《生物学》课程参考书籍文献（英文版，Chapter 20 - Chapter 31）Part VI Evolution、Part VII Ecology and Behavior、Part VIII The Global Environment

21.2 Natural selection can produce evolutionary change As we saw in chapter 20, a variety of different processes can with the larger bills in figure 21.8 feed on seeds that they result in evolutionary change. Nonetheless, in agreement crush in their beaks, whereas the two with narrower bills with Darwin, most evolutionary biologists would agree that eat insects. One specie a fruit eater. another a cactus natural selection is the process responsible for most of the eater, yet another a"vampire"that creeps up on seabirds major evolutionary changes that have occurred through and uses its sharp beak to drink their blood. Perhaps most time. Although we cannot travel back through time, a vari- remarkable are the tool users, woodpecker finches that pick ety of modern-day evidence confirms the power of natural up a twig, cactus thorn, or leaf stalk, trim it into sh selection as an agent of evolutionary change. These data their bills, and then poke it into dead branches to pry out come from both the field and the laboratory and from nat- ural and human-altered situations The correspondence between the beaks of the 13 finch species and their food source immediately suggested to The beaks of darwin's finches Darwin that evolution had shaped them Darwins finches are a classic example of evolution by nat- Seeing this gradation and diversity of structure in one ural selection. Darwin collected 3 1 specimens of finch from small, intimately related group of birds, one might really hree islands when he visited the galapagos Islands off the fancy that from an original paucity of birds in this archi- coast of Ecuador in 1835. Darwin, not an expert on birds, pelago, one species has been taken and modified for dif- ferent ends.” had trouble identifying the specimens, believing by ng their bills that his collection contained wrens, "gross beaks, "and blackbirds. You can see Darwin's sketches of Was Darwin Wrong four of these birds in figure 21.8 If Darwins suggestion that the beak of an ancestral finch had been "modified for different ends"is correct then it The Importance of the Beak ought to be possible to see the different species of finches Upon Darwins return to England, ornithologist John cting out their evolutionary roles, each using their bills to Gould examined the finches. Gould recognized that Dar- acquire their particular food specialty. The four species win's collection was in fact a closely related group of dis that crush seeds within their bills, for example, should feed on different seeds, those with stouter beaks specializing on tinct species, all similar to one another except for their harder-to-crush seeds bills. In all, there were 13 species. The two ground finches FIGURE 21.8 Darwin,'s own sketches of galapagos finches. From Darwin's Journal of Researches: (1) large ground finch Geospiza d finch spiza fortis; (3)small tree finch 3 Camarbyncbus parvulus:(+)warbler finch Certhidea olivacea 444 Part vi evolution

As we saw in chapter 20, a variety of different processes can result in evolutionary change. Nonetheless, in agreement with Darwin, most evolutionary biologists would agree that natural selection is the process responsible for most of the major evolutionary changes that have occurred through time. Although we cannot travel back through time, a vari￾ety of modern-day evidence confirms the power of natural selection as an agent of evolutionary change. These data come from both the field and the laboratory and from nat￾ural and human-altered situations. The Beaks of Darwin’s Finches Darwin’s finches are a classic example of evolution by nat￾ural selection. Darwin collected 31 specimens of finch from three islands when he visited the Galápagos Islands off the coast 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 Darwin’s sketches of four of these birds in figure 21.8. The Importance of the Beak Upon Darwin’s return to England, ornithologist John Gould examined the finches. Gould recognized that Dar￾win’s collection was in fact a closely related group of dis￾tinct species, all similar to one another except for their bills. In all, there were 13 species. The two ground finches with the larger bills in figure 21.8 feed on seeds that they crush in their beaks, whereas the two with narrower bills eat insects. One species is a fruit eater, another a cactus eater, yet another a “vampire” that creeps up on seabirds and uses its sharp beak to drink their blood. Perhaps most remarkable are the tool users, woodpecker finches that pick up a twig, cactus thorn, or leaf stalk, trim it into shape with their bills, and then poke it into dead branches to pry out grubs. The correspondence between the beaks of the 13 finch species and their food source immediately suggested to Darwin that evolution had shaped them: “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy 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 Darwin’s suggestion that the beak of an ancestral finch had been “modified for different ends” is correct, then it ought to be possible to see the different species of finches acting out their evolutionary roles, each using their bills to acquire their particular food specialty. The four species that crush seeds within their bills, for example, should feed on different seeds, those with stouter beaks specializing on harder-to-crush seeds. 444 Part VI Evolution 21.2 Natural selection can produce evolutionary change. FIGURE 21.8 Darwin’s own sketches of Galápagos finches. From Darwin’s Journal of Researches: (1) large ground finch Geospiza magnirostris; (2) medium ground finch Geospiza fortis; (3) small tree finch Camarhynchus parvulus; (4) warbler finch Certhidea olivacea

Many biologists visited the Galapagos after Darwin, but it was 100 years before any tried this key test of his hypothesis. When the great naturalist David Lack finally set out to do this in 1938, observing the birds closely for a full five months. his observations seemed to contradict Darwin's proposal! Lack often observed many different let year pecies of finch feeding together on the same seeds. His data indicated that the stout-beaked species and the/a/ slender-beaked species were feeding on the very same array of seeds Dry year Dry year Dry year We now know that it was Lack's misfortune to study the birds during a wet year, when food was plentiful. The finch's beak is of little importance in such flush times; small seeds are so abundant that birds of all species are able to get enough to eat. Later work has revealed a very different picture during leaner, dry years, when few seeds are avail able and the difference between survival and starvation de- 1980 pends on being able to efficiently gather enough to eat. In such times, having beaks designed to be maximally effective for a particular type of food becomes critical and the FIGURE 21.9 species diverge in their diet, each focusing on the type of f Evidence that natural selection alters beak size in Geospiza food to which it is specialized fortis. In dry years, when only large, tough seeds are available, the mean beak size increases. In wet years, when many small seeds are available. smaller beaks become more common. A Closer look beaks of galapagos finches are adaptations to different food sources proved to be patience. Starting in 1973, Peter and gene frequencies, but rather are simply a response to diet- Rosemary Grant of Princeton University and generations perhaps during lean times the birds become malnourished of their students have studied the medium ground finch and then grow stouter beaks, for example. To rule out this Geospiza fortis on a tiny island in the center of the Galapa- possibility, the grants measured the relation of parent bill gos called Daphne Major. These finches feed preferentially size to offspring bill size, examining many broods over sev on small tender seeds, produced in abundance by plants in eral years. The depth of the bill was passed down faithfully wet years. The birds resort to larger, drier seeds, which are from one generation to the next, regardless of environmen harder to crush, only when small seeds become depleted tal conditions, suggesting that the differences in bill size in- during long periods of dry weather, when plants produce deed reflected genetic differences few seeds The grants quantified beak shape among the medium ground finches of Daphne Major by carefully measuri Darwin Was Right After All on individual birds. Measuring many birds every year, they netic changes, as now seems likely, and these changes can were able to assemble for the first time a detailed portrait be predicted by the pattern of dry years, then Darwin was of evolution in action. The Grants found that beak depth right after all-natural selection does seem to be operating changed from one year to the next in a predictable fashion. to adjust the beak to its food supply. Birds with stout beaks During droughts, plants produced few seeds and all avail- have an advantage durin le small seeds quickly were eaten, leaving large seeds as the large, dry seeds that are the only food available. When he major remaining source of food. As a result, birds with small seeds become plentiful once again with the return of large beaks survived better, because they were better able wet weather, a smaller beak proves a more efficient tool for to break open these large seeds. Consequently, the average harvesting the more abundant smaller seeds beak depth of birds in the population increased the next year, only to decrease again when wet seasons returned (figure 21.9) Among Darwins finches, natural selection adjusts the Could these changes in beak dimension reflect the ac- shape of the beak in response to the nature of the tion of natural selection? An alternative possibility might available food supply, adjustments that can be seen to be that the changes in beak depth do not reflect changes be occurring even today. Chapter 21 The Evidence for Evolution 445

Many biologists visited the Galápagos after Darwin, but it was 100 years before any tried this key test of his hypothesis. When the great naturalist David Lack finally set out to do this in 1938, observing the birds closely for a full five months, his observations seemed to contradict Darwin’s proposal! Lack often observed many different species of finch feeding together on the same seeds. His data indicated that the stout-beaked species and the slender-beaked species were feeding on the very same array of seeds. We now know that it was Lack’s misfortune to study the birds during a wet year, when food was plentiful. The finch’s beak is of little importance in such flush times; small seeds are so abundant that birds of all species are able to get enough to eat. Later work has revealed a very different picture 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. In such times, having beaks designed to be maximally effective for a particular type of food becomes critical and the species diverge in their diet, each focusing on the type of food to which it is specialized. A Closer Look The key to successfully testing Darwin’s proposal that the beaks of Galápagos finches are adaptations to different food sources proved to be patience. Starting in 1973, Peter and Rosemary Grant of Princeton University and generations of their students have studied the medium ground finch Geospiza fortis on a tiny island in the center of the Galápa￾gos called Daphne Major. These finches feed preferentially on small tender seeds, produced in abundance by plants in wet years. The birds resort to larger, drier seeds, which are harder to crush, only when small seeds become depleted during long periods of dry weather, when plants produce few seeds. The Grants quantified beak shape among the medium ground finches of Daphne Major by carefully measuring beak depth (width of beak, from top to bottom, at its base) on individual birds. Measuring many birds every year, they were able to assemble for the first time a detailed portrait of evolution in action. The Grants found that beak depth changed 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 as the major remaining source of food. As a result, birds with large beaks survived better, because they were better able to break open these large seeds. Consequently, the average beak depth of birds in the population increased the next year, 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 might be that the changes in beak depth do not reflect changes in gene frequencies, but rather are simply a response to diet— perhaps during lean times the birds become malnourished and then grow stouter beaks, for example. To rule out this possibility, the Grants measured the relation of parent bill size to offspring bill size, examining many broods over sev￾eral years. The depth of the bill was passed down faithfully from 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 can be predicted by the pattern of dry years, then Darwin was right after all—natural selection does seem to be operating to adjust the beak to its food supply. Birds with stout beaks have an advantage during dry periods, for they can break the large, dry seeds that are the only food available. When small seeds become plentiful once again with the return of wet weather, a smaller beak proves a more efficient tool for harvesting the more abundant smaller seeds. Among Darwin’s finches, natural selection adjusts the shape of the beak in response to the nature of the available food supply, adjustments that can be seen to be occurring even today. Chapter 21 The Evidence for Evolution 445 1977 1980 1982 1984 Dry year Dry year Dry year Wet year Beak depth FIGURE 21.9 Evidence that natural selection alters beak size in Geospiza fortis. In dry years, when only large, tough seeds are available, the mean beak size increases. In wet years, when many small seeds are available, smaller beaks become more common

Peppered Moths and Industrial Melanism When the environment changes, natural selection often may favor new traits in a species. The example of the Dar- vin's finches clearly indicates how natural variation can lead to evolutionary change. Humans are greatly altering the environment in many ways; we should not be surprised to see organisms attempting to adapt to these new condi- ons. One classic example concerns the peppered moth, Biston betularia. Until the mid-nineteenth century, almost every individual of this species captured in Great Britain ad light-colored wings with black speckling(her name"peppered "moth). From that time on, individuals with dark-colored wings increased in frequency in the moth populations near industrialized centers until they ade up almost 100%of these populations. Black individu- als had a dominant allele that was present but very rare in populations before 1850. Biologists soon noticed that inin- dustrialized regions where the dark moths were common the tree trunks were darkened almost black by the soot of pollution. Dark moths were much less conspicuous restin on them than were light moths. In addition, the air pollu tion that was spreading in the industrialized regions had killed many of the light-colored lichens on tree trunks aking the trunks darker. Selection for Melanism Can Darwin's theory explain the increase in the frequency of the dark allele? Why did dark moths gain a survival ad IGURE 21.10 vantage around 18502 An amateur moth collector named Tutt's hypothesis explaining industrial melanism. These photographs show color variants of the peppered moth J.W. Tutt proposed what became the most commonly Biston betularia. Tutt proposed that the dark moth is more colored moths. He suggested that peppered forms were moth is more visible to predators on bark blackened bp. ight accepted hypothesis explaining the decline of the light- visible to predators on unpolluted trees(top), while the more visible to predators on sooty trees that have lost industrial pollution(bottom) their lichens. Consequently, birds ate the peppered moths esting on the trunks of trees during the day. The black forms, in contrast, were at an advantage because they the dark ones. This indicated that where the tree trunks were camouflaged(figure 21.10). Although Tutt initially were still light-colored, light moths had a much better had no evidence, British ecologist Bernard Kettlewell chance of survival. Kettlewell later solidified his argument tested the hypothesis in the 1950s by rearing populations by placing hidden blinds in the woods and actually filming of peppered moths with equal numbers of dark and light birds eating the moths. Sometimes the birds Kettlewell ob- individuals. Kettlewell then released these populations served actually passed right over a moth that was the same into two sets of woods: one, near heavily polluted Birm- color as its background ingham, the other, in unpolluted Dorset. Kettlewell set up rings of traps around the woods to see how many of botl kinds of moths survived. To evaluate his results, he had Industrial Melanism marked the released moths with a dot of paint on the un- Industrial melanism is a term used to describe the evolu- derside of their wings, where birds could not see it. tionary process in which darker individuals come to pre- In the polluted area near Birmingham, Kettlewell dominate over lighter individuals since the industrial revo- rapped 19% of the light moths, but 40% of the dark ones. lution as a result of natural selection. The process is widely This indicated that dark moths had a far better chance of believed to have taken place because the dark organisms are surviving in these polluted woods, where the tree trunks better concealed from their predators in habitats that have were dark. In the relatively unpolluted Dorset woods, Ket- been darkened by soot and other forms of industrial pollu- dewell recovered 12.5% of the light moths but only 6% of tion, as suggested by Kettlewell's research 446 Part vi Evolution

Peppered Moths and Industrial Melanism When the environment changes, natural selection often may favor new traits in a species. The example of the Dar￾win’s finches clearly indicates how natural variation can lead to evolutionary change. Humans are greatly altering the environment in many ways; we should not be surprised to see organisms attempting to adapt to these new condi￾tions. One classic example concerns the peppered moth, Biston betularia. Until the mid-nineteenth century, almost every individual of this species captured in Great Britain had light-colored wings with black specklings (hence the name “peppered” moth). From that time on, individuals with dark-colored wings increased in frequency in the moth populations near industrialized centers until they made up almost 100% of these populations. Black individu￾als had a dominant allele that was present but very rare in populations 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 of pollution. Dark moths were much less conspicuous resting on them than were light moths. In addition, the air pollu￾tion that was spreading in the industrialized regions had killed many of the light-colored lichens on tree trunks, making the trunks darker. Selection for Melanism Can Darwin’s theory explain the increase in the frequency of the dark allele? Why did dark moths gain a survival ad￾vantage around 1850? An amateur moth collector named J. W. Tutt proposed what became the most commonly accepted hypothesis explaining the decline of the light￾colored moths. He suggested that peppered forms were more visible to predators on sooty trees that have lost their lichens. Consequently, birds ate the peppered moths resting on the trunks of trees during the day. The black forms, in contrast, were at an advantage because they were camouflaged (figure 21.10). Although Tutt initially had no evidence, British ecologist Bernard Kettlewell tested the hypothesis in the 1950s by rearing populations of peppered moths with equal numbers of dark and light individuals. Kettlewell then released these populations into two sets of woods: one, near heavily polluted Birm￾ingham, the other, in unpolluted Dorset. Kettlewell set up rings of traps around the woods to see how many of both kinds of moths survived. To evaluate his results, he had marked 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, Kettlewell trapped 19% of the light moths, but 40% of the dark ones. This indicated that dark moths had a far better chance of surviving in these polluted woods, where the tree trunks were 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 trunks were still light-colored, light moths had a much better chance of survival. Kettlewell later solidified his argument by placing hidden blinds in the woods and actually filming birds eating the moths. Sometimes the birds Kettlewell ob￾served actually passed right over a moth that was the same color 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 widely believed to have taken place because the dark organisms are better concealed from their predators in habitats that have been darkened by soot and other forms of industrial pollu￾tion, as suggested by Kettlewell’s research. 446 Part VI Evolution FIGURE 21.10 Tutt’s hypothesis explaining industrial melanism. These photographs show color variants of the peppered moth, Biston betularia. Tutt proposed that the dark moth is more visible to predators on unpolluted trees (top), while the light moth is more visible to predators on bark blackened by industrial pollution (bottom)

Dozens of other species of moths have hanged in the same way as the peppered moth in industrialized areas throughout Eurasia and north america with dark forms becoming more common from the mid-nineteenth century onward as indus trialization spread Selection against melanism In the second half of the twentieth cen-2 tion of pollution controls, these trends 30 tury, with the widespread implementa are reversing, not only for the peppered moth in many areas in England but also for many other species of moths throughout the northern continents 757983 These examples provide some of the best Year documented instances of changes in al- lelic frequencies of natural populations as FIGuRE 2111 a result of natural selection due to specific Selection against melanism. The circles indicate the frequency of melanic bisto factors in the environment moths at Caldy Common in England, sampled continuously from 1959 to In England, the pollution promoting 1995. Diamonds indicate frequencies in Michigan from 1959 to 1962 and from industrial melanism began to reverse 1994 to 1995 lation in 1956. Beginning in 1959, the Moths"in ]ournal f Heredity, vol 87, 1996, Oxford University Press.ppered following enactment of Clean Air legis- Source: Data from grant, et al, "Parallel Rise and Fall of Melanic Pe Biston population at Caldy Common outside Liverpool has been sampled f the melanic dark) form has dropped from a high of 94% in 1960 to its current(1994)low of 19%(figure appear to correlate with changes in tree lichens. At Caldy 21.11). Similar reversals have been documented at Common, the light form of the peppered moth began its numerous other locations throughout England. The drop increase in frequency long before lichens began to reappear correlates well with a drop in air pollution, particularly on the trees. At the Detroit field station, the lichens never with tree-darkening sulfur dioxide and suspended changed significantly as the dark moths first became domi particulates nant and then declined over the last 30 years. In fact, Interestingly, the same reversal of industrial melanism tigators have not been able to find peppered moths or appears to have occurred in America during the same time trot trees at all, whether covered with lichens or that it was happening in England. Industrial melanism in Wherever the moths rest during the day, it does not appear the American subspecies of the peppered moth was not as to be on tree bark. Some evidence suggests they rest on widespread as in England, but it has been well-documented leaves on the treetops, but no one is sure at a rural field station near Detroit. Of 576 peppered moths The action of selection may depend less on the presence collected there from 1959 to 1961. 515 were melanic. a fre- of lichens and more on other differences in the environ quency of 89%. The American Clean Air Act, passed in ment resulting from industrial pollution. Pollution tends to 1963, led to significant reductions in air pollution. Resam- cover all objects in the environment with a fine layer of pled in 1994, the Detroit field station peppered moth pop- particulate dust, which tends to decrease how much light ulation had only 15% melanic moths(see figure 21.11) surfaces reflect. In addition, pollution has a particularly se- The moths in Liverpool and Detroit, both part of the same vere effect on birch trees, which are light in color. Both ef- natural experiment, exhibit strong evidence of natural se- fects would tend to make the environment darker and thus would favor darker color in moths Reconsidering the Target of Natural Selection Natural selection has favored the dark form of the peppered moth in areas subject to severe air pollution Tutt's hypothesis, widely accepted in the light of Ket perhaps because on darkened trees they are less easily tlewell's studies, is currently being reevaluated. The prob- seen by moth-eating birds. Selection has in turn favored lem is that the recent selection against melanism does not the light form as pollution has abated. Chapter 21 The Evidence for Evolution 447

Dozens of other species of moths have changed in the same way as the peppered moth in industrialized areas throughout Eurasia and North America, with dark forms becoming more common from the mid-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 trends are reversing, not only for the peppered moth in many areas in England, but also for many other species of moths throughout the northern continents. These examples provide some of the best documented instances of changes in al￾lelic frequencies of natural populations as a result of natural selection due to specific factors in the environment. In England, the pollution promoting industrial melanism began to reverse following enactment of Clean Air legis￾lation in 1956. Beginning in 1959, the Biston population at Caldy Common outside Liverpool has been sampled each year. The frequency of the melanic (dark) form has dropped from a high of 94% in 1960 to its current (1994) low of 19% (figure 21.11). Similar reversals have been documented at numerous other locations throughout England. The drop correlates well with a drop in air pollution, particularly with tree-darkening sulfur dioxide and suspended particulates. Interestingly, the same reversal of industrial melanism appears to have occurred in America during the same time that it was happening in England. Industrial melanism in the American subspecies of the peppered moth was not as widespread as in England, but it has been well-documented at a rural field station near Detroit. Of 576 peppered moths collected there from 1959 to 1961, 515 were melanic, a fre￾quency of 89%. The American Clean Air Act, passed in 1963, 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 same natural experiment, exhibit strong evidence of natural se￾lection. Reconsidering the Target of Natural Selection Tutt’s hypothesis, widely accepted in the light of Ket￾tlewell’s 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 Caldy Common, the light form of the peppered moth began its increase in frequency long before lichens began to reappear on the trees. At the Detroit field station, the lichens never changed 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 appear to be on tree bark. Some evidence suggests they rest on leaves on the treetops, but no one is sure. The action of selection may depend less on the presence of lichens and more on other differences in the environ￾ment resulting from industrial pollution. Pollution tends to cover all objects in the environment with a fine layer of particulate dust, which tends to decrease how much light surfaces 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 thus would favor darker color in moths. Natural selection has favored the dark form of the peppered moth in areas subject to severe air pollution, perhaps because on darkened trees they are less easily seen by moth-eating birds. Selection has in turn favored the light form as pollution has abated. Chapter 21 The Evidence for Evolution 447 Year 0 10 20 30 40 60 50 80 70 90 100 Percentage of melanic moths 59 63 67 71 75 79 83 87 91 95 FIGURE 21.11 Selection against melanism. The circles indicate the frequency of melanic Biston moths at Caldy Common in England, sampled continuously from 1959 to 1995. Diamonds indicate frequencies in Michigan from 1959 to 1962 and from 1994 to 1995. Source: Data from Grant, et al., “Parallel Rise and Fall of Melanic Peppered Moths” in Journal of Heredity, vol. 87, 1996, Oxford University Press

Artificial selection Humans have imposed selection upon plants and animals since the dawn of civilization. Just as in natural selection, rtificial selection operates by favoring individuals with cer- population tain phenotypic traits, allowing them to reproduce and pass their genes into the next generation. Assuming that pheno- High typic differences are genetically determined, such selection population should lead to evolu ndeed. it has. Arti ficial selection, imposed in laboratory experiments, agricul ≌≥E2 ture,and the domestication process, has produced substan- tial change in almost every case in which it has been pplied. This success is strong proof that selection is an ef- fective evolutionary process 0102030405060708090100110 Bristle number in Drosophila Laboratory Experiments FIGURE 21.12 With the rise of genetics as a field of science in the 1920s Artificial selection in the laboratory. In this experiment,one and 1930 researchers began conducting experiments to bristles and the other for high numbers. Note that not only laboratory did the means of the populations change greatly in 35 fruit fly, Drosophila melanogaster. Geneticists have imposed generations, but also that all individuals in both experimental selection on just about every conceivable aspect of the fruit populations lie outside the range of the initial population fly-including body size, eye color, growth rate, life span, and exploratory behavior-with a consistent result: selec- tion for a trait leads to strong and predictable evolutionary In one classic experiment, scientists selected for fruit flies with many bristles(stiff, hairlike structures) on their abdomen. At the start of the experiment, the average num ber of bristles was 9.5. Each generation, scientists picke out the 20% of the population with the greatest number of bristles and allowed them to reproduce, thus establishing the next generation. After 86 generations of such selection, he average number of bristles had quadrupled, to nearly 0. In a similar experiment, fruit flies were selected for ei ther the most or the fewest numbers of bristles. Within 35 generations, the populations did not overlap at all in range of variation(figure 21. 12) Teosinte intermediates Similar experiments have been conducted on a wide va- riety of other laboratory organisms. For example, by select- FIGURE 21.13 ing for rats that were resistant to tooth decay, scientists Corn looks very different from its ancestor. The tassels and were able to increase in less than 20 generations the aver- seeds of a wild grass, such as teosinte, evolved into the male age time for onset of decay from barely over 100 days to tassels and female ears of modern corn greater than 500 days griculture crop plants. In 1896, agricultural scientists began selecting on oil content of corn kernels, which initially was 4.5%.As Similar methods have been practiced in agriculture for in the fruit fly experiments, the top 20% of all individuals many centuries. Familiar livestock, such as cattle and pigs, were allowed to reproduce. In addition, a parallel experi- and crops, like corn and strawberries, are greatly different ment selected for the individuals with the lowest oil con from their wild ancestors(figure 21. 13). These differences tent. By 1986, at which time 90 generations had passed, av have resulted from generations of selection for desirable erage oil content had increased approximately 450% in the traits like milk production and corn stalk size high-content experiment; by contrast, oil content in the An experimental study with corn demonstrates the abil- low experiment had decreased to about 0.5%, a level at ity of artificial selection to rapidly produce major change in which it is difficult to get accurate reading 448 Part vi Evolution

Artificial Selection Humans have imposed selection upon plants and animals since 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 pass their genes into the next generation. Assuming that pheno￾typic differences are genetically determined, such selection should 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 been applied. 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 1920s and 1930s, researchers began conducting experiments to test the hypothesis that selection can produce evolutionary change. A favorite subject was the now-famous laboratory fruit fly, Drosophila melanogaster. Geneticists have imposed selection on just about every conceivable aspect of the fruit fly—including body size, eye color, growth rate, life span, and exploratory behavior—with a consistent result: selec￾tion for a trait leads to strong and predictable evolutionary response. In one classic experiment, scientists selected for fruit flies with many bristles (stiff, hairlike structures) on their abdomen. At the start of the experiment, the average num￾ber of bristles was 9.5. Each generation, scientists picked out the 20% of the population with the greatest number of bristles and allowed them to reproduce, thus establishing the next generation. After 86 generations of such selection, the average number of bristles had quadrupled, to nearly 40. In a similar experiment, fruit flies were selected for ei￾ther the most or the fewest numbers of bristles. Within 35 generations, the populations did not overlap at all in range of 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, scientists were able to increase in less than 20 generations the aver￾age time for onset of decay from barely over 100 days to greater than 500 days. Agriculture Similar methods have been practiced in agriculture for many centuries. Familiar livestock, such as cattle and pigs, and crops, like corn and strawberries, are greatly different from their wild ancestors (figure 21.13). These differences have resulted from generations of selection for desirable traits 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 selecting on oil content of corn kernels, which initially was 4.5%. As in the fruit fly experiments, the top 20% of all individuals were 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 the high-content experiment; by contrast, oil content in the low experiment had decreased to about 0.5%, a level at which it is difficult to get accurate readings. 448 Part VI Evolution Mean Mean Mean High population Bristle number in Drosophila 0 10 20 30 40 50 60 70 80 90 100 110 Number of individuals Low population Initial population FIGURE 21.12 Artificial selection in the laboratory. In this experiment, one population of Drosophila was selected for low numbers of bristles and the other for high numbers. Note that not only did the means of the populations change greatly in 35 generations, but also that all individuals in both experimental populations lie outside the range of the initial population. Teosinte Intermediates Modern corn FIGURE 21.13 Corn looks very different from its ancestor. The tassels and seeds of a wild grass, such as teosinte, evolved into the male tassels and female ears of modern corn