404 Chapter 16

1 FOCUSObjectives16.3.1 Identify the condition nec-

essary for a new species toevolve.

16.3.2 Describe the process of spe-ciation in the Galápagosfinches.

Vocabulary PreviewIntroduce students to the Vocabularyterms by explaining that speciation,or the formation of new species,comes about because of one or moretypes of reproductive isolation:behavioral, geographic, or temporalisolation.

Reading StrategySuggest that students make twographic organizers as they read tosummarize the information in thetwo parts of the section. For exam-ple, they might make a concept mapto show the types of isolating mecha-nisms that lead to speciation and aflowchart to show how speciation ofDarwin’s finches occurred.

2 INSTRUCT

IsolatingMechanismsAddress MisconceptionsStudents may think that being ableto mate more widely rather thanjust within a species would be anevolutionary advantage, not a dis-advantage. Explain that attemptingto mate with a member of anotherspecies almost always results inreproductive failure and thereforewastes time and energy that mighthave been spent mating effectivelywith a conspecific. Also, even if mat-ing does take place and offspring areproduced, they are likely to be lesswell adapted to the niche of either ofthe parent species and, thus, selectedagainst.

Factors such as natural selection and chance events canchange the relative frequencies of alleles in a population.

But how do these changes lead to the formation of new species,or ?

Recall that biologists define a species as a group of organismsthat breed with one another and produce fertile offspring. Thismeans that individuals in the same species share a common gene pool. Because a population of individuals has a shared genepool, a genetic change that occurs in one individual can spreadthrough the population as that individual and its offspringreproduce. If a genetic change increases fitness, that allele willeventually be found in many individuals of that population.

Isolating MechanismsGiven this genetic definition of species, what must happen for aspecies to evolve into two new species? The gene pools of twopopulations must become separated for them to become newspecies. As new species evolve, populations becomereproductively isolated from each other. When the mem-bers of two populations cannot interbreed and produce fertileoffspring, has occurred. At that point,the populations have separate gene pools. They respond tonatural selection or genetic drift as separate units. Reproductiveisolation can develop in a variety of ways, including behavioralisolation, geographic isolation, and temporal isolation.

Behavioral Isolation One type of isolating mechanism,occurs when two populations are capa-

ble of interbreeding but have differences in courtship rituals orother reproductive strategies that involve behavior. For exam-ple, the eastern and western meadowlarks shown in Figure

16–11 are very similar birds whose habitatsoverlap in the center of the United States.Members of the two species will not mate witheach other, however, partly because they usedifferent songs to attract mates. Eastern meadowlarks will not respond to westernmeadowlark songs, and vice versa.

behavioral isolation,

reproductive isolation

speciation

16–3 The Process of Speciation

Key Concepts• What factors are involved in

the formation of new species?• Describe the process of

speciation in the Galápagosfinches.

Vocabularyspeciationreproductive isolationbehavioral isolation geographic isolationtemporal isolation

Reading Strategy: Using Visuals Before youread, preview Figure 16–16.As you read about speciation ofDarwin’s finches, notice whathappens at each step in thediagram.

Figure 16–11 The eastern meadowlark (left) and westernmeadowlark (right) have overlapping ranges. They do notinterbreed, however, because they have different mating songs.Applying Concepts What type of reproductive isolation doesthis situation illustrate?

SECTION RESOURCES

Print:

• Teaching Resources, Section Review 16–3,Enrichment

• Reading and Study Workbook A, Section 16–3• Adapted Reading and Study Workbook B,

Section 16–3• Issues and Decision Making, Issues and

Decisions 16• Lesson Plans, Section 16–3

Technology:

• iText, Section 16–3• Transparencies Plus, Section 16–3• BioDetectives Videotapes, “The Galápagos

Islands: A Glimpse Into the Past”

Tim

eSaver

Section 16–3

Build Science SkillsApplying Concepts Divide theclass into several groups, and chal-lenge each group to brainstorm ascenario in which a small populationof a species becomes geographicallyisolated from the remainder of thespecies long enough to evolve into aseparate species. Urge groups to con-sider both natural events and humanactivities when they brainstorm waysthat geographic isolation could comeabout. Have each group elect aspokesperson to describe their sce-nario to the class. In each case, ask:Why did the geographically isolated population evolve into adifferent species? (Answers will varydepending on scenarios. Studentsmight say, for example, that the isolat-ed population was genetically differentto begin with because of founder effectand that it became even more differentthrough time due to different selectivepressures.)

Use VisualsFigure 16–12 Have students usethe key to locate the range of eachtype of squirrel. Point out how theColorado River effectively isolates thetwo types of squirrels geographically,despite the closeness of their ranges.

Build Science SkillsInferring Have students infer thereproductive characteristics of speciesmost likely to be affected by tempo-ral isolation. Ask: What must be trueabout the reproductive behavior ofspecies that are isolated by tempo-ral isolation? (Their reproductivebehavior must be limited to a certaintime of day or a certain season.)

Evolution of Populations 405

Geographic Isolation With twopopulations are separated by geographic barriers such as rivers,mountains, or bodies of water. The Abert squirrel in Figure 16–12,for example, lives in the Southwest. About 10,000 years ago, theColorado River split the species into two separate populations.Two separate gene pools formed. Genetic changes that appearedin one group were not passed to the other. Natural selectionworked separately on each group and led to the formation of adistinct subspecies, the Kaibab squirrel. The Abert and Kaibabsquirrels have very similar anatomical and physiological char-acteristics, indicating that they are closely related. However, theKaibab squirrel differs from the Abert squirrel in significantways, such as fur coloring.

Geographic barriers do not guarantee the formation of newspecies, however. Separate lakes may be linked for a time duringa flood, or a land bridge may temporarily form between islands,enabling separated populations to mix. If two formerly sepa-rated populations can still interbreed, they remain a singlespecies. Also, any potential geographic barrier may separatecertain types of organisms but not others. A large river will keepsquirrels and other small rodents apart, but it does not neces-sarily isolate bird populations.

Temporal Isolation A third isolating mechanism is in which two or more species reproduce

at different times. For example, three similar species of orchidall live in the same rain forest. Each species releases pollen only on a single day. Because the three species release pollen on different days, they cannot pollinate one another.

How can temporal isolation lead to speciation?

temporal isolation,

geographic isolation,

Figure 16–12 When twopopulations of a species becomereproductively isolated, newspecies can develop. The Kaibabsquirrel evolved from the Abertsquirrel. The Kaibab squirrels wereisolated from the main populationby the Colorado River.

UTAH

NEWMEXICOARIZONA

COLORADO

GrandCanyon

LakePowell

LakeMead

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ado

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r

Rio

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nde

Gila River

Range ofKaibab squirrel

Range ofAbert squirrel

0

0 250 Miles

250 Kilometers125

125Abert squirrel

Kaibab squirrel

Inclusion/Special NeedsCreate a flowchart showing in a simple way thesteps that occurred in the speciation of Darwin’sfinches. Include the following steps: foundersarriving; founders remaining geographically isolat-ed; directional selection occurring in differentenvironments; reproductive isolation developing;and interspecific competition leading to improvedadaptation to the species’ niches.

Advanced LearnersInvite students who need extra challenges toresearch the process of speciation in Hawaiianbirds called honeycreepers. Like Darwin’s finches,honeycreepers underwent adaptive radiation aftercolonizing islands with vacant niches. Ask studentsto share their findings with the class and to pointout similarities and differences between theHawaiian and Galápagos cases.

Answers to . . . If two populations

reproduce at different times, they areunlikely to reproduce with each other.Eventually they may become separatespecies.

Figure 16–11 Behavioral isolation

406 Chapter 16

Testing NaturalSelection in NatureUse VisualsFigure 16–13 Point out that thewoodpecker finch uses its beak tohold a cactus spine, which it pokesinto holes in trees in order to spearinsects. Ask: What tool does its beakresemble? (Students might say pliersor needle-nosed pliers.) If anotherspecies of fruit-eating finch wasdiscovered, what type of beak doyou think it would have? (Studentsare likely to infer that it would have abeak like the vegetarian tree finch,which also eats fruit.)

Build Science SkillsApplying Concepts Point out thatthe Grants used the scientific methodin their research on the GalápagosIslands. Challenge students to recallthe steps typically involved in the sci-entific method. Assign a student torecord the steps on the board. (Ask aquestion, gather information and formhypothesis, experiment, record andanalyze data, draw conclusion) Then,have students describe each step ofthe scientific method as it applies tothe Grants’ research. (For example,the Grants’ problem was to demon-strate natural selection in action. Theirhypotheses were that there wasenough inheritable variation in beaksize and shape to provide raw materialfor natural selection and that variationin beak size and shape produced differ-ences in fitness.)

Testing Natural Selection in NatureNow that you know the basic mechanisms of evolutionary change,you might wonder if these processes can be observed in nature.The answer is yes. In fact, some of the most important studiesshowing natural selection in action involve descendants of thefinches that Darwin observed in the Galápagos Islands.

Those finch species looked so different from one anotherthat when Darwin first saw them, he did not realize they wereall finches. He thought they were blackbirds, warblers, andother kinds of birds. The species he examined differed greatlyin the sizes and shapes of their beaks and in their feedinghabits, as shown in Figure 16–13. Some species fed on smallseeds, while others ate large seeds with thick shells. Onespecies used cactus spines to pry insects from dead wood. Onespecies, not shown here, even pecked at the tails of large seabirds and drank their blood!

Once Darwin discovered that these birds were all finches, hehypothesized that they had descended from a common ancestor.Over time, he proposed, natural selection shaped the beaks ofdifferent bird populations as they adapted to eat different foods.

That was a reasonable hypothesis. But was there any way totest it? No one thought so, until the work of Peter and RosemaryGrant from Princeton University proved otherwise. For morethan twenty years, the Grants, shown in Figure 16–14, have beencollaborating to band and measure finches on the GalápagosIslands. They realized that Darwin’s hypothesis relied on twotestable assumptions. First, in order for beak size and shape toevolve, there must be enough heritable variation in those traits toprovide raw material for natural selection. Second, differences inbeak size and shape must produce differences in fitness thatcause natural selection to occur.

Common Name of Finch Species

Vegetariantree finch

Large insectivoroustree finch

Woodpeckerfinch

Cactus groundfinch

Sharp-beakedground finch

Large groundfinch

Main Food Fruits Insects Insects Cacti Seeds Seeds

FeedingAdaptation

Parrotlikebeak

Graspingbeak

Uses cactusspines

Large crushingbeak

Pointedcrushing beak

Largecrushing beak

Habitat Trees Trees Trees Ground Ground Ground

Shape of Head and Beak

Galápagos Islands Finches

� Figure 16–13 Detailedgenetic studies have shown thatthese finches evolved from a specieswith a more-or-less general-purposebeak. Formulating HypothesesSuggest how one of these beaks couldhave resulted from natural selection.

For: Links on speciation

Visit: www.SciLinks.orgWeb Code: cbn-5163

NSTA

Fruit fly speciationThe North American fruit fly Rhagoletis pomonel-la appears to be in the process of speciation.Before the 1800s, R. pomonella infested onlyhawthorn trees. Then, when apple trees wereintroduced to North America, the fruit fly beganinfesting them as well. Today, the species exists

in separate populations on each type of fruittree. The different populations do not inter-breed and have some genetic differences.Although they are still one species, they appearto be on their way to becoming separatespecies.

FACTS AND FIGURES

16–3 (continued)

NSTA

Download a worksheet on speciationfor students to complete, and findadditional teacher support fromNSTA SciLinks.

Build Science SkillsInferring Challenge students toassume the role of an evolutionarybiologist. First, have them choose apopulation that they will study. Then,ask them to list the kinds of observa-tions they would need to make todetermine if the population wereundergoing natural selection. (Youwould need to observe evidence ofinheritable variation in the populationand evidence that different phenotypesvary in fitness.) Ask: What might youobserve if speciation were occur-ring? (You might observe that variouspopulations of the species had becomeseparated so they no longer shared thesame gene pool. You also mightobserve genetic differences between thepopulations.)

Use VisualsFigure 16–15 Challenge studentsto predict how the graph would bedifferent if birds with medium-sizedbeaks were more likely to survive. (Itwould resemble a normal curve.) Ask:What type of natural selectionwould this represent? (Stabilizingselection)

Evolution of Populations 407

The Grants tested these hypotheses on the medium groundfinch on Daphne Major, one of the Galápagos islands. Thisisland is large enough to support good-sized finch populations,yet small enough to enable the Grants to catch and identifynearly every bird belonging to the species under study.

Variation The Grants first identified and measured as manyindividual birds as possible on the island. They recordedwhich birds were still living and which had died, whichhad succeeded in breeding and which had not. Foreach individual, they also recorded anatomicalcharacteristics such as wing length, leg length, beaklength, beak depth, beak color, feather colors, andtotal mass. Many of these characteristics appearedin bell-shaped distributions typical of polygenictraits. These data indicate that there is great varia-tion of heritable traits among the Galápagos finches.

Natural Selection Other researchers who hadvisited the Galápagos did not see the different finchescompeting or eating different foods. During the rainyseason, when these researchers visited, there is plenty of food.Under these conditions, finches often eat the most availabletype of food. During dry-season drought, however, some foodsbecome scarce, and others disappear altogether. At that time,differences in beak size can mean the difference between lifeand death. To survive, birds become feeding specialists. Eachspecies selects the type of food its beak handles best. Birdswith big, heavy beaks, for example, select big, thick seeds thatno other species can crack open.

The Grants’ most interesting discovery was that individualbirds with different-sized beaks had different chances of survivalduring a drought. When food for the finches was scarce, individ-uals with the largest beaks were more likely to survive, as shownin Figure 16–15. Beak size also plays a role in mating behavior,because big-beaked birds tend to mate with other big-beakedbirds. The Grants observed that average beak size in thatfinch population increased dramatically over time. Thischange in beak size is an example of directional selectionoperating on an anatomical trait.

By documenting natural selection in the wild, theGrants provided evidence of the process of evolution: Thenext generation of finches had larger beaks than did thegeneration before selection had occurred. An importantresult of this work was their finding that natural selectiontakes place frequently—and sometimes very rapidly.Changes in the food supply on the Galápagos causedmeasurable fluctuations in the finch populations over aperiod of only decades. This is markedly different from theslow, gradual evolution that Darwin envisioned.

What type of natural selection did the Grantsobserve in the Galápagos?

Beak Size (mm)

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Bird Survival Based on Beak Size

7 8 9 10 11 12 13

Figure 16–14Peter and RosemaryGrant have demon-strated that naturalselection is still a

force in the evolutionof the Galápagos finches. ApplyingConcepts How doestheir researchdemonstrate naturalselection?

� Figure 16–15 This graphshows the survival rate of onespecies of ground-feeding finches,the medium ground finch,Geospiza fortis. Using Tables andGraphs What trend does this graphshow?

Answers to . . . Directional selection

Figure 16–13 Sample answer: Forthe large ground finch, natural selec-tion favored birds with large, heavybeaks that could crush tough seeds.

Figure 16–14 Their research showsthat birds with larger beaks have a bet-ter chance of surviving during adrought and that average beak sizehas increased dramatically over time asa result.

Figure 16–15 The larger a bird’sbeak, the greater its chances ofsurvival.

Birds and flies in HawaiiIn the process of adaptive radiation on theGalápagos Islands, one species of finches gave riseto many species of finches. Similarly, five millionyears of adaptive radiation in Hawaiian birdsknown as honeycreepers resulted in a wide arrayof beak shapes and as many as 43 species.However, human actions have resulted in theextinction of most of these species.

Diverging physical traits do not cause all spe-ciation. In Hawaii, a variety of courtship songshas separated the native Drosophila fruit fly intomore than 500 species! Some HawaiianDrosophila sound more like cicadas than flies.Others make a cricketlike noise. Still othersmake a sound like that of a North American fruitfly, but they create the sound by vibrating theirabdomen instead of their wings.

FACTS AND FIGURES

408 Chapter 16

Speciation inDarwin’s Finches

Build Science SkillsApplying Concepts Challenge stu-dents to apply the concept offounder effect to the example ofDarwin’s finches. Ask: How did thefirst finches to arrive on theGalápagos compare geneticallywith finches on the mainland?(They had only a tiny fraction of thetotal genetic variability of the mainlandfinches and, through chance, may nothave been genetically representative ofmainland finches.)

Speciation in Darwin’s FinchesThe Grants’ work demonstrates that finch beak size can bechanged by natural selection. If we combine this informationwith other evolutionary concepts you have learned in thischapter, we can show how natural selection can lead to specia-tion. We can devise a hypothetical scenario for the evolution ofall Galápagos finches from a single group of founding birds.

Speciation in the Galápagos finches occurred byfounding of a new population, geographic isolation,changes in the new population’s gene pool, reproductiveisolation, and ecological competition.

Founders Arrive Many years ago, a few finches from theSouth American mainland—species A—flew or were blown to one of the Galápagos Islands, as shown in Figure 16–16. Finchesare small birds that do not usually fly far over open water. Thesebirds may have gotten lost, or they may have been blown offcourse by a storm. Once they arrived on one of the islands, theymanaged to survive and reproduce.

Geographic Isolation Later on, some birds from species Acrossed to another island in the Galápagos group. Because thesebirds do not usually fly over open water, they rarely move fromisland to island. Thus, finch populations on the two islands wereessentially isolated from each other and no longer shared acommon gene pool.

How did finches arrive in the Galápagos Islands?

How Are These Fish Related?A research team studied two lakes in an area thatsometimes experiences flooding. Each lake con-tained two types of similar fish: a dull brown formand an iridescent gold form. The team wonderedhow all the fish were related, and they consideredthe two hypotheses diagrammed on the right.

1. Interpreting Graphics Study the twodiagrams. What does hypothesis A indicateabout the ancestry of the fish in Lake 1 and Lake 2?What does hypothesis B indicate?

2. Comparing and Contrasting According to thetwo hypotheses, what is the key difference in theway the brown and gold fish populations mighthave formed?

3. Drawing Conclusions A DNA analysis showedthat the brown and gold fish from Lake 1 are themost closely related. Which hypothesis does thisevidence support?

4. Asking Questions To help determine whetherthe brown and gold fish are members of separatespecies, what question might scientists ask?

Lake 1

Hypothesis A

Lake 2

Hypothesis B

A

Lake 2

Lake 1

Lake 2

B

G

A = Possible ancestor

B = Contemporary brown form

G = Contemporary gold form

Shows possibleline of descent

AB

G

B

G

GA

A

B

To find out moreabout ongoingresearch on theGalápagos, view

the segment “The Galápagos Islands: AGlimpse Into the Past,” on Videotape Two.

16–3 (continued)

Explain to students that the arrows inthe diagrams imply evolutionary rela-tionships. For example, in hypothesisA, both B and G are presumed tohave evolved from A.Answers1. Hypothesis A suggests that Lake 1and Lake 2 brown fishes are unrelat-ed, and the gold fishes from the twolakes are unrelated. This differs fromhypothesis B, which suggests that Lake1 gold fishes evolved from Lake 2 goldfishes and that Lake 2 brown fishesevolved from Lake 1 brown fishes.2. In hypothesis A, the brown (B) andgold (G) fish populations eachevolved independently. In hypothesisB, the brown (B) fish populations inboth lakes evolved from fish type A inLake 1, and the gold (G) fish popula-tions in both lakes evolved from fishtype A in Lake 2.3. This evidence supports hypothesis A.4. Scientists would need to askwhether fishes from the two lakescan interbreed and produce fertileoffspring. If not, then they are mem-bers of different species.

Encourage students toview the segment “TheGalápagos Islands: A

Glimpse Into the Past” on Videotape Two.

Not just finchesAmong birds on the Galápagos Islands,Darwin’s finches are better known than themockingbirds, but it was the four species ofmockingbirds that first drew Darwin’s attentionto the diversity of species on the islands. Darwindiscovered that three of the mockingbird

species were each confined to a single island,and it surprised him to learn that the islandswere all similar and within sight of one another.Like Darwin’s finches, the mockingbird specieshad evolved differences primarily in the size andshape of their beaks that reflected the differentfood sources they relied upon.

FACTS AND FIGURES

Make ConnectionsEnvironmental Science Introducestudents to the concept of adaptiveradiation, which is the evolution ofmany diversely adapted species froma common ancestor. Explain thatDarwin’s finches could not haveevolved into so many differentspecies if the niche resources theyfound had already been put to useby other birds. Point out that theGalápagos are volcanic islands that atone time had many available nicheresources for land birds. Ask: Besidesthe formation of islands by volca-noes, how might an area come tosupport so many niches establishedby invading species through adap-tive radiation? (One possible answeris the occurrence of a natural disaster,such as a forest fire or flood, thatdestroys habitats. As the original habi-tats gradually return, the speciesreestablish niches.)

Build Science SkillsInferring Point out the role of eco-logical competition in the speciationof Darwin’s finches. Ask: How doescompetition increase the differ-ences between species? (Individualsthat are more specialized and less likethe other species have less competitionand higher fitness. This leads to anincrease in the differences between thespecies.)

Use VisualsFigure 16–16 Challenge studentsto create new captions for thesequence of drawings to explain howanother species of Darwin’s finches,such as the vegetarian tree finch,might have evolved. Ask volunteersto share their captions with the class.

Evolution of Populations 409

Changes in the Gene Pool Over time, populations oneach island became adapted to their local environments. Theplants growing on the first island may have produced smallthin-shelled seeds, whereas the plants on the second island mayhave produced larger thick-shelled seeds. On the second island,directional selection would favor individuals with larger, heavierbeaks. These birds could crack open and eat the large seedsmore easily. Thus, birds with large beaks would be better able tosurvive on the second island. Over time, natural selection wouldhave caused that population to evolve larger beaks, forming aseparate population, B.

Reproductive Isolation Now, imagine that a few birdsfrom the second island cross back to the first island. Will thepopulation-A birds breed with the population-B birds? Probablynot. These finches choose their mates carefully. As part of theircourtship behavior, they inspect a potential partner’s beak veryclosely. Finches prefer to mate with birds that have the same-sized beak as they do. In other words, big-beaked birds prefer tomate with other big-beaked birds, and smaller-beaked birdsprefer to mate with other smaller-beaked birds. Because thebirds on the two islands have different-sized beaks, it is likelythat they would not choose to mate with each other. Thus,differences in beak size, combined with mating behavior, couldlead to reproductive isolation. The gene pools of the two birdpopulations remain isolated from each other—even whenindividuals live together in the same place. The two populationshave now become separate species.

Ecological Competition As these two new species livetogether in the same environment (the first island), they com-pete with each other for available seeds. During the dry season,individuals that are most different from each other have thehighest fitness. The more specialized birds have less competitionfor certain kinds of seeds and other foods, and the competitionamong individual finches is also reduced. Over time, speciesevolve in a way that increases the differences between them. Thespecies-B birds on the first island may evolve into a new species, C.

Continued Evolution This process of isolation on differentislands, genetic change, and reproductive isolation probablyrepeated itself time and time again across the entire Galápagosisland chain. Over many generations, it produced the 13 differ-ent finch species found there today. Use the steps in this illus-tration to explain how other Darwin finches, such as thevegetarian tree finch that feeds on fruit, might have evolved.

� Figure 16–16 Speciation in the Galápagos finches occurredby founding of new populations, geographic isolation, gene poolchanges, reproductive isolation, and ecological competition. Smallgroups of finches moved from one island to another, became reproduc-tively isolated, and evolved into new species.

A

A

A

B

A

GalapagosIslands

S O U T HA M E R I C A

Founders ArriveA few finches travel from South America to one of the islands. There, they survive and reproduce.

Geographic Isolation Some birds from species A cross to a second island. The two populations nolonger share a gene pool.

Changes in the Gene Pool Seed sizes on the second island favor birds with larger beaks. The populationon the second island evolves into a population, B, with larger beaks. Eventually, populations A and B evolve into separate species.

´

They flew, or were blown,from South America.

410 Chapter 16

Studying EvolutionSince DarwinBuild Science SkillsApplying Concepts Challenge stu-dents to explain in two or threesentences the process of speciation,using concepts of Mendelian inheri-tance. Ask volunteers to share theirstatements with the class.

3 ASSESSEvaluate UnderstandingCall on students at random to nameand give examples of each of theways reproductive isolation comesabout.

ReteachHave students write on a note card abrief summary of each step in thespeciation of Darwin’s finches. Then,have students shuffle the cards andtry to put them back in the correctorder.

Studying Evolution Since DarwinIt is useful to review and critique the strengths and weaknesses ofevolutionary theory. Darwin made bold assumptions about her-itable variation, the age of Earth, and relationships among organ-isms. New data from genetics, physics, and biochemistry couldhave proved him wrong on many counts. They didn’t. Scientific evi-dence supports the theory that living species descended with mod-ification from common ancestors that lived in the ancient past.

Limitations of Research The Grants’ research clearlyshows the effects of directional selection in nature. The Grants’data also show how competition and climate change affectnatural selection. The work does have limitations. For example,while the Grants observed changes in the size of the finches’beaks, they did not observe the formation of a new species.Scientists predict that as new fossils are found, they will con-tinue to expand our understanding of how species evolved.

Unanswered Questions The studies of the Grants fit into anenormous body of scientific work supporting the theory of evolu-tion. Millions of fossils show that life has existed on Earth for morethan 3 billion years and that organisms have changed dramati-cally over this time. These fossils form just a part of the evidencesupporting the conclusion that life has evolved. Remember that ascientific theory is defined as a well-tested explanation thataccounts for a broad range of observations. Evolutionary theoryfits this definition. To be sure, many new discoveries have led tonew hypotheses that refine and expand Darwin’s original ideas.No scientist suggests that all evolutionary processes are fullyunderstood. Many unanswered questions remain.

Why is understanding evolution important? Because evolu-tion continues today, driving changes in the living world such asdrug resistance in bacteria and viruses, and pesticide resistancein insects. Evolutionary theory helps us understand andrespond to these changes in ways that improve human life.

� Figure 16–17 Paleontologistsstudy fossils to find clues aboutprevious life forms.

1. Key Concept How isreproductive isolation related tothe formation of new species?

2. Key Concept What typeof isolating mechanism wasimportant in the formation ofGalápagos finch species?

3. Explain how behavior can play arole in the evolution of species.

4. What recent research findingssupport Darwin’s theory ofevolution?

5. Critical Thinking InferringSuppose that a drought on anisland eliminates all but plants thatproduce large, tough seeds. All thefinches on the island have verysmall beaks. How might thisenvironmental change impact thesurvival of this finch population?

SummarizingWrite a paragraph thatsummarizes the Grants’research with Galápagosfinches. Your summary shouldinclude the main points of theresearch. Hint: The firstsentence in your summarymight state the Grants’hypothesis.

16–3 Section Assessment

16–3 (continued)

If your class subscribes to the iText,use it to review the Key Conceptsin Section 16–3.

16–3 Section Assessment1. For new species to evolve, populations must

be reproductively isolated from each other.2. Geographic isolation 3. Behavioral isolation can occur when two

populations that could otherwise interbreeddo not because of differences in behavior.This can lead to the evolution of differentspecies.

4. Peter and Rosemary Grant have provided evi-dence of the process of evolution throughtheir studies of changes of beak size andshape in Galápagos finches.

5. Only those finches that can find another foodsource or those with beaks large enough tocrack the seeds will survive.

Assess students’ paragraphs on thebasis of their completeness, accu-racy, and organization. Theparagraphs should begin with atopic sentence, such as the Grants’hypothesis, followed by sentencesproviding details related to thetopic sentence. Paragraphs shouldalso include the conclusions theGrants reached based on theirresearch.