaping frompredators, finding mates, and locatingshelter. Only some individuals survivethe competition and produce offspring.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. Vari-ations are then 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 hypothe-sized 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 mechanism forchange in populations. It occurs whenorganisms with favorable variations


Figure 15.2Darwin proposed the idea of natural selection to explainhow species change over time.

In nature, organ-isms produce moreoffspring than cansurvive. Fishes, forexample, cansometimes laymillions of eggs.

A

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

B

Individuals withcertain usefulvariations, such asspeed, survive intheir environment,passing thosevariations to thenext generation.

C

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

D

0393-0403 C15S1 BDOL-829900 8/4/04 6:01 AM Page 395

survive, reproduce, and pass their vari-ations to the next generation. Orga-nisms without these variations are lesslikely to survive and reproduce. As aresult, each generation consists largelyof offspring from parents with thesevariations that aid survival.

Darwin was not the only one torecognize the significance of naturalselection for populations. As a resultof his studies on islands near Indo-nesia in the Pacific Ocean, AlfredRussel Wallace, another British natu-ralist, reached a similar conclusion.After Wallace wrote Darwin to sharehis ideas about natural selection,Darwin and Wallace had their sim-ilar ideas jointly presented to the scientific community. Soon thereafter,

Darwin published the first book aboutevolution called On the Origin ofSpecies by Means of Natural Selection in1859. The ideas detailed in Darwin’sbook are a basic unifying theme ofbiology today.

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 ofthe issues is that evolutionaryprocesses are difficult for humans toobserve directly. The short scale ofhuman life spans makes it difficult tocomprehend evolutionary processes


Some ancestral rats may have avoided predators betterthan others because of variations such as the size of teethand claws.

BThe ancestors of today’s common mole-ratsprobably resembled African rock rats.

A

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

Ancestral rats that survived passed theirvariations to offspring. After many gen-erations, most of the population’s indivi-duals would have these adaptations.

C

Over time, natural selection produced modern mole-rats.Their blindness may have evolved because vision had nosurvival advantage for them.

D

0393-0403 C15S1 BDOL-829900 8/4/04 6:01 AM Page 396

that occur over millions of years.Despite this, much data about thebiological world has been gatheredfrom many sources. These data arebest explained by evolution. Almostall of today’s biologists accept thetheory of evolution by natural selec-tion. The advent of genetics hasadded yet more data to our under-standing of evolution. This meansthat the change in the gene pool of apopulation over time can be added toour modern definition of evolution.Use Problem-Solving Lab 15.1 to ana-lyze data from a study of pepperedmoths.

Summarize themain ideas of natural selection.

Adaptations: Evidencefor Evolution

Have you noticed that some plantshave thorns and some plants don’t?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 aids anorganism’s chances of survival in itsenvironment. Thorns are an adapta-tion 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 help youunderstand how natural selection hasaffected them. Mole-rats that liveunderground in darkness are blind.These blind mole-rats have manyadaptations that enable them to live

successfully underground. Look atFigure 15.3 to see how these modernmole-rat adaptations might haveevolved over millions of years fromcharacteristics of their ancestors.


Interpret DataHow can natural selection beobserved? In some organisms thathave a short life cycle, biologistshave observed the evolution ofadaptations to rapid environmentalchanges. Scientists studied camou-flage adaptations in a population oflight- and dark-colored peppered moths,Biston betularia. The moths sometimesrested on trees that grew in both thecountry and the city. Moths are usually speckled gray-brown,and dark moths, which occur occasionally, are black. Somebirds eat peppered moths. Urban industrial pollution hadblackened the bark of city trees with soot. In the photo, yousee a city tree with dark bark similar to the color of one of themoths.

Solve the ProblemScientists raised more than 3000 caterpillars to provide adultmoths. They marked the wings of the moths these caterpillarsproduced so they would recapture only their moths. In a seriesof trials in the country and the city, they released and recap-tured the moths. The number of moths recaptured in a trialindicates how well the moths survived in the environment.Examine the table below.

Thinking CriticallyInterpret Data Calculate the percentage of moths recapturedin each experiment, and explain any differences in survival ratesin the country and the city moths in terms of natural selection.

Biston betularia

Comparison of Country and City Moths

Numbers of Numbers ofLocationLight Moths Dark Moths

Country Released 496 488

Recaptured 62 34

City Released 137 493

Recaptured 18 136

M.W. Tweedie/Photo Researchers

0393-0403 C15S1 BDOL-829900 8/4/04 6:02 AM Page 397

AA

BB

Figure 15.4Mimicry and camouflage are protective adaptations oforganisms. The colors and body shape of a yellow jacketwasp (A) and a harmless syrphid fly (B) are similar. Preda-tors avoid both insects. Camouflage enables organisms,such as this leaf frog (C), to blend with their surroundings.

The structural adaptations of com-mon mole-rats include large teeth andclaws. These are body parts that helpmole-rats survive in their environ-ment by, for example, enabling themto dig better tunnels. Structural adap-tations such as the teeth and claws ofmole-rats are often used to defendagainst predators. Some adaptationsof other organisms that keep preda-tors from approaching include a rose’sthorns or a porcupine’s quills.

Some other structural adaptationsare subtle. Mimicry is a structuraladaptation that enables one species toresemble another species. In one formof mimicry, a harmless species hasadaptations that result in a physicalresemblance to a harmful species.Predators that avoid the harmfulspecies also avoid the similar-looking,harmless species. See if you can tellthe difference between a harmless flyand the wasp it mimics when you lookat Figures 15.4A and B.

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

CC

398

Formulate ModelsCamouflage Provides an Adaptive Advantage Camou-flage is a structural adaptation that allows organisms to blendwith their surroundings. In this activity, you’ll discover hownatural selection can result in camouflage adaptations inorganisms.

Procedure! Working with a partner, punch 100 dots from a sheet of

white paper with a paper hole punch. Repeat with a sheetof black paper. These dots will represent black and whiteinsects.

@ Scatter both white and black dots on a sheet of blackpaper.

# 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. Observe What color dots were most often collected?2. Infer How does color affect the survival rate of insects?3. Hypothesize What might happen over many generations

to a similar population in nature?

(l)R. Calentine/Visuals Unlimited (c)John Colwell/Grant Heilman Photography (r)S.L. & J.T. Collins/Photo Researchers

0393-0403 C15S1 BDOL-829900 8/4/04 6:02 AM Page 398

Non-resistantbacterium

Resistantbacterium

Antibiotic


species of wasps all have harmfulstings and similar coloration andbehavior. Predators may learn quicklyto avoid any organism with their gen-eral appearance.

Another subtle adaptation is camouflage (KA muh flahj), anadaptation that enables species toblend with their surroundings, asshown in Figure 15.4C. Becausewell-camouflaged organisms are noteasily found by predators, they sur-vive to reproduce. Try MiniLab 15.1to experience how camouflage canhelp an organism survive and adaptto it’s environment.

Explain and illustrate how mimicry and camou-flage can cause populations tochange over time.

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 of themedicines developed during the twen-tieth century to fight bacterial diseasesare no longer effective? When theantibiotic drug penicillin was discov-ered about 50 years ago, it was called awonder drug because it killed many

types of disease-causing bacteria andsaved many lives. Today, penicillin nolonger affects as many species of bacte-ria because some species have evolvedphysiological (fih zee uh LAH jih kul)adaptations to prevent being killed bypenicillin. Look at Figure 15.5 to seehow resistance develops in bacteria.

Physiological adaptations are chan-ges in an organism’s metabolic pro-cesses. In addition to species of bacteria, scientists have observed theseadaptations in species of insects andweeds that are pests. After years ofexposure to specific pesticides, manyspecies of insects and weeds havebecome resistant to these chemicalsthat used to kill them.

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 they

Figure 15.5The development ofbacterial resistance toantibiotics is direct evi-dence for evolution.Infer What problemscan antibiotic-resistant bacteriacause?

The bacteria in a popula-tion vary in their ability toresist antibiotics.

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

B The resistant bacterialive and produce moreresistant bacteria.

C

0393-0403 C15S1 BDOL-829900 8/4/04 6:03 AM Page 399

provide a record of early life and evo-lutionary history. For example, pale-ontologists conclude from fossils thatthe ancestors of whales were probablyland-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 thesame with fossils. Although paleon-tologists do not have fossils for all thechanges that have occurred, they canstill understand the overall picture ofhow most groups evolved.

Fossils are found throughout theworld. As the fossil record becomesmore complete, the sequences of evo-lution become clearer. For example,in Table 15.1 you can see how pale-ontologists have charted the evolu-tionary path that led to today’s camel

after piecing together fossil skulls,teeth, and limb bones.

AnatomyLook at the forelimb bones of the

animals shown in Figure 15.6.Although the bones of each forelimbare modified for their function, thebasic arrangement of the bones ineach limb is similar. Evolutionarybiologists view such structural simi-larities as evidence that organismsevolved from a common ancestor. Itwould be unlikely for so many ani-mals to have similar structures if eachspecies arose separately. Structuralfeatures with a common evolution-ary origin are called homologous structures. Homologous structurescan be similar in arrangement, infunction, or in both.

The structural or functional simi-larity of a body feature doesn’t alwaysmean that two species are closelyrelated. In Figure 15.7, you can com-pare the wing of a butterfly with the


Table 15.1 Camel Evolution

Age Paleocene Eocene Oligocene Miocene Present65 million 54 million 33 million 23 millionyears ago years ago years ago years ago

Organism

Skull andteeth

Limb bones

Table 15.1Fossils are used by scientists to understand how camels evolved.

0393-0403 C15S1 BDOL-829900 8/4/04 6:04 AM Page 400

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 evidence ofevolution. For example, insect andbird wings probably evolved sepa-rately when their different ancestorsadapted independently to similar waysof life.

Another type of body feature thatsuggests an evolutionary relationship is a vestigial (veh STIH jee ul)structure—a body structure in a present-day organism that no longerserves its original purpose, but wasprobably useful to an ancestor. A struc-ture becomes vestigial when the speciesno longer needs the feature for its orig-inal function, yet it is still inherited aspart of the body plan for the species.


Whale forelimb

Figure 15.6The forelimbs ofcrocodiles, whales,and birds are homo-logous structures.The bones of eachare modified fortheir function.

Crocodile forelimb

Bird wing

Figure 15.7Insect and bird wings are similar infunction but not in structure. Bonesare the framework of bird wings,whereas a tough material called chitincomposes insect wings.

(t)Dave Watts/Tom Stack & Associates (b)Stephen Dalton/Photo Researchers

0393-0403 C15S1 BDOL-829900 8/4/04 6:05 AM Page 401


Figure 15.9In other stages of embryonicdevelopment, these organ-isms look different. However,at some point they look similar. Hypothesize thestrengths and weaknessesof embryology as evidence for evolution.

vestigial from the Latin word vestigium, mean-ing “sign”; Theforelimbs ofostriches are ves-tigial structures.

Figure 15.8Vestigial structures, such aspelvic bones in the baleenwhale, are evidence of evolu-tion because they show struc-tural change over time.

Many organisms,including the whale inFigure 15.8, have vestigialstructures. The eyes of blindmole-rats and cave fish are vestigialstructures because they are no longerused for sight. Two flightless birds—an extinct elephant bird and anAfrican ostrich—have extremelyreduced forelimbs. Their ancestorsprobably foraged on land for foodand nested on the ground. As a result,over time, the ancestral birds becamequite large and unable to fly, featuresevident 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 their

embryos? 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 15.9. At thisstage of development, all the embryoshave a tail and pharyngeal pouches. Infish, these pouches develop into thesupports for the gills, while in mam-mals, reptiles, and birds, they developinto parts of ears, jaws, and throat. It isthe shared features in the youngembryos that suggest evolution from adistant, common ancestor.

BiochemistryBiochemistry also provides strong

evidence for evolution. Nearly all

Fish Reptile Bird MammalDr. R. Kessel and Dr. G. Shih N. Bromhall Dr. J.D. Cunningham Dr. F. Hossler

0393-0403 C15S1 BDOL-829900 8/4/04 6:06 AM Page 402

Understanding Main Ideas1. Briefly review, analyze, and critique 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 are resistant tothe drugs used to treat the disease. Explain howthis could be an example of natural selectionoccurring.

6. Get the Big Picture Fossils indicate that whalesevolved from ancestors that had legs. Using yourknowledge of natural selection, sequence thesteps that may have occurred during the evolutionof whales from their terrestrial, doglike ancestors.For more help, refer to Get the Big Picture in theSkill Handbook.

SKILL REVIEWSKILL REVIEW

403

organisms share DNA, ATP, andmany enzymes among their bio-chemical molecules. One enzyme,cytochrome c, occurs in organisms asdiverse as bacteria and bison. Bio-logists compared the differences thatexist among species in the amino acidsequence of cytochrome c. Data fromthese biochemical studies are shownin Table 15.2. The data show thenumber of amino acid substitutions in the amino acid sequences for the different organisms. Organisms thatare biochemically similar have fewerdifferences in their amino acidsequences. Groups that share moresimilarities are interpreted as being

more closely related or as sharing acloser ancestor. The evolutionaryrelationships of some of the groups inthe table are shown in Figure 15.10.

Since Darwin’s time, scientists haveconstructed evolutionary diagramsthat show levels of relationshipsamong species. In the 1970s, somebiologists began to use RNA andDNA nucleotide sequences to con-struct evolutionary diagrams. Today,scientists combine data from fossils,comparative anatomy, embryology,and biochemistry in order to interpretthe evolutionary relationships amongspecies.

Table 15.2 Biochemical Similarities of Organisms

Percent Substitutions Comparison of Organisms of Amino Acids in

Cytochrome c Residues

Two orders of mammals 5 and 10

Birds vs. mammals 8–12

Amphibians vs. birds 14–18

Fish vs. land vertebrates 18–22

Insects vs. vertebrates 27–34

Algae vs. animals 57

AlgaeFungi

InsectFish

FrogMammals

Birds

ca.bdol.glencoe.com/self_check_quiz

Figure 15.10This drawing shows theevolutionary relationshipof some of the groups inthe table.

0393-0403 C15S1 BDOL-829900 8/4/04 6:07 AM Page 403

http://ca.bdol.glencoe.com/self_check_quiz

15.2SECTION PREVIEWObjectivesSummarize the effects of the different types of natural selection ongene pools.Relate changes in geneticequilibrium to mechanismsof speciation.Explain the role of natu-ral selection in convergentand divergent evolution.

Review Vocabularygene: DNA segment that

controls protein produc-tion and the cell cycle(p. 211)

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


Population Genetics and EvolutionWhen Charles Darwin developed his theory of natural selection in the

1800s, he did so without knowing about genes. Since Darwin’s time, sci-entists have learned a great deal about genes and modified Darwin’s ideasaccordingly. At first, genetic information was used to explain the variationamong individuals of a population. Then, studies of the complex behav-ior of genes in populations of plants and animals developed into the fieldof study called population genetics. The principles of today’s modern the-ory of evolution are rooted in population genetics and other related fieldsof study and are expressed in genetic terms.

Populations, not individuals, evolveCan individuals evolve? That is, can an organism respond to natural

selection by acquiring or losing characteristics? Recall that genes deter-mine most of an individual’s features, such as tooth shape or flower color.If an organism has a feature—called a phenotype in genetic terms—thatis poorly adapted to its environment, the organism may be unable to sur-vive and reproduce. However, within its lifetime, it cannot evolve a newphenotype by natural selection in response to its environment.

Mechanisms of Evolution

Interspecies CompetitionUsing Prior Knowledge You may recognize the birds shownhere as meadowlarks. These birds range throughout much of the United States. Meadowlarkslook so similar that it’s often difficult to tell them apart. Al-though they are closely related and occupy the same ranges in parts of the central United States,these different meadowlarks do not normally interbreed and areclassified as distinct species.Infer Using your knowledge of birds and animal behavior, infer what prevents competition between two meadowlark species that occupy the same area.

Western meadowlark—Sturnella neglecta

Eastern meadowlark—Sturnella magna

(l)Rod Planck/Tom Stack & Associates (r)Ron Austing/Photo Researchers

Standard 7d Students know variation within a species increases the likelihoodthat at least some members of a species will survive under changed environmental conditions.

California Standards

0404-0419 C15S2 BDOL-829900 8/4/04 6:03 AM Page 404

Rather, natural selection acts on therange of phenotypes in a population.Recall that a population consists of allthe members of a species that live in anarea. Each member has the genes thatcharacterize the traits of the species,and these genes exist as pairs of alleles.Just as all of the individuals make upthe population, all of the genes of thepopulation’s individuals make up thepopulation’s genes. Evolution occurs asa population’s genes and their frequen-cies change 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 any spe-cific allele in the gene pool is called theallelic frequency. Scientists calculatethe allelic frequency of an allele in thesame way that a baseball player calcu-lates a batting average. They refer to apopulation in which the frequency ofalleles remains the same over genera-tions as being in genetic equilibrium.In the Connection to Math at the end ofthe chapter, you can read about the

mathematical description of geneticequilibrium. You can study the effect ofnatural selection on allelic frequenciesin the BioLab at the end of the chapter.

Look at the population of snap-dragons shown in Figure 15.11. 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 when thefrequency 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.

15.2 MECHANISMS OF EVOLUTION 405

Figure 15.11Incomplete dominanceproduces three pheno-types: red flowers (RR),white flowers (R'R'),and pink flowers (RR').Although the pheno-type frequencies of thegenerations vary, theallelic frequencies forthe 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

0404-0419 C15S2 BDOL-829900 8/4/04 6:04 AM Page 405

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 bychance. Of the mutations that affectorganisms, many are lethal, and theorganisms do not survive. Thus, le-thal mutations are quickly eliminated.However, occasionally, a mutationresults in a useful variation, and thenew gene becomes part of the popula-tion’s gene pool by the process of nat-ural 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 organisms.This is because the genes of the orig-inal ancestors represent only a smallfraction of the gene pool of the entirespecies and are the only genes avail-able to pass on to offspring. The dis-tinctive forms of life that Darwinfound in the Galápagos 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 be-lief systems. For example, in Lan-caster County, Pennsylvania, there is an Amish population of about 12 000 people who have a uniquelifestyle and marry other members oftheir community. By chance, at leastone of the original 30 Amish settlersin this community carried a recessiveallele that results in short arms andlegs and extra fingers and toes in off-spring, Figure 15.12. 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 individualsin and out of a population. The trans-port of genes by migrating individuals


Papilio ajax ajax Papilio ajax ampliata

Figure 15.12Genetic drift can resultin an increase of rarealleles in a small popu-lation. Notice the childhas six fingers on eachhand.

Figure 15.13These swallowtail but-terflies live in differentareas of North America.Despite their slightvariations, they caninterbreed to producefertile offspring.

(t)CORBIS/Bettmann-UPI (b)Matt Meadows

0404-0419 C15S2 BDOL-829900 8/4/04 6:04 AM Page 406

is called gene flow. When an individualleaves a population, its genes are lostfrom the gene pool. When individualsenter a population, their genes areadded to the pool.

Mutation, genetic drift, and geneflow may significantly affect the evolu-tion of small and isolated gene pools,such as those on islands. However, theireffect is often insignificant in larger, lessisolated gene pools. Natural selection isusually the most significant factor thatcauses changes in established genepools—small or large.

Natural selection acts on variations

As you’ve learned, traits have varia-tion, as shown in the butterflies pic-tured in Figure 15.13. Try measuringvariations in MiniLab 15.2.

Recall that some variations increaseor decrease an organism’s chance ofsurvival in an environment. Thesevariations can be inherited and arecontrolled by alleles. Thus, the allelicfrequencies in a population’s genepool will change over generations dueto the natural selection of variations.There are three different types of nat-ural selection that act on variation:stabilizing, directional, and disruptive.

Explain why popula-tions that are in genetic equilibriumare not evolving.


Papilio ajax curvifascia Papilio ajax ehrmann

Collect DataDetecting 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. Collect and Organize Data Was there variation among

the lengths of peanut shells? Use class data to supportyour answer.

2. Draw Conclusions If larger peanut shells were a selec-tive advantage, would this be stabilizing, directional, ordisruptive selection? Explain your answer.

Data Table

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

Checks

My Data—Number of Shells

Class Data—Number of Shells

(t)Elaine Shay (b)Matt Meadows

0404-0419 C15S2 BDOL-829900 8/4/04 6:05 AM Page 407

Stabilizing selection is naturalselection that favors average individ-uals in a population, as shown inFigure 15.14. 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, inthis environment, average-sized spi-ders are more likely to survive—theyhave a selective advantage, or are“selected for.”

Directional selection occurs whennatural selection favors one of theextreme variations of a trait. For exam-ple, imagine a population of woodpeck-ers pecking holes in trees to feed on theinsects living under the bark. Supposethat a species of insect that lives deep intree tissues invades the trees in a wood-pecker population’s territory. Onlywoodpeckers with long beaks couldfeed on that insect. Therefore, thelong-beaked woodpeckers in the popu-lation would have a selective advantageover woodpeckers with very short oraverage-sized beaks.

Finally, in disruptive selection,individuals with either extreme of a trait’s variation are selected for.Consider, for example, a population ofmarine organisms called limpets. Theshell color of limpets ranges fromwhite, 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. On the other hand, birds easily see tan-colored limpets on either the light ordark backgrounds. Disruptive selec-tion tends to eliminate the intermedi-ate phenotypes.


Figure 15.14Different types of natural selection act over the rangeof a trait’s variation. The red, bell-shaped curve indicatesa trait’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 type of selection reduces variation in a population.

A

Directional selection favors one of the extreme variations of a traitand can lead to the rapid evolution of a population.

B

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

C

0404-0419 C15S2 BDOL-829900 8/4/04 6:05 AM Page 408

Figure 15.15When geographic isola-tion divides a popula-tion of tree frogs, theindividuals no longermate across popula-tions. Explain andIllustrate How couldgeographic isolationresult in naturalselection and possi-bly new species?

Natural selection can significantlyalter the genetic equilibrium of a pop-ulation’s gene pool over time. Sig-nificant changes in the gene poolcould 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 interbreedto produce fertile offspring in nature.The evolution of new species, a processcalled speciation (spee shee AY shun),occurs when members of similar popu-lations no longer interbreed to producefertile offspring within their naturalenvironment.

Physical barriers can prevent interbreeding

In nature, physical barriers canbreak large populations into smallerones. Lava from volcanic erup-tions can isolate populations. Sea-level changes along continentalshelves can create islands. The waterthat surrounds an island isolates itspopulations. Geographic isolationoccurs whenever a physical barrierdivides a population.

A new species can evolve when apopulation has been geographicallyisolated. For example, imagine apopulation of tree frogs living in arain forest, Figure 15.15. If smallpopulations of tree frogs were geo-graphically isolated, they would nolonger be able to interbreed andexchange genes. Over time, eachsmall population might adapt to itsenvironment through natural selec-tion and develop its own gene pool.Eventually, the gene pools of eachpopulation might become so different

speciation fromthe Latin wordspecies, meaning“kind”; Speciationis a process thatproduces twospecies from one.

Tree frogs are asingle population.

A The formation of a river maydivide the frogs into twopopulations. A new form mayappear in one population.

B Over time, the dividedpopulations may become twospecies that may no longerinterbreed, even if reunited.

C

0404-0419 C15S2 BDOL-829900 8/4/04 6:06 AM Page 409

that they could no longer interbreedwith the other populations. In thisway, natural selection results in newspecies.

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 ofreproductive isolation. Two exam-ples are given here. One type occurswhen the genetic material of thepopulations becomes so differentthat fertilization cannot occur. Somegeographically separated popula-tions of salamanders in Californiahave this type of reproductive isola-tion. Another type of reproductiveisolation is behavioral. For example,if one population of tree frogs matesin the 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 ofplants and some species of animalshave evolved in the same geographicarea as a result of polyploidy (PAH lihploy dee), illustrated in Figure 15.16.Any individual or species with a mul-tiple of the normal set of chromo-somes is known as a polyploid.

Mistakes during mitosis or meiosiscan result in polyploid individuals. Forexample, if chromosomes do not sepa-rate properly during the first meio-tic division, diploid (2n) gametes can be produced instead of the normal haploid (n) gametes. Polyploidy mayresult in immediate reproductive isola-tion. When a polyploid mates with an individual of the normal species, theresulting zygotes may not develop nor-mally because of the difference in chromosome numbers. In other cases,the zygotes develop into adults thatprobably cannot reproduce. However,polyploids within a population mayinterbreed and form a separate species.

Polyploids can arise from within aspecies or from hybridization between


polyploidy fromthe Greek wordpolys, 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 15.16Many flowering plants,such as this Californiatarweed, are poly-ploids—individuals thatresult from mistakesmade during meiosis.

Al Lowry/Photo Researchers

0404-0419 C15S2 BDOL-829900 8/4/04 6:07 AM Page 410

species. Many flowering plant speciesand many important crop plants, suchas wheat, cotton, and apples, originatedby polyploidy.

Speciation ratesAlthough polyploid speciation takes

only one generation, most other mech-anisms of speciation do not occur asquickly. What is the usual rate ofspeciation?

Scientists once argued that evolu-tion occurs at a slow, steady rate, with small, adaptive changes gradu-ally accumulating over time in popu-lations. Gradualism is the idea thatspecies originate through a gradualchange of adaptations. Some evidencefrom the fossil record supports gradu-alism. For example, fossil evidenceshows that sea lilies evolved slowlyand steadily over time.

In 1972, Niles Eldredge and Ste-phen J. Gould proposed a differenthypothesis known as punctuatedequilibrium. This hypothesis arguesthat speciation occurs relativelyquickly, in rapid bursts, with long peri-ods of genetic equilibrium in between.According to this hypothesis, environ-mental changes, such as higher tem-peratures or the introduction of acompetitive species, lead to rapidchanges in a small population’s genepool that is reproductively isolatedfrom the main population. Speciationhappens quickly—in about 10 000years or less. Like gradualism, punctu-ated equilibrium is supported by fossilevidence as shown in Figure 15.17.

Biologists generally agree that bothgradualism and punctuated equilib-rium can result in speciation, depend-ing on the circumstances. It shouldn’t


Millions

of

Year

s A

go

about 55 million years agoAncestral species

6

5

4

3

2

Loxodontaafricana Elephas

maximus

Mammuthusprimigenius

Primelephas

Mammuthus

Elephas

Loxodonta

1

0

Figure 15.17The fossil record of ele-phant evolution sup-ports the view ofpunctuated equilib-rium. Several elephantspecies may haveevolved from an ances-tral population in ashort time.

0404-0419 C15S2 BDOL-829900 8/4/04 6:07 AM Page 411

Kauai

Niihau

Oahu

Lanai

Kahoolawe

Molokai

Maui

Hawaii

Possible ancestral Laysan finch

AmakihiExtinct mamo

Akialoa

Akepa

LiwiAkiapolaau

Akikiki Palila Ou

Grosbeakfinch

Apapane

Crestedhoneycreeper

Mauiparrotbill

Figure 15.18Evolutionary biologistshave suggested thatthe ancestors of allHawaiian Island honey-creepers migrated fromNorth America about 5 million years ago. Asthis ancestral bird pop-ulation settled in thediverse Hawaiianniches, adaptive radia-tion occurred.

surprise you to see scientists offeralternative hypotheses to explain ob-servations. The nature of science issuch that new evidence or new ideascan modify theories.

Patterns of EvolutionBiologists have observed differ-

ent 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 or havelived on the Hawaiian Islands, amongthem a group of birds called Hawaiianhoneycreepers. This group of birds isinteresting because, although similarin body size and shape, they differsharply in color and beak shape.

Different species of honeycreepersevolved to occupy their own niches.

Despite their differences, scientistshypothesize that honeycreepers, asshown in Figure 15.18, 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 GalápagosIslands are a typical example of adap-tive radiation.

Adaptive radiation is a type of divergent evolution, the pattern ofevolution in which species that oncewere similar to an ancestral spe-cies diverge, or become increasingly


0404-0419 C15S2 BDOL-829900 8/4/04 6:07 AM Page 412

Understanding Main Ideas1. Explain and illustrate why the evolution of resist-

ance to antibiotics in bacteria is an example ofdirectional natural selection.

2. How can geographic isolation change a popula-tion’s gene pool?

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

4. How do gradualism and punctuated equilibriumdiffer? How are they similar? Include in youranswer the patterns of extinction observed inboth theories.

Thinking Critically5. Hummingbird moths are night-flying insects

whose behavior and appearance are similar to those of hummingbirds. Explain how these twoorganisms demonstrate convergent evolution.

6. Experiment Biologists discovered two squirrelspecies living on opposite sides of the GrandCanyon. They hypothesize that the species evolvedfrom a common ancestor. What observations orexperiments could provide evidence for thishypothesis? For more help, refer to Experiment in the Skill Handbook.

SKILL REVIEWSKILL REVIEW


Figure 15.19Unrelated species of plants such as the organ pipe cactus (A) and this Euphorbia (B) share a similar fleshy body type and no leaves.

BB

distinct. Divergent evolution occurswhen populations change as they adaptto different environmental conditions,eventually resulting in new species.

Different species can look alikeA pattern of evolution in which dis-

tantly related organisms evolve similartraits is called convergent evolution.Convergent evolution occurs whenunrelated species occupy similar envi-ronments in different parts of theworld. Because they share similarenvironmental pressures, they sharesimilar pressures of natural selection.

For example, in Figure 15.19 you see an organ pipe cactus (familyCactaceae) that grows in the desertsof North and South America and aplant of the family Euphorbiaceaethat looks similar and lives in Africandeserts. Although these plants areunrelated species, their environmentsare similar. You can see that they bothhave fleshy bodies and no leaves. Thatconvergent evolution has apparentlyoccurred in unrelated species, is fur-ther evidence for natural selection.

Compare and contrast convergent and divergentevolution.

AA

ca.bdol.glencoe.com/self_check_quiz(l)Stephen J. Krasemann/DRK Photo (r)Patti Murray/Earth Scenes

0404-0419 C15S2 BDOL-829900 8/4/04 6:08 AM Page 413

http://ca.bdol.glencoe.com/self_check_quiz


Before YouBegin

Evolution can be describedas the change in allelic fre-quencies of a gene poolover time. Natural selectioncan place pressure on spe-cific phenotypes and causea change in the frequencyof the alleles that producethe phenotypes. In thisactivity, you will simulatethe effects of eagle preda-tion on a population ofrabbits, where GG repre-sents the homozygous con-dition for gray fur, Gg isthe heterozygous conditionfor gray fur, and gg repre-sents the homozygous con-dition for white fur.

Natural Selection and Allelic Frequency

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 over time.■ Use the Internet to collect and compare data from other

students.

Materialscolored pencils (2) paper baggraph paper pinto beanswhite navy beans

Safety PrecautionsCAUTION: Clean up spilled beans immediately to prevent anyone from slipping.

Skill HandbookIf you need help with this lab, refer to the Skill Handbook.

1. Copy the data table shown on the next page.2. Place 50 pinto beans and 50 white navy beans into the

paper bag.3. Shake the bag. Remove two beans. These represent one

rabbit’s genotype. Set the pair aside, and continue toremove 49 more pairs.

4. Arrange the beans on a flat surface in two columns repre-senting the two possible rabbit phenotypes, gray (geno-types GG or Gg) and white (genotype gg).

5. Examine your columns. Remove 25 percent of the grayrabbits and 100 percent of the white rabbits. These num-bers represent a random selection pressure on your rab-bit population. If the number you calculate is a fraction,remove a whole rabbit to make whole numbers.

PROCEDUREPROCEDURE

PREPARATIONPREPARATION

Matt Meadows

0404-0419 C15S2 BDOL-829900 8/4/04 6:09 AM Page 414

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


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 numberof beans of one type by 100. Record these data.

8. Begin the next generation by placing 100 beans into thebag. The proportions of pinto and navy beans should bethe same as the percentages you calculated in step 7.

9. Repeat steps 3 through 8, collecting data for five generations.

10. Go to topost your data.

11. Graph the frequencies of each allele over fivegenerations. Plot the frequency of the allele onthe vertical axis and the number of the generationon the horizontal axis. Use a different colored pencilfor each allele.

12. Return all materials to theirproper places for reuse.CLEANUP AND DISPOSAL

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Analyze Data Did either allele disappear?Why or why not?

2. Think Critically What does your graph showabout allelic frequencies and natural selection?

3. Infer What would happen to the allelic fre-quencies if the number of eagles declined?

4. Explain any differences inallelic frequencies you observed between yourdata and the data from the Internet. Whatadvantage is there to having a large amountof data? What problems might there be inusing data from the Internet?

ERROR ANALYSIS

Graph Find this BioLab using the link below,and post your data in the data table pro-vided for this activity. Using the additionaldata from other students, analyze the com-bined data, and complete your graph.

ca.bdol.glencoe.com/internet_lab

Photodisc

ca.bdol.glencoe.com/internet_lab

0404-0419 C15S2 BDOL-829900 8/4/04 6:11 AM Page 415

http://ca.bdol.glencoe.com/internet_lab

http://ca.bdol.glencoe.com/internet_lab


Mathematics and Evolution

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

doctor, independently discovered how the fre-quency of a trait’s alleles in a population couldbe described mathematically.

Suppose that in a population of pea plants, 36 plants are homozygous dominant for the talltrait (TT), 48 plants are heterozygous tall (Tt),and 16 plants are short plants (tt). In the homo-zygous tall plants, there are (36) (2), or 72, T alleles and in the heterozygous plants thereare 48 T alleles, for a total of 120 T alleles in the population. There are 48 t alleles in the het-erozygous plants plus (16) (2), or 32, t alleles inthe 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 is80/200, or 0.4.

The Hardy-Weinberg principle TheHardy-Weinberg principle states that the fre-quency of the alleles for a trait in a stable popula-tion will not vary. This statement is expressed asthe equation p � q � 1, where p is the frequencyof one allele for the trait, and q is the frequencyof the other allele. The sum of the frequencies ofthe alleles always includes 100 percent of thealleles, 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 geno-types in a population: homozygous dominantindividuals (p2), heterozygous individuals (2pq),and recessive individuals (q2). For example, inthe pea plant population described above, thefrequency of the genotypes would be

(0.6) (0.6) � 2(0.6) (0.4) � (0.4) (0.4) � 1

The frequency of the homozygous tall geno-type is 0.36, the heterozygous genotype is 0.48,and the short genotype is 0.16.

In any sexually reproducing, large population,genotype frequencies will remain constant if nomutations occur, random mating occurs, no nat-ural selection occurs, and no genes enter or leavethe population.

Implications of the principle The Hardy-Weinberg principle is useful for several reasons.First, it explains that the genotypes in populationstend to remain the same. Second, because a reces-sive allele may be masked by its dominant allele,the equation is useful for determining the recessiveallele’s frequency in the population. Finally, theHardy-Weinberg principle is useful in studyingnatural populations to determine how much natu-ral selection may be occurring in the population.

Draw Conclusions The general population ofthe United States is getting taller. Assuming thatheight is a genetic trait, does this observation vio-late the Hardy-Weinberg principle? Explain youranswer.

To find out more about the Hardy-Weinberg principle, visit

A population of penquins

ca.bdol.glencoe.com/math

G.L. Kooyman/Animals Animals

0404-0419 C15S2 BDOL-829900 8/4/04 6:12 AM Page 416

http://ca.bdol.glencoe.com/math

Section 15.1Vocabularyanalogous structure

(p. 401)artificial selection

(p. 395)camouflage (p. 399)embryo (p. 402)homologous structure

(p. 400)mimicry (p. 398)natural selection

(p. 395)vestigial structure

(p. 401)

Vocabularyadaptive radiation

(p. 412)allelic frequency (p. 405)convergent evolution

(p. 413)directional selection

(p. 408)disruptive selection

(p. 408)divergent evolution

(p. 412)gene pool (p. 405)genetic drift (p. 406)genetic equilibrium

(p. 405)geographic isolation

(p. 409)gradualism (p. 411)polyploid (p. 410)punctuated equilibrium

(p. 411)reproductive isolation

(p. 410)speciation (p. 409)stabilizing selection

(p. 408)

Mechanisms ofEvolution

STUDY GUIDESTUDY GUIDE

CHAPTER 15 ASSESSMENT 417

NaturalSelection andthe Evidencefor Evolution

To help you review theevidence for evolution, use the Organi-zational Study Fold on page 393.

Section 15.2Key Concepts■ Evolution can occur only when a popula-

tion’s genetic equilibrium changes. Muta-tion, 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.

■ Gradualism is the hypothesis that speciesoriginate through a gradual change inadaptations. The alternative hypothesis,punctuated equilibrium, argues that speciation occurs in relatively rapid bursts,followed by long periods of genetic equili-brium. Evidence for both evolutionaryrates can be found in the fossil record.

ca.bdol.glencoe.com/vocabulary_puzzlemaker

Key Concepts■ 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 are likelyto survive, reproduce, and pass these varia-tions 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.

0404-0419 C15S2 BDOL-829900 8/4/04 6:13 AM Page 417

http://ca.bdol.glencoe.com/vocabulary_puzzlemaker

418 CHAPTER 15 ASSESSMENT ca.bdol.glencoe.com/chapter_test

Review the Chapter 15 vocabulary words listed inthe Study Guide on page 417. Match the wordswith the definitions below.

1. adaptation that enables an individual toblend with its surroundings

2. mechanism for change in a population3. process of evolution of a new species4. hypothesis that speciation occurs in rapid

bursts

5. Which of the structures shown below isNOT homologous with the others?A. B. C. D.

6. Which type of natural selection favors aver-age individuals in a population?A. directional C. stabilizingB. disruptive D. divergent

7. Which of the following pairs of terms isNOT related?A. analogous structures—butterfly and bird

wingsB. evolution—natural selectionC. vestigial structure—eyes in blind fishD. adaptive radiation—convergent evolution

8. 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

9. 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.

10. Open Ended How might the bright colorsof poisonous species aid in their survival?

11. Open Ended Explain the different ways inwhich a new species can evolve as a result ofnatural selection. Give examples of speciesthat illustrate and support your conclusions.

12. Open Ended Explain why the forelimbs of acat, a bat, and a whale would be homologousstructures.

13. Examples of divergent evolution are especially evident in island archipelagoes. Visit

to find out aboutsome examples of divergent evolution.Chose an island group, such as theGalápagos Islands or the Hawaiian Islands.Make a map of the islands and drawings of agroup of organisms that shows divergentevolution. Your drawings should show atleast three species that are closely related.Explain you findings to the class.

14. Draw Conclusions What can be concludedfrom the observation that many organismshave the same chemical enzymes that workin exactly the same way?

15. Observe and Infer Describe adaptive radia-tion as a form of divergent evolution.

REAL WORLD BIOCHALLENGE

ca.bdol.glencoe.com

0404-0419 C15S2 BDOL-829900 8/4/04 6:13 AM Page 418

http://ca.bdol.glencoe.com/chapter_test

http://ca.bdol.glencoe.com

CHAPTER 15 ASSESSMENT 419ca.bdol.glencoe.com/standardized_test

Multiple ChoiceUse the graph below to answer question 18.

18. Which pattern of natural selection is illus-trated above?A. disruptive selectionB. stabilizing selectionC. directional selectionD. random selection

19. Which of the following provides for instantreproductive isolation?A. polyploidyB. most geographic barriers such as rivers

and mountain beltsC. climate changesD. behavior changes in females of the species

20. Which type of adaptation develops quicklyin bacteria that are antibiotic resistant?A. structural adaptationB. physical adaptationC. physiological adaptationD. phenotypic adaptation

Use the illustration below to answer questions 21 and 22.

21. The pattern of evolution shown above indicates ________.A. convergent evolutionB. divergent evolutionC. stabilizing selectionD. geographic isolation

22. The pattern of evolution shown above mostclosely resembles that of ________.A. Hawaiian honeycreepers and Galápagos

finchesB. organ pipe cactus and EuphorbiaC. bat wings and butterfly wingsD. forelimbs of whales and crocodiles

Divergence

Multiple-descendant species

Ancestral species

Extinction

Time

Freq

uen

cy

Trait value

Distribution Before Selection

Freq

uen

cy

Trait value

Distribution After Selection

Constructed Response/Grid InRecord your answers on your answer document.

23. Open Ended Give an example of two species that evolved due to geographic isolation.24. Open Ended How is it possible for both Darwin’s idea of gradualism and Eldredge and Gould’s

idea of punctuated equilibrium to be valid?

16. Explain In terms of natural selection, whydo many municipalities no longer routinelyspray insecticides to kill mosquitoes duringthe summer months?

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

0404-0419 C15S2 BDOL-829900 8/4/04 6:14 AM Page 419

http://ca.bdol.glencoe.com/standardized_test

chapter 15: the theory of...

Documents