Darwiin

439

21The Evidence for

Evolution

Concept Outline

21.1 Fossil evidence indicates that evolution hasoccurred.

The Fossil Record. When fossils are arranged in theorder of their age, a continual series of change is seen, newchanges being added at each stage.The Evolution of Horses. The record of horse evolutionis particularly well-documented and instructive.

21.2 Natural selection can produce evolutionarychange.

The Beaks of Darwins Finches. Natural selectionfavors stouter bills in dry years, when large tough-to-crushseeds are the only food available to finches. Peppered Moths and Industrial Melanism. Naturalselection favors dark-colored moths in areas of heavypollution, while light-colored moths survive better inunpolluted areas.Artificial Selection. Artificial selection practiced inlaboratory studies, agriculture, and domesticationdemonstrate that selection can produce substantialevolutionary change.

21.3 Evidence for evolution can be found in otherfields of biology.

The Anatomical Record. When anatomical features ofliving animals are examined, evidence of shared ancestry isoften apparent.The Molecular Record. When gene or proteinsequences from organisms are arranged, species thought tobe closely related based on fossil evidence are seen to bemore similar than species thought to be distantly related.Convergent and Divergent Evolution. Evolution favorssimilar forms under similar circumstances.

21.4 The theory of evolution has proven controversial.

Darwins Critics. Critics have raised seven objections toDarwins theory of evolution by natural selection.

Of all the major ideas of biology, the theory that to-days organisms evolved from now-extinct ancestors(figure 21.1) is perhaps the best known to the general pub-lic. This is not because the average person truly under-stands the basic facts of evolution, but rather because manypeople mistakenly believe that it represents a challenge totheir religious beliefs. Similar highly publicized criticismsof evolution have occurred ever since Darwins time. Forthis reason, it is important that, during the course of yourstudy of biology, you address the issue squarely: Just whatis the evidence for evolution?

FIGURE 21.1A window into the past. The fossil remains of the now-extinct reptile Mesosaurus found in Permian sediments inAfrica and South America provided one of the earliest cluesto a former connection between the two continents.Mesosaurus was a freshwater species and so clearly incapableof a transatlantic swim. Therefore, it must have lived in thelakes and rivers of a formerly contiguous landmass thatlater became divided as Africa and South America driftedapart in the Cretaceous.

Dating Fossils

By dating the rocks in which fossils occur, we can get an ac-curate idea of how old the fossils are. In Darwins day,rocks were dated by their position with respect to one an-other (relative dating); rocks in deeper strata are generallyolder. Knowing the relative positions of sedimentary rocksand the rates of erosion of different kinds of sedimentaryrocks in different environments, geologists of the nine-teenth century derived a fairly accurate idea of the relativeages of rocks.

Today, rocks are dated by measuring the degree ofdecay of certain radioisotopes contained in the rock (ab-solute dating); the older the rock, the more its isotopes havedecayed. Because radioactive isotopes decay at a constantrate unaltered by temperature or pressure, the isotopes in arock act as an internal clock, measuring the time since therock was formed. This is a more accurate way of datingrocks and provides dates stated in millions of years, ratherthan relative dates.

A History of Evolutionary Change

When fossils are arrayed according to their age, fromoldest to youngest, they often provide evidence of succes-sive evolutionary change. At the largest scale, the fossilrecord documents the progression of life through time,from the origin of eukaryotic organisms, through theevolution of fishes, the rise of land-living organisms, thereign of the dinosaurs, and on to the origin of humans(figure 21.2).

440 Part VI Evolution

At its core, the case for evolution is built upon two pillars:first, evidence that natural selection can produce evolution-ary change and, second, evidence from the fossil recordthat evolution has occurred. In addition, information frommany different areas of biologyincluding fields as differ-ent as embryology, anatomy, molecular biology, and bio-geography (the study of the geographic distribution ofspecies)can only be interpreted sensibly as the outcomeof evolution.

The Fossil RecordThe most direct evidence that evolution has occurred isfound in the fossil record. Today we have a far more com-plete understanding of this record than was available inDarwins time. Fossils are the preserved remains of once-living organisms. Fossils are created when three eventsoccur. First, the organism must become buried in sedi-ment; then, the calcium in bone or other hard tissue mustmineralize; and, finally, the surrounding sediment musteventually harden to form rock. The process of fossilizationprobably occurs rarely. Usually, animal or plant remainswill decay or be scavenged before the process can begin. Inaddition, many fossils occur in rocks that are inaccessible toscientists. When they do become available, they are oftendestroyed by erosion and other natural processes beforethey can be collected. As a result, only a fraction of thespecies that have ever existed (estimated by some to be asmany as 500 million) are known from fossils. Nonetheless,the fossils that have been discovered are sufficient to pro-vide detailed information on the course of evolutionthrough time.

21.1 Fossil evidence indicates that evolution has occurred.

Millions of years ago

Eukaryotes

Vertebrates

Colonizationof land

Reptiles

Amphibians

Mammalsand

dinosaursFlowering plants

and first birds

Firsthominids

1002003004005006001500

Extinctionof the

dinosaurs

FIGURE 21.2Timeline of the history of life as revealed by the fossil record.

Gaps in the Fossil Record

This is not to say that the fossilrecord is complete. Given the lowlikelihood of fossil preservation andrecovery, it is not surprising thatthere are gaps in the fossil record.Nonetheless, paleontologists (thescientists who study fossils) continueto fill in the gaps in the fossil record.While many gaps interrupted thefossil record in Darwins era, eventhen, scientists knew of the Ar-chaeopteryx fossil transitional betweendinosaurs and birds. Today, the fos-sil record is far more complete, par-ticularly among the vertebrates; fos-sils have been found linking all themajor groups. Recent years haveseen spectacular discoveries closingsome of the major remaining gaps inour understanding of vertebrate evo-lution. For example, recently a four-legged aquatic mammal was discov-ered that provides important insightsconcerning the evolution of whalesand dolphins from land-living,hoofed ancestors (figure 21.3). Simi-larly, a fossil snake with legs has shedlight on the evolution of snakes,which are descended from lizardsthat gradually became more andmore elongated with simultaneousreduction and eventual disappear-ance of the limbs.

On a finer scale, evolutionarychange within some types of animalsis known in exceptional detail. Forexample, about 200 million yearsago, oysters underwent a changefrom small curved shells to larger,flatter ones, with progressively flat-ter fossils being seen in the fossilrecord over a period of 12 millionyears (figure 21.4). A host of otherexamples all illustrate a record ofsuccessive change. The demonstra-tion of this successive change is oneof the strongest lines of evidencethat evolution has occurred.

The fossil record provides a clearrecord of the major evolutionarytransitions that have occurredthrough time.

Chapter 21 The Evidence for Evolution 441

Present

10 millionyears ago

20 millionyears ago

30 millionyears ago

40 millionyears ago

50 millionyears ago

60 millionyears ago Hypothetical

mesonychid skeleton

Modern toothed whales

Ambulocetus natansprobably walked on land (as do modern sea lions) and swam by flexing its backbone and paddling with its hind limbs (as do modern otters)

Rodhocetus kasrani's reduced hind limbs could not have aided it in walking or swimming. Rodhocetus swam with an up-and-down motion, as do modern whales

FIGURE 21.3Whale missing links. The recent discoveries of Ambulocetus and Rodhocetus have filledin the gaps between the mesonychids, the hypothetical ancestral link between thewhales and the hoofed mammals, and present-day whales.

G. arcuataobliquata

G. arcuataincurva G. mecullochii G. gigantea

FIGURE 21.4Evolution of shell shape in oysters. Over 12 million years of the Early JurassicPeriod, the shells of this group of coiled oysters became larger, thinner, and flatter.These animals rested on the ocean floor in a special position called the lifeposition, and it may be that the larger, flatter shells were more stable in disruptivewater movements.

The Evolution of HorsesOne of the best-studied cases in the fossil record concernsthe evolution of horses. Modern-day members of theEquidae include horses, zebras, donkeys and asses, all ofwhich are large, long-legged, fast-running animals adaptedto living on open grasslands. These species, all classified inthe genus Equus, are the last living descendants of a longlineage that has produced 34 genera since its origin in theEocene Period, approximately 55 million years ago. Exam-ination of these fossils has provided a particularly well-documented case of how evolution has proceeded by adap-tation to changing environments.

The First Horse

The earliest known members of the horse family, species inthe genus Hyracotherium, didnt look much like horses atall. Small, with short legs and broad feet (figure 21.5), thesespecies occurred in wooded habitats, where they probablybrowsed on leaves and herbs and escaped predators bydodging through openings in the forest vegetation. Theevolutionary path from these diminutive creatures to theworkhorses of today has involved changes in a variety oftraits, including:

Size. The first horses were no bigger than dogs, withsome considerably smaller. By contrast, modern equidscan weigh more than a half ton. Examination of the fos-sil record reveals that horses changed little in size fortheir first 30 million years, but since then, a number ofdifferent lineages exhibited rapid and substantial in-creases. However, trends toward decreased size werealso exhibited among some branches of the equid evolu-tionary tree (figure 21.6).Toe reduction. The feet of modern horses have a sin-gle toe, enclosed in a tough, bony hoof. By contrast,Hyracotherium had four toes on its front feet and threeon its hindfeet. Rather than hooves, these toes were en-cased in fleshy pads. Examination of the fossils clearlyshows the transition through time: increase in length ofthe central toe, development of the bony hoof, and re-duction and loss of the other toes (figure 21.7). As withbody size, these trends occurred concurrently on severaldifferent branches of the horse evolutionary tree. At thesame time as these developments, horses were evolvingchanges in the length and skeletal structure of the limbs,leading to animals capable of running long distances athigh speeds.Tooth size and shape. The teeth of Hyracotheriumwere small and relatively simple in shape. Through time,horse teeth have increased greatly in length and have de-veloped a complex pattern of ridges on their molars andpremolars (figure 21.7). The effect of these changes is toproduce teeth better capable of chewing tough andgritty vegetation, such as grass, which tends to wear

teeth down. Accompanying these changes have been al-terations in the shape of the skull that strengthened theskull to withstand the stresses imposed by continualchewing. As with body size, evolutionary change has notbeen constant through time. Rather, much of the changein tooth shape has occurred within the past 20 millionyears.

All of these changes may be understood as adaptations tochanging global climates. In particular, during the late


FIGURE 21.5Hyracotherium sandrae, one of the earliest horses, was thesize of a housecat.

Body

size

(kg)

Millions of years ago

50

100

150

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250

300

350

400

450

500

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60 55 50 45 40 35 30 25 20 15 10 5 0

Equus

Hyracotherium

Mesohippus

Merychippus

Nannippus

FIGURE 21.6Evolutionary change in body size of horses. Lines show thebroad outline of evolutionary relationships. Although mostchange involved increases in size, some decreases alsooccurred.

Miocene and early Oligocene (approximately 20 to 25 mil-lion years ago), grasslands became widespread in NorthAmerica, where much of horse evolution occurred. Ashorses adapted to these habitats, long-distance and high-speed locomotion probably became more important to es-cape predators and travel great distances. By contrast, thegreater flexibility provided by multiple toes and shorterlimbs, which was advantageous for ducking through com-plex forest vegetation, was no longer beneficial. At thesame time, horses were eating grasses and other vegetationthat contained more grit and other hard substances, thusfavoring teeth and skulls better suited for withstandingsuch materials.

Evolutionary Trends

For many years, horse evolution was held up as an exampleof constant evolutionary change through time. Some evensaw in the record of horse evolution evidence for a progres-sive, guiding force, consistently pushing evolution to movein a single direction. We now know that such views aremisguided; evolutionary change over millions of years israrely so simple.

Rather, the fossils demonstrate that, although there havebeen overall trends evident in a variety of characteristics,evolutionary change has been far from constant and uni-form through time. Instead, rates of evolution have variedwidely, with long periods of little change and some periodsof great change. Moreover, when changes happen, theyoften occur simultaneously in different lineages of thehorse evolutionary tree. Finally, even when a trend exists,exceptions, such as the evolutionary decrease in body sizeexhibited by some lineages, are not uncommon. These pat-terns, evident in our knowledge of horse evolution, are usu-ally discovered for any group of plants and animals forwhich we have an extensive fossil record, as we shall seewhen we discuss human evolution in chapter 23.

Horse Diversity

One reason that horse evolution was originally conceivedof as linear through time may be that modern horse diver-sity is relatively limited. Thus, it is easy to mentally pic-ture a straight line from Hyracotherium to modern-dayEquus. However, todays limited horse diversityonlyone surviving genusis unusual. Indeed, at the peak ofhorse diversity in the Miocene, as many as 13 genera ofhorses could be found in North America alone. Thesespecies differed in body size and in a wide variety of othercharacteristics. Presumably, they lived in different habi-tats and exhibited different dietary preferences. Had thisdiversity existed to modern times, early workers presum-ably would have had a different outlook on horse evolu-tion, a situation that is again paralleled by the evolution ofhumans.

The extensive fossil record for horses provides adetailed view of the evolutionary diversification of thisgroup from small forest dwellers to the large and fastmodern grassland species.


Hyracotherium

Mesohippus

Merychippus

Pliohippus

Equus

FIGURE 21.7Evolutionary changes in horses through time.

As we saw in chapter 20, a variety of different processes canresult in evolutionary change. Nonetheless, in agreementwith Darwin, most evolutionary biologists would agree thatnatural selection is the process responsible for most of themajor evolutionary changes that have occurred throughtime. Although we cannot travel back through time, a vari-ety of modern-day evidence confirms the power of naturalselection as an agent of evolutionary change. These datacome from both the field and the laboratory and from nat-ural and human-altered situations.

The Beaks of Darwins FinchesDarwins finches are a classic example of evolution by nat-ural selection. Darwin collected 31 specimens of finch fromthree islands when he visited the Galpagos Islands off thecoast of Ecuador in 1835. Darwin, not an expert on birds,had trouble identifying the specimens, believing by examin-ing their bills that his collection contained wrens, gross-beaks, and blackbirds. You can see Darwins sketches offour of these birds in figure 21.8.

The Importance of the Beak

Upon Darwins return to England, ornithologist JohnGould examined the finches. Gould recognized that Dar-wins collection was in fact a closely related group of dis-tinct species, all similar to one another except for theirbills. In all, there were 13 species. The two ground finches

with the larger bills in figure 21.8 feed on seeds that theycrush in their beaks, whereas the two with narrower billseat insects. One species is a fruit eater, another a cactuseater, yet another a vampire that creeps up on seabirdsand uses its sharp beak to drink their blood. Perhaps mostremarkable are the tool users, woodpecker finches that pickup a twig, cactus thorn, or leaf stalk, trim it into shape withtheir bills, and then poke it into dead branches to pry outgrubs.

The correspondence between the beaks of the 13 finchspecies and their food source immediately suggested toDarwin that evolution had shaped them:

Seeing this gradation and diversity of structure in onesmall, intimately related group of birds, one might reallyfancy that from an original paucity of birds in this archi-pelago, one species has been taken and modified for dif-ferent ends.

Was Darwin Wrong?

If Darwins suggestion that the beak of an ancestral finchhad been modified for different ends is correct, then itought to be possible to see the different species of finchesacting out their evolutionary roles, each using their bills toacquire their particular food specialty. The four speciesthat crush seeds within their bills, for example, should feedon different seeds, those with stouter beaks specializing onharder-to-crush seeds.


21.2 Natural selection can produce evolutionary change.

FIGURE 21.8Darwins own sketches of Galpagosfinches. From Darwins Journal ofResearches: (1) large ground finch Geospizamagnirostris; (2) medium ground finchGeospiza fortis; (3) small tree finchCamarhynchus parvulus; (4) warbler finchCerthidea olivacea.

Many biologists visited the Galpagos after Darwin,but it was 100 years before any tried this key test of hishypothesis. When the great naturalist David Lack finallyset out to do this in 1938, observing the birds closely fora full five months, his observations seemed to contradictDarwins proposal! Lack often observed many differentspecies of finch feeding together on the same seeds. Hisdata indicated that the stout-beaked species and theslender-beaked species were feeding on the very samearray of seeds.

We now know that it was Lacks misfortune to study thebirds during a wet year, when food was plentiful. Thefinchs beak is of little importance in such flush times; smallseeds are so abundant that birds of all species are able toget enough to eat. Later work has revealed a very differentpicture during leaner, dry years, when few seeds are avail-able and the difference between survival and starvation de-pends on being able to efficiently gather enough to eat. Insuch times, having beaks designed to be maximally effectivefor a particular type of food becomes critical and thespecies diverge in their diet, each focusing on the type offood to which it is specialized.

A Closer Look

The key to successfully testing Darwins proposal that thebeaks of Galpagos finches are adaptations to different foodsources proved to be patience. Starting in 1973, Peter andRosemary Grant of Princeton University and generationsof their students have studied the medium ground finchGeospiza fortis on a tiny island in the center of the Galpa-gos called Daphne Major. These finches feed preferentiallyon small tender seeds, produced in abundance by plants inwet years. The birds resort to larger, drier seeds, which areharder to crush, only when small seeds become depletedduring long periods of dry weather, when plants producefew seeds.

The Grants quantified beak shape among the mediumground finches of Daphne Major by carefully measuringbeak depth (width of beak, from top to bottom, at its base)on individual birds. Measuring many birds every year, theywere able to assemble for the first time a detailed portraitof evolution in action. The Grants found that beak depthchanged from one year to the next in a predictable fashion.During droughts, plants produced few seeds and all avail-able small seeds quickly were eaten, leaving large seeds asthe major remaining source of food. As a result, birds withlarge beaks survived better, because they were better ableto break open these large seeds. Consequently, the averagebeak depth of birds in the population increased the nextyear, only to decrease again when wet seasons returned(figure 21.9).

Could these changes in beak dimension reflect the ac-tion of natural selection? An alternative possibility mightbe that the changes in beak depth do not reflect changes in

gene frequencies, but rather are simply a response to dietperhaps during lean times the birds become malnourishedand then grow stouter beaks, for example. To rule out thispossibility, the Grants measured the relation of parent billsize to offspring bill size, examining many broods over sev-eral years. The depth of the bill was passed down faithfullyfrom one generation to the next, regardless of environmen-tal conditions, suggesting that the differences in bill size in-deed reflected genetic differences.

Darwin Was Right After All

If the year-to-year changes in beak depth indeed reflect ge-netic changes, as now seems likely, and these changes canbe predicted by the pattern of dry years, then Darwin wasright after allnatural selection does seem to be operatingto adjust the beak to its food supply. Birds with stout beakshave an advantage during dry periods, for they can breakthe large, dry seeds that are the only food available. Whensmall seeds become plentiful once again with the return ofwet weather, a smaller beak proves a more efficient tool forharvesting the more abundant smaller seeds.

Among Darwins finches, natural selection adjusts theshape of the beak in response to the nature of theavailable food supply, adjustments that can be seen tobe occurring even today.


1977 1980 1982 1984

Dry year Dry year Dry year

Wet year

Beak

dep

th

FIGURE 21.9Evidence that natural selection alters beak size in Geospizafortis. In dry years, when only large, tough seeds are available, themean beak size increases. In wet years, when many small seeds areavailable, smaller beaks become more common.

Peppered Moths and IndustrialMelanismWhen the environment changes, natural selection oftenmay favor new traits in a species. The example of the Dar-wins finches clearly indicates how natural variation canlead to evolutionary change. Humans are greatly alteringthe environment in many ways; we should not be surprisedto see organisms attempting to adapt to these new condi-tions. One classic example concerns the peppered moth,Biston betularia. Until the mid-nineteenth century, almostevery individual of this species captured in Great Britainhad light-colored wings with black specklings (hence thename peppered moth). From that time on, individualswith dark-colored wings increased in frequency in themoth populations near industrialized centers until theymade up almost 100% of these populations. Black individu-als had a dominant allele that was present but very rare inpopulations before 1850. Biologists soon noticed that in in-dustrialized regions where the dark moths were common,the tree trunks were darkened almost black by the soot ofpollution. Dark moths were much less conspicuous restingon them than were light moths. In addition, the air pollu-tion that was spreading in the industrialized regions hadkilled many of the light-colored lichens on tree trunks,making the trunks darker.

Selection for Melanism

Can Darwins theory explain the increase in the frequencyof the dark allele? Why did dark moths gain a survival ad-vantage around 1850? An amateur moth collector namedJ. W. Tutt proposed what became the most commonlyaccepted hypothesis explaining the decline of the light-colored moths. He suggested that peppered forms weremore visible to predators on sooty trees that have losttheir lichens. Consequently, birds ate the peppered mothsresting on the trunks of trees during the day. The blackforms, in contrast, were at an advantage because theywere camouflaged (figure 21.10). Although Tutt initiallyhad no evidence, British ecologist Bernard Kettlewelltested the hypothesis in the 1950s by rearing populationsof peppered moths with equal numbers of dark and lightindividuals. Kettlewell then released these populationsinto two sets of woods: one, near heavily polluted Birm-ingham, the other, in unpolluted Dorset. Kettlewell set uprings of traps around the woods to see how many of bothkinds of moths survived. To evaluate his results, he hadmarked the released moths with a dot of paint on the un-derside of their wings, where birds could not see it.

In the polluted area near Birmingham, Kettlewelltrapped 19% of the light moths, but 40% of the dark ones.This indicated that dark moths had a far better chance ofsurviving in these polluted woods, where the tree trunkswere dark. In the relatively unpolluted Dorset woods, Ket-tlewell recovered 12.5% of the light moths but only 6% of

the dark ones. This indicated that where the tree trunkswere still light-colored, light moths had a much betterchance of survival. Kettlewell later solidified his argumentby placing hidden blinds in the woods and actually filmingbirds eating the moths. Sometimes the birds Kettlewell ob-served actually passed right over a moth that was the samecolor as its background.

Industrial Melanism

Industrial melanism is a term used to describe the evolu-tionary process in which darker individuals come to pre-dominate over lighter individuals since the industrial revo-lution as a result of natural selection. The process is widelybelieved to have taken place because the dark organisms arebetter concealed from their predators in habitats that havebeen darkened by soot and other forms of industrial pollu-tion, as suggested by Kettlewells research.


FIGURE 21.10Tutts hypothesis explaining industrial melanism. Thesephotographs show color variants of the peppered moth,Biston betularia. Tutt proposed that the dark moth is morevisible to predators on unpolluted trees (top), while the lightmoth is more visible to predators on bark blackened byindustrial pollution (bottom).

Dozens of other species of moths havechanged in the same way as the pepperedmoth in industrialized areas throughoutEurasia and North America, with darkforms becoming more common from themid-nineteenth century onward as indus-trialization spread.

Selection against Melanism

In the second half of the twentieth cen-tury, with the widespread implementa-tion of pollution controls, these trendsare reversing, not only for the pepperedmoth in many areas in England, but alsofor many other species of mothsthroughout the northern continents.These examples provide some of the bestdocumented instances of changes in al-lelic frequencies of natural populations asa result of natural selection due to specificfactors in the environment.

In England, the pollution promotingindustrial melanism began to reversefollowing enactment of Clean Air legis-lation in 1956. Beginning in 1959, theBiston population at Caldy Commonoutside Liverpool has been sampledeach year. The frequency of the melanic(dark) form has dropped from a high of94% in 1960 to its current (1994) low of 19% (figure21.11). Similar reversals have been documented atnumerous other locations throughout England. The dropcorrelates well with a drop in air pollution, particularlywith tree-darkening sulfur dioxide and suspendedparticulates.

Interestingly, the same reversal of industrial melanismappears to have occurred in America during the same timethat it was happening in England. Industrial melanism inthe American subspecies of the peppered moth was not aswidespread as in England, but it has been well-documentedat a rural field station near Detroit. Of 576 peppered mothscollected there from 1959 to 1961, 515 were melanic, a fre-quency of 89%. The American Clean Air Act, passed in1963, led to significant reductions in air pollution. Resam-pled in 1994, the Detroit field station peppered moth pop-ulation had only 15% melanic moths (see figure 21.11)!The moths in Liverpool and Detroit, both part of the samenatural experiment, exhibit strong evidence of natural se-lection.

Reconsidering the Target of Natural Selection

Tutts hypothesis, widely accepted in the light of Ket-tlewells studies, is currently being reevaluated. The prob-lem is that the recent selection against melanism does not

appear to correlate with changes in tree lichens. At CaldyCommon, the light form of the peppered moth began itsincrease in frequency long before lichens began to reappearon the trees. At the Detroit field station, the lichens neverchanged significantly as the dark moths first became domi-nant and then declined over the last 30 years. In fact, inves-tigators have not been able to find peppered moths on De-troit trees at all, whether covered with lichens or not.Wherever the moths rest during the day, it does not appearto be on tree bark. Some evidence suggests they rest onleaves on the treetops, but no one is sure.

The action of selection may depend less on the presenceof lichens and more on other differences in the environ-ment resulting from industrial pollution. Pollution tends tocover all objects in the environment with a fine layer ofparticulate dust, which tends to decrease how much lightsurfaces reflect. In addition, pollution has a particularly se-vere effect on birch trees, which are light in color. Both ef-fects would tend to make the environment darker and thuswould favor darker color in moths.

Natural selection has favored the dark form of thepeppered moth in areas subject to severe air pollution,perhaps because on darkened trees they are less easilyseen by moth-eating birds. Selection has in turn favoredthe light form as pollution has abated.


Year

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Perc

enta

ge o

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hs

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FIGURE 21.11Selection against melanism. The circles indicate the frequency of melanic Bistonmoths at Caldy Common in England, sampled continuously from 1959 to1995. Diamonds indicate frequencies in Michigan from 1959 to 1962 and from1994 to 1995.Source: Data from Grant, et al., Parallel Rise and Fall of Melanic PepperedMoths in Journal of Heredity, vol. 87, 1996, Oxford University Press.

Artificial SelectionHumans have imposed selection upon plants and animalssince the dawn of civilization. Just as in natural selection,artificial selection operates by favoring individuals with cer-tain phenotypic traits, allowing them to reproduce and passtheir genes into the next generation. Assuming that pheno-typic differences are genetically determined, such selectionshould lead to evolutionary change and, indeed, it has. Arti-ficial selection, imposed in laboratory experiments, agricul-ture, and the domestication process, has produced substan-tial change in almost every case in which it has beenapplied. This success is strong proof that selection is an ef-fective evolutionary process.

Laboratory Experiments

With the rise of genetics as a field of science in the 1920sand 1930s, researchers began conducting experiments totest the hypothesis that selection can produce evolutionarychange. A favorite subject was the now-famous laboratoryfruit fly, Drosophila melanogaster. Geneticists have imposedselection on just about every conceivable aspect of the fruitflyincluding body size, eye color, growth rate, life span,and exploratory behaviorwith a consistent result: selec-tion for a trait leads to strong and predictable evolutionaryresponse.

In one classic experiment, scientists selected for fruitflies with many bristles (stiff, hairlike structures) on theirabdomen. At the start of the experiment, the average num-ber of bristles was 9.5. Each generation, scientists pickedout the 20% of the population with the greatest number ofbristles and allowed them to reproduce, thus establishingthe next generation. After 86 generations of such selection,the average number of bristles had quadrupled, to nearly40. In a similar experiment, fruit flies were selected for ei-ther the most or the fewest numbers of bristles. Within 35generations, the populations did not overlap at all in rangeof variation (figure 21.12).

Similar experiments have been conducted on a wide va-riety of other laboratory organisms. For example, by select-ing for rats that were resistant to tooth decay, scientistswere able to increase in less than 20 generations the aver-age time for onset of decay from barely over 100 days togreater than 500 days.

Agriculture

Similar methods have been practiced in agriculture formany centuries. Familiar livestock, such as cattle and pigs,and crops, like corn and strawberries, are greatly differentfrom their wild ancestors (figure 21.13). These differenceshave resulted from generations of selection for desirabletraits like milk production and corn stalk size.

An experimental study with corn demonstrates the abil-ity of artificial selection to rapidly produce major change in

crop plants. In 1896, agricultural scientists began selectingon oil content of corn kernels, which initially was 4.5%. Asin the fruit fly experiments, the top 20% of all individualswere allowed to reproduce. In addition, a parallel experi-ment selected for the individuals with the lowest oil con-tent. By 1986, at which time 90 generations had passed, av-erage oil content had increased approximately 450% in thehigh-content experiment; by contrast, oil content in thelow experiment had decreased to about 0.5%, a level atwhich it is difficult to get accurate readings.


Mea

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Bristle number in Drosophila0 10 20 30 40 50 60 70 80 90 100 110

Num

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ofi

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Initialpopulation

FIGURE 21.12Artificial selection in the laboratory. In this experiment, onepopulation of Drosophila was selected for low numbers ofbristles and the other for high numbers. Note that not onlydid the means of the populations change greatly in 35generations, but also that all individuals in both experimentalpopulations lie outside the range of the initial population.

Teosinte Intermediates Modern corn

FIGURE 21.13Corn looks very different from its ancestor. The tassels andseeds of a wild grass, such as teosinte, evolved into the maletassels and female ears of modern corn.

Domestication

Artificial selection has also been responsiblefor the great variety of breeds of cats, dogs(figure 21.14), pigeons, cattle and other do-mestic animals. In some cases, breeds havebeen developed for particular purposes. Grey-hound dogs, for example, were bred by select-ing for maximal running abilities, with the endresult being an animal with long legs and tail(the latter used as a rudder), an arched back (toincrease the length of its stride), and greatmuscle mass. By contrast, the odd proportionsof the ungainly basset hound resulted from se-lection for dogs that could enter narrow holesin pursuit of rabbits and other small game. Inother cases, breeds have been developed pri-marily for their appearance, such as the manycolorful and ornamented varieties of pigeonsor the breeds of cats.

Domestication also has led to unintentionalselection for some traits. In recent years, aspart of an attempt to domesticate the silverfox, Russian scientists each generation havechosen the most docile animals and allowedthem to reproduce. Within 40 years, the vastmajority of foxes born were exceptionallydocile, not only allowing themselves to be pet-ted, but also whimpering to get attention andsniffing and licking their caretakers. In manyrespects, they had become no different thandomestic dogs! However, it was not only be-havior that changed. These foxes also began to exhibit dif-ferent color patterns, floppy ears, curled tails, and shorterlegs and tails. Presumably, the genes responsible for docilebehavior have other effects as well (the phenomenon ofpleiotropy discussed in the last chapter); as selection has fa-vored docile animals, it has also led to the evolution ofthese other traits.

Can Selection Produce Major EvolutionaryChanges?

Given that we can observe the results of selection operatingover relatively short periods of time, most scientists believethat natural selection is the process responsible for the evo-lutionary changes documented in the fossil record. Somecritics of evolution accept that selection can lead to changeswithin a species, but contend that such changes are rela-tively minor in scope and not equivalent to the substantialchanges documented in the fossil record. In other words, itis one thing to change the number of bristles on a fruit flyor the size of a corn stalk, and quite another to produce anentirely new species.

This argument does not fully appreciate the extent ofchange produced by artificial selection. Consider, for ex-

ample, the breeds of dogs, all of which have been pro-duced since wolves were first domesticated, perhaps10,000 years ago. If the various dog breeds did not existand a paleontologist found fossils of animals similar todachshunds, greyhounds, mastiffs, Chihuahuas, andpomeranians, there is no question that they would be con-sidered different species. Indeed, these breeds are so dif-ferent that they would probably be classified in differentgenera. In fact, the diversity exhibited by dog breeds faroutstrips the differences observed among wild members ofthe family Canidaesuch as coyotes, jackals, foxes, andwolves. Consequently, the claim that artificial selectionproduces only minor changes is clearly incorrect. Indeed,if selection operating over a period of only 10,000 yearscan produce such substantial differences, then it wouldseem powerful enough, over the course of many millionsof years, to produce the diversity of life we see around ustoday.

Artificial selection often leads to rapid and substantialresults over short periods of time, thus demonstratingthe power of selection to produce major evolutionarychange.


Greyhound

Mastiff

Dachshund

Chihuahua

FIGURE 21.14Breeds of dogs. The differences between these dogs are greater than thedifferences displayed between any wild species of canids.

The AnatomicalRecordMuch of the power of the theory ofevolution is its ability to provide asensible framework for understandingthe diversity of life. Many observa-tions from a wide variety of fields ofbiology simply cannot be understoodin any meaningful way except as a re-sult of evolution.

Homology

As vertebrates evolved, the samebones were sometimes put to differ-ent uses. Yet the bones are still seen,their presence betraying their evolu-tionary past. For example, the fore-limbs of vertebrates are all homolo-gous structures, that is, structureswith different appearances and func-tions that all derived from the samebody part in a common ancestor. Youcan see in figure 21.15 how the bonesof the forelimb have been modifiedin different ways for different verter-bates. Why should these very differ-ent structures be composed of thesame bones? If evolution had not oc-curred, this would indeed be a riddle.But when we consider that all ofthese animals are descended from acommon ancestor, it is easy to under-stand that natural selection has modi-fied the same initial starting blocks toserve very different purposes.

Development

Some of the strongest anatomical evi-dence supporting evolution comesfrom comparisons of how organismsdevelop. In many cases, the evolu-tionary history of an organism can beseen to unfold during its develop-ment, with the embryo exhibitingcharacteristics of the embryos of itsancestors (figure 21.16). For example,early in their development, human embryos possess gillslits, like a fish; at a later stage, every human embryo has along bony tail, the vestige of which we carry to adulthoodas the coccyx at the end of our spine. Human fetuses even

possess a fine fur (called lanugo) during the fifth month ofdevelopment. These relict developmental forms suggeststrongly that our development has evolved, with new in-structions layered on top of old ones.


21.3 Evidence for evolution can be found in other fields of biology.

Human Cat Bat Porpoise Horse

FIGURE 21.15Homology among the bones of the forelimb. Although these structures showconsiderable differences in form and function, the same basic bones are present inthe forelimbs of humans, cats, bats, porpoises, and horses.

Gill slits

Tail

Fish Reptile Bird Human

Tail

Gill slits

FIGURE 21.16Our embryos show our evolutionary history. The embryos of various groups ofvertebrate animals show the features they all share early in development, such asgill slits (in purple) and a tail.

The observation that seeminglydifferent organisms may exhibitsimilar embryological forms pro-vides indirect but convincing evi-dence of a past evolutionary rela-tionship. Slugs and giant oceansquids, for example, do not bearmuch superficial resemblance toeach other, but the similarity oftheir embryological forms pro-vides convincing evidence thatthey are both mollusks.

Vestigial Structures

Many organisms possess vestigialstructures that have no apparentfunction, but that resemble struc-tures their presumed ancestorshad. Humans, for example, possessa complete set of muscles for wig-gling their ears, just as a coyotedoes (table 21.1). Boa constrictorshave hip bones and rudimentary hind legs. Manatees (atype of aquatic mammal often referred to as sea cows)have fingernails on their fins (which evolved from legs).Figure 21.17 illustrates the skeleton of a baleen whale,which contains pelvic bones, as other mammal skeletonsdo, even though such bones serve no known function in thewhale. The human vermiform appendix is apparently vesti-gial; it represents the degenerate terminal part of thececum, the blind pouch or sac in which the large intestinebegins. In other mammals such as mice, the cecum is thelargest part of the large intestine and functions in storageusually of bulk cellulose in herbivores. Although some sug-gestions have been made, it is difficult to assign any currentfunction to the vermiform appendix. In many respects, it isa dangerous organ: quite often it becomes infected, leadingto an inflammation called appendicitis; without surgical re-

moval, the appendix may burst, allowing the contents ofthe gut to come in contact with the lining of the body cav-ity, a potentially fatal event. It is difficult to understand ves-tigial structures such as these as anything other than evolu-tionary relicts, holdovers from the evolutionary past. Theyargue strongly for the common ancestry of the members ofthe groups that share them, regardless of how differentthey have subsequently become.

Comparisons of the anatomy of different living animalsoften reveal evidence of shared ancestry. In someinstances, the same organ has evolved to carry outdifferent functions, in others, an organ loses its functionaltogether. Sometimes, different organs evolve insimilar ways when exposed to the same selectivepressures.


FIGURE 21.17Vestigial features. The skeleton of a baleen whale, a representative of the group ofmammals that contains the largest living species, contains pelvic bones. These bonesresemble those of other mammals, but are only weakly developed in the whale and haveno apparent function.

Table 21.1 Some Vestigial Traits in Humans

Trait Description

Ear-wiggling muscles Three small muscles around each ear that are large and important in some mammals, such as dogs, turning the ears toward a source of sound. Few people can wiggle their ears, and none can turn them toward sound.

Tail Present in human and all vertebrate embryos. In humans, the tail is reduced; most adults only have three to five tiny tail bones and, occasionally, a trace of a tail-extending muscle.

Appendix Structure which presumably had a digestive function in some of our ancestors, like the cecum of some herbivores. In humans, it varies in length from 515 cm, and some people are born without one.

Wisdom teeth Molars that are often useless and sometimes even trapped in the jawbone. Some people never develop wisdom teeth.

Based on a suggestion by Dr. Leslie Dendy, Department of Science and Technology, University of New Mexico, Los Alamos.

The Molecular RecordTraces of our evolutionary past arealso evident at the molecular level. Ifyou think about it, the fact that organ-isms have evolved successively fromrelatively simple ancestors implies thata record of evolutionary change is pre-sent in the cells of each of us, in ourDNA. When an ancestral species givesrise to two or more descendants, thosedescendants will initially exhibit fairlyhigh overall similarity in their DNA.However, as the descendants evolve in-dependently, they will accumulatemore and more differences in theirDNA. Consequently, organisms thatare more distantly related would be ex-pected to accumulate a greater numberof evolutionary differences, whereastwo species that are more closely re-lated should share a greater portion oftheir DNA.

To examine this hypothesis, weneed an estimate of evolutionary rela-tionships that has been developedfrom data other than DNA (it wouldbe a circular argument to use DNA toestimate relationships and then con-clude that closely related species aremore similar in their DNA than aredistantly related species). Such an hypothesis of evolu-tionary relationships is provided by the fossil record,which indicates when particular types of organismsevolved. In addition, by examining the anatomical struc-tures of fossils and of modern species, we can infer howclosely species are related to each other.

When degree of genetic similarity is compared withour ideas of evolutionary relationships based on fossils, aclose match is evident. For example, when the human he-moglobin polypeptide is compared to the correspondingmolecule in other species, closely related species arefound to be more similar. Chimpanzees, gorillas, orang-utans, and macaques, vertebrates thought to be moreclosely related to humans, have fewer differences fromhumans in the 146-amino-acid hemoglobin chain thando more distantly related mammals, like dogs. Nonmam-malian vertebrates differ even more, and nonvertebratehemoglobins are the most different of all (figure 21.18).Similar patterns are also evident when the DNA itself iscompared. For example, chimps and humans, which arethought to have descended from a common ancestor thatlived approximately 6 million years ago, exhibit few differ-ences in their DNA.

Why should closely related species be similar in DNA?Because DNA is the genetic code that produces the struc-ture of living organisms, one might expect species that aresimilar in overall appearance and structure, such as humansand chimpanzees, to be more similar in DNA than aremore dissimilar species, such as humans and frogs. This ex-pectation would hold true even if evolution had not oc-curred. However, there are some noncoding stretches ofDNA (sometimes called junk DNA) that have no func-tion and appear to serve no purpose. If evolution had notoccurred, there would be no reason to expect similar-appearing species to be similar in their junk DNA. How-ever, comparisons of such stretches of DNA provide thesame results as for other parts of the genome: more closelyrelated species are more similar, an observation that onlymakes sense if evolution has occurred.

Comparison of the DNA of different species providesstrong evidence for evolution. Species deduced fromthe fossil record to be closely related are more similarin their DNA than are species thought to be moredistantly related.


Number of amino acid differences between this hemoglobin polypeptide and a human one 10 20 30 40 50 60 70

67

125

45

32

8

80 90 100 110 120

Tim

e

LampreyFrogBirdDogMacaqueHuman

FIGURE 21.18Molecules reflect evolutionary divergence. You can see that the greater theevolutionary distance from humans (white cladogram), the greater the number ofamino acid differences in the vertebrate hemoglobin polypeptide.

Convergent andDivergent EvolutionDifferent geographical areas some-times exhibit groups of plants and an-imals of strikingly similar appearance,even though the organisms may beonly distantly related. It is difficult toexplain so many similarities as the re-sult of coincidence. Instead, naturalselection appears to have favored par-allel evolutionary adaptations in simi-lar environments. Because selectionin these instances has tended to favorchanges that made the two groupsmore alike, their phenotypes haveconverged. This form of evolutionarychange is referred to as convergentevolution, or sometimes, parallelevolution.

The Marsupial-PlacentalConvergence

In the best-known case of conver-gent evolution, two major groups ofmammals, marsupials and placentals,have evolved in a very similar way,even though the two lineages havebeen living independently on sepa-rate continents. Australia separatedfrom the other continents more than50 million years ago, after marsupi-als had evolved but before the ap-pearance of placental mammals. As aresult, the only mammals in Aus-tralia (other than bats and a few col-onizing rodents) have been marsupi-als, members of a group in which theyoung are born in a very immaturecondition and held in a pouch untilthey are ready to emerge into theoutside world. Thus, even thoughplacental mammals are the dominant mammalian groupthroughout most of the world, marsupials retained su-premacy in Australia.

What are the Australian marsupials like? To an aston-ishing degree, they resemble the placental mammals livingtoday on the other continents (figure 21.19). The similaritybetween some individual members of these two sets ofmammals argues strongly that they are the result of conver-gent evolution, similar forms having evolved in different,isolated areas because of similar selective pressures in simi-lar environments.

Homology versus Analogy

How do we know when two similar characters are homolo-gous and when they are analogous? As we have seen, adap-tation favoring different functions can obscure homologies,while convergent evolution can create analogues that ap-pear as similar as homologues. There is no hard-and-fastanswer to this question; the determination of homologuesis often a thorny issue in biological classification. As wehave seen in comparing vertebrate embryos, and again incomparing slugs and squids, studies of embryonic develop-ment often reveal features not apparent when studyingadult organisms. In general, the more complex two struc-tures are, the less likely they evolved independently.


Niche Placental Mammals Australian Marsupials

Burrower

Mole

Lesser anteater

Mouse

Lemur

Flying squirrel

Ocelot

WolfTasmanianwolf

Tasmanian "tiger cat"

Flying phalanger

Spottedcuscus

Numbat (anteater)

Marsupial mole

Marsupialmouse

Anteater

Mouse

Climber

Glider

Cat

Wolf

FIGURE 21.19Convergent evolution. Marsupials in Australia resemble placental mammals in therest of the world. They evolved in isolation after Australia separated from othercontinents.

Darwin and Patterns of Recent Divergence

Darwin was the first to present evidence that animals andplants living on oceanic islands resemble most closely theforms on the nearest continenta relationship that onlymakes sense as reflecting common ancestry. The Galpagosturtle in figure 21.20 is more similar to South Americanturtles than to those of any other continent. This kind ofrelationship strongly suggests that the island forms evolvedfrom individuals that came from the adjacent mainland atsome time in the past. Thus, the Galpagos finches of fig-ure 21.8 have different beaks than their South Americanrelatives. In the absence of evolution, there seems to be nological explanation of why individual kinds of island plantsand animals would be clearly related to others on the near-est mainland, but still have some divergent features. AsDarwin pointed out, this relationship provides strong evi-dence that macroevolution has occurred.

A similar resemblance to mainland birds can be seen inan island finch Darwin never sawa solitary finch speciesliving on Cocos Island, a tiny, remote volcanic island lo-cated 630 kilometers to the northeast of the Galpagos.This finch does not resemble the finches of Europe, Aus-tralia, Africa, or North America. Instead, it resembles thefinches of Costa Rica, 500 kilometers to the east.

Of course, because of adaptation to localized habitats, is-land forms are not identical to those on the nearby conti-nents. The turtles have evolved different shell shapes, forexample; those living in moist habitats have dome-shaped

shells while others living in dry places have low, saddle-backed shells with the front of the shell bent up to exposethe head and neck. Similarly, the Galpagos finches haveevolved from a single presumptive ancestor into 13 species,each specialized in a different way. These Galpagos turtlesand finches have evolved in concert with the continentalforms, from the same ancestors, but the two lineages havediverged rather than converged.

It is fair to ask how Darwin knew that the Galpagostortoises and finches do not represent the convergence ofunrelated island and continental forms (analogues) ratherthan the divergence of recently isolated groups (homo-logues). While either hypothesis would argue for naturalselection, Darwin chose divergence of homologues as by farthe simplest explanation, because the turtles and finchesdiffer by only a few traits, and are similar in many.

In sum total, the evidence for macroevolution is over-whelming. In the next chapter, we will consider Darwinsproposal that microevolutionary changes have led directlyto macroevolutionary changes, the key argument in his the-ory that evolution occurs by natural selection.

Evolution favors similar forms under similarcircumstances. Convergence is the evolution of similarforms in different lineages when exposed to the sameselective pressures. Divergence is the evolution ofdifferent forms in the same lineage when exposed todifferent selective pressures.


FIGURE 21.20A Galpagos tortoise most closely resembles South American tortoises. Isolated on these remote islands, the Galpagos tortoisehas evolved distinctive forms. This natural experiment is being terminated, however. Since Darwins time, much of thenatural habitat of the larger islands has been destroyed by human intrusion. Goats introduced by settlers, for example, havedrastically altered the vegetation.

Darwins CriticsIn the century since he proposed it, Darwin's theory ofevolution by natural selection has become nearly univer-sally accepted by biologists, but has proven controversialamong the general public. Darwin's critics raise seven prin-cipal objections to teaching evolution:

1. Evolution is not solidly demonstrated. Evolutionis just a theory, Darwin's critics point out, as if theorymeant lack of knowledge, some kind of guess. Scien-tists, however, use the word theory in a very differentsense than the general public does. Theories are thesolid ground of science, that of which we are mostcertain. Few of us doubt the theory of gravity becauseit is "just a theory."

2. There are no fossil intermediates. No one eversaw a fin on the way to becoming a leg, critics claim,pointing to the many gaps in the fossil record in Dar-win's day. Since then, however, most fossil intermedi-ates in vertebrate evolution have indeed been found.A clear line of fossils now traces the transition be-tween whales and hoofed mammals, between reptilesand mammals, between dinosaurs and birds, betweenapes and humans. The fossil evidence of evolutionbetween major forms is compelling.

3. The intelligent design argument. The organs ofliving creatures are too complex for a random process tohave producedthe existence of a clock is evidence of theexistence of a clockmaker. Biologists do not agree.The intermediates in the evolution of the mam-malian ear can be seen in fossils, and many interme-diate eyes are known in various invertebrates.These intermediate forms arose because they havevaluebeing able to detect light a little is betterthan not being able to detect it at all. Complexstructures like eyes evolved as a progression of slightimprovements.

4. Evolution violates the Second Law of Thermody-namics. A jumble of soda cans doesn't by itself jumpneatly into a stackthings become more disorganized dueto random events, not more organized. Biologists pointout that this argument ignores what the second lawreally says: disorder increases in a closed system,which the earth most certainly is not. Energy contin-ually enters the biosphere from the sun, fueling lifeand all the processes that organize it.

5. Proteins are too improbable. Hemoglobin has 141amino acids. The probability that the first one would beleucine is 1/20, and that all 141 would be the ones they areby chance is (1/20)141, an impossibly rare event. This isstatistical foolishnessyou cannot use probability toargue backwards. The probability that a student in aclassroom has a particular birthday is 1/365; arguing

this way, the probability that everyone in a class of 50would have the birthdays they do is (1/365)50, and yetthere the class sits.

6. Natural selection does not imply evolution. Noscientist has come up with an experiment where fish evolveinto frogs and leap away from predators. Is microevolu-tion (evolution within a species) the mechanism thathas produced macroevolution (evolution amongspecies)? Most biologists that have studied the prob-lem think so. Some kinds of animals produced by ar-tificial selection are remarkably distinctive, such asChihuahuas, dachshunds, and greyhounds. While alldogs are in fact the same species and can interbreed,laboratory selection experiments easily create formsthat cannot interbreed and thus would in nature beconsidered different species. Thus, production of rad-ically different forms has indeed been observed, re-peatedly. To object that evolution still does not ex-plain really major differences, like between fish andamphibians, simply takes us back to point 2thesechanges take millions of years, and are seen clearly inthe fossil record.

7. The irreducible complexity argument. The in-tricate molecular machinery of the cell cannot be ex-plained by evolution from simpler stages. Because eachpart of a complex cellular process like blood clotting is es-sential to the overall process, how can natural selectionfashion any one part? What's wrong with this argu-ment is that each part of a complex molecular ma-chine evolves as part of the system. Natural selectioncan act on a complex system because at every stageof its evolution, the system functions. Parts that im-prove function are added, and, because of laterchanges, become essential. The mammalian bloodclotting system, for example, has evolved from muchsimpler systems. The core clotting system evolved atthe dawn of the vertebrates 600 million years ago,and is found today in lampreys, the most primitivefish. One hundred million years later, as vertebratesevolved, proteins were added to the clotting systemmaking it sensitive to substances released from dam-aged tissues. Fifty million years later, a third compo-nent was added, triggering clotting by contact withthe jagged surfaces produced by injury. At eachstage as the clotting system evolved to become morecomplex, its overall performance came to depend onthe added elements. Thus, blood clotting has be-come "irreducibly complex"as the result of Dar-winian evolution.

Darwins theory of evolution has proven controversialamong the general public, although the commonlyraised objections are without scientific merit.




Chapter 21Summary Questions Media Resources

21.1 Fossil evidence indicates that evolution has occurred.

Fossils of many extinct species have never beendiscovered. Nonetheless, the fossil record is completeenough to allow a detailed understanding of theevolution of life through time. The evolution of themajor vertebrate groups is quite well known.

Although evolution of groups like horses may appearto be a straight-line progression, in fact there havebeen many examples of parallel evolution, and evenreversals from overall trends.

1. Why do gaps exist in the fossilrecord? What lessons can belearned from the fossil record ofhorse evolution?2. How did scientists date fossilsin Darwins day? Why arescientists today able to daterocks more accurately?

Natural populations provide clear evidence ofevolutionary change.

Darwins finches have different-sized beaks, whichare adaptations to eating different kinds of seeds. Inparticularly dry years, natural selection favors birdswith stout beaks within one species, Geospiza fortis. Asa result, the average bill size becomes larger in thenext generation.

The British populations of the peppered moth, Bistonbetularia, consisted mostly of light-colored individualsbefore the Industrial Revolution. Over the last twocenturies, populations that occur in heavily pollutedareas, where the tree trunks are darkened with soot,have come to consist mainly of dark-colored(melanic) individualsa result of rapid naturalselection.

3. Why did the average beak sizeof the medium ground finchincrease after a particularly dryyear?4. Why did the frequency oflight-colored moths decreaseand that of dark-colored mothsincrease with the advent ofindustrialism? What is industrialmelanism?5. What can artificial selectiontell us about evolution? Isartificial selection a goodanalogy for the selection thatoccurs in nature?

21.2 Natural selection can produce evolutionary change.

Several indirect lines of evidence argue thatmacroevolution has occurred, including successivechanges in homologous structures, developmentalpatterns, vestigial structures, parallel patterns ofevolution, and patterns of distribution.

When differences in genes or proteins are examined,species that are thought to be closely related based onthe fossil record may be more similar than speciesthought to be distantly related.

6. What is homology? How doesit support evolutionary theory?7. What is convergentevolution? Give examples.8. How did Darwins studies ofisland populations provideevidence for evolution?

21.3 Evidence for evolution can be found in other fields of biology.

The objections raised by Darwins critics are easilyanswered.

9. Is Darwinism really science?Explain.


On Science Article:featherd Dinosaurs

Book Reviews: In Search of Deep Time

by Gee Digging Dinosaurs by

Horner

Activity: Evolution ofFish

Exploration:Evolution of theHeart

Molecular Clock

Activity: Divergence

Student Research:Evolution of InsectDiets

On Science Articles: Darwinism at the

Cellular Level Was Darwin Wrong?

On Science Article:AnsweringEvolutions Critics

Bioethics Case Study:Creationism

Book Reviews: Mr. Darwins Shooter

by McDonald

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