《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

VI Evolution Catching evolution in action A hundred years ago Charles Darwin's theory of evolution natural selection was taught as the foundation of biology in public schools throughout the United States. Then something happened. In the 1920s, conservative religious groups began to argue against the teaching of evolution in our nations schools. Darwinism, they said, contradicted the revealed word of god in the bible and thus was a direct attack on their religious beliefs. Many of you will have read about the 1925 Scopes"monkey trial"or seen the move about it, Inberit the Wind. In the backwash of this contro- The evolution of protective coloration in guppies. In versy, evolution for the first time in this century disap below waterfalls where predation is high, guppies are dn ools peared from the schools. Textbook publishers and local colored. In the absence of the highly predatory pike cichlid school boards, in a wish to avoid the dispute, simply chose guppies in pools above waterfalls are much more colorful and not to teach evolution. By 1959, 100 years after Darwins attractive to females. The evolution of these differences can be book, a famous American geneticist cried in anguish, "A hundred years without Darwin is enough! "What he meant was that the theory of evolution by natural selection has be- come the central operating concept of the science of biol ogy,organic evolution being one of the most solidly vali omy and history, relying on observation and deduction dated facts of science. How could we continue to hide this rather than experiment and induction to examine ideas truth from our children, crippling their understanding of about past events. science? Nonetheless, evolutionary biology is not entirely an ob- In the 1970s, Darwin reappeared in our nation's schools, servational science. Darwin was right about many things part of the wave of concern about science that followed but one area in which he was mistaken concerns the pace Sputnik. Not for long, however. Cries from creationists for at which evolution occurs. Darwin thought that evolution equal time in the classroom soon had evolution out of our occurred at a very slow, almost imperceptible, pace. How classrooms again. Only in recent years, amid considerable ever, in recent years many case studies of natural popula uproar, have states like California succeeded in reforming tions have demonstrated that in some circumstances evolu- their school curriculums, focusing on evolution as the cen- tionary change can occur rapidly. In these instances, it is tral principle of biology. In other states, teaching Darwin ossible to establish experimental studies to directly test remains controversial volutionary hypotheses. Although laboratory studies on While Darwin's proposal that evolution occurs as the fruit flies and other organisms have been common for esult of natural selection remains controversial in many more than 50 years, it has only been in recent years that local school boards, it is accepted by practically every biol- scientists have started conducting experimental studies of gist who has examined it seriously. In this section, we will evolution in nature review the evidence supporting Darwins theory. Evolu To conduct experimental tests of evolution, it is first ne tionary biology is unlike most other fields of biology in essary to identify a population in nature upon which strong which hypotheses are tested directly with experimental selection might be operating( see above). Then, by manipt methods. To study evolution, we need to investigate what lating the strength of the selection, an investigator can pre- hat the past, sometimes many millions of years dict what outcome selection might produce, then look and ago. In this way, evolutionary biology is similar to astron- see the actual effect on the population 419

419 Catching evolution in action A hundred years ago Charles Darwin’s theory of evolution by natural selection was taught as the foundation of biology in public schools throughout the United States. Then something happened. In the 1920s, conservative religious groups began to argue against the teaching of evolution in our nation's schools. Darwinism, they said, contradicted the revealed word of God in the Bible and thus was a direct attack on their religious beliefs. Many of you will have read about the 1925 Scopes "monkey trial" or seen the move about it, Inherit the Wind. In the backwash of this contro￾versy, evolution for the first time in this century disap￾peared from the schools. Textbook publishers and local school boards, in a wish to avoid the dispute, simply chose not to teach evolution. By 1959, 100 years after Darwin's book, a famous American geneticist cried in anguish, "A hundred years without Darwin is enough!" What he meant was that the theory of evolution by natural selection has be￾come the central operating concept of the science of biol￾ogy, organic evolution being one of the most solidly vali￾dated facts of science. How could we continue to hide this truth from our children, crippling their understanding of science? In the 1970s, Darwin reappeared in our nation's schools, part of the wave of concern about science that followed Sputnik. Not for long, however. Cries from creationists for equal time in the classroom soon had evolution out of our classrooms again. Only in recent years, amid considerable uproar, have states like California succeeded in reforming their school curriculums, focusing on evolution as the cen￾tral principle of biology. In other states, teaching Darwin remains controversial. While Darwin’s proposal that evolution occurs as the result of natural selection remains controversial in many local school boards, it is accepted by practically every biol￾ogist who has examined it seriously. In this section, we will review the evidence supporting Darwin’s theory. Evolu￾tionary biology is unlike most other fields of biology in which hypotheses are tested directly with experimental methods. To study evolution, we need to investigate what happened in the past, sometimes many millions of years ago. In this way, evolutionary biology is similar to astron￾omy and history, relying on observation and deduction rather than experiment and induction to examine ideas about past events. Nonetheless, evolutionary biology is not entirely an ob￾servational science. Darwin was right about many things, but one area in which he was mistaken concerns the pace at which evolution occurs. Darwin thought that evolution occurred at a very slow, almost imperceptible, pace. How￾ever, in recent years many case studies of natural popula￾tions have demonstrated that in some circumstances evolu￾tionary change can occur rapidly. In these instances, it is possible to establish experimental studies to directly test evolutionary hypotheses. Although laboratory studies on fruit flies and other organisms have been common for more than 50 years, it has only been in recent years that scientists have started conducting experimental studies of evolution in nature. To conduct experimental tests of evolution, it is first nec￾essary to identify a population in nature upon which strong selection might be operating (see above). Then, by manipu￾lating the strength of the selection, an investigator can pre￾dict what outcome selection might produce, then look and see the actual effect on the population. Part VI Evolution The evolution of protective coloration in guppies. In pools below waterfalls where predation is high, guppies are drab colored. In the absence of the highly predatory pike cichlid, guppies in pools above waterfalls are much more colorful and attractive to females. The evolution of these differences can be experimentally tested

Evolutionary change in spot number. Populations transported to the low-predation environment quickly increased in number of spots, whereas selection in more dangerous environments, like the predator-filled pool above rigbt, led to less conspicuous fish The Experiment the case, then a founder effect would occur in which the new population was established solely by individuals with Guppies offer an excellent experimental opportunity. The uppy, Poecilia reticulata, is found in small streams in north- genes for large size The only way to rule out such alternative possibilities is eastern South America and the nearby island of Trinidad. to conduct a controlled In Trinidad, guppies are found in many mountain streams experiment. The first experime One interesting feature of several streams is that they have were conducted in large pools in laboratory greenhouses waterfalls. Amazingly, guppies are capable of colonizing At the start of the experiment, a group of 2000 guppies portions of the stream above the waterfall. During flood later, pike cichlids were added to four of the pools and killi- easons, rivers sometimes overflow their banks, creating fish(which rarely prey on guppies) to another four, with secondary channels that move through the forest. Durin these ns, guppies may be able to move upstream and e remain ng pools left as“ no predator” controls. invade pools above waterfalls. By contrast, not all species are capable of such dispersal and thus are only found in The Results these streams below the first waterfall. One species whose Fourteen months later(which corresponds to 10 guppy distribution is restricted by waterfalls is the pike cichlid, Crenicicbla alta, a voracious predator that feeds on other generations), the scientists compared the populations. The fish, including guppies guppies in the killifish and control pools were indistin- Because of these barriers to dispersal, guppies can be guishable, brightly colored and large. In contrast, the gup- ies in the pike cichlid pools were smaller and drab in col- found in two very different environments. In pools just oration. These results established that predation can lead to below the waterfalls, predation is a substantial risk and rates of survival are relatively low. By contrast, in similar pools ments reflect what occurs in natur a result, guppy populations above and below waterfalls have pies in pools below a waterfall, but not above it. As in other Trinidadian streams, the pike cichlid was present in the guppies exhibit drab coloration. Moreover, they tend to re produce at a younger age lower pools, but only the killifish was found above the wa- terfalls. The scientists then transplanted guppies to the Perhaps as a result of shunting energy to reproduction upper pools and returned at several-year intervals to moni- tor the populations. Despite originating from population rather than growth, the fish in high-predation pools attain in which predation levels were high, the transplanted popu- relatively smaller adult sizes. By contrast, male fish above lations rapidly evolved the traits characteristic of low-pre- the waterfall display gaudy colors that they use to court fe- dation guppies: they matured late, attained greater size and Although the differences between guppies living above brighter colors,. Control populations in the lower pools,by and below the waterfalls are consistent with the hypothesis contrast, continued to mature early and at smaller size These results demonstrate that substantial evolutionar that they represent evolutionary responses to differences in the strength of predation, alternative explanations are pos hange can occur in less than 12 years sible. Perhaps, for example, only very large fish are capable To explore this concept further go to our interactive lab of moving past the waterfall to colonize pools. If this were atwww.mhhe.com/raven6 420 Part VI Evolution

420 Part VI Evolution The Experiment Guppies offer an excellent experimental opportunity. The guppy, Poecilia reticulata, is found in small streams in north￾eastern South America and the nearby island of Trinidad. In Trinidad, guppies are found in many mountain streams. One interesting feature of several streams is that they have waterfalls. Amazingly, guppies are capable of colonizing portions of the stream above the waterfall. During flood seasons, rivers sometimes overflow their banks, creating secondary channels that move through the forest. During these occasions, guppies may be able to move upstream and invade pools above waterfalls. By contrast, not all species are capable of such dispersal and thus are only found in these streams below the first waterfall. One species whose distribution is restricted by waterfalls is the pike cichlid, Crenicichla alta, a voracious predator that feeds on other fish, including guppies. Because of these barriers to dispersal, guppies can be found in two very different environments. In pools just below the waterfalls, predation is a substantial risk and rates of survival are relatively low. By contrast, in similar pools just above the waterfall, few predators prey on guppies. As a result, guppy populations above and below waterfalls have evolved many differences. In the high-predation pools, guppies exhibit drab coloration. Moreover, they tend to re￾produce at a younger age. The differences suggest the action of natural selection. Perhaps as a result of shunting energy to reproduction rather than growth, the fish in high-predation pools attain relatively smaller adult sizes. By contrast, male fish above the waterfall display gaudy colors that they use to court fe￾males. Adults there mature later and grow to larger sizes. Although the differences between guppies living above and below the waterfalls are consistent with the hypothesis that they represent evolutionary responses to differences in the strength of predation, alternative explanations are pos￾sible. Perhaps, for example, only very large fish are capable of moving past the waterfall to colonize pools. If this were the case, then a founder effect would occur in which the new population was established solely by individuals with genes for large size. The only way to rule out such alternative possibilities is to conduct a controlled experiment. The first experiments were conducted in large pools in laboratory greenhouses. At the start of the experiment, a group of 2000 guppies were divided equally among 10 large pools. Six months later, pike cichlids were added to four of the pools and killi￾fish (which rarely prey on guppies) to another four, with the remaining pools left as “no predator” controls. The Results Fourteen months later (which corresponds to 10 guppy generations), the scientists compared the populations. The guppies in the killifish and control pools were indistin￾guishable, brightly colored and large. In contrast, the gup￾pies in the pike cichlid pools were smaller and drab in col￾oration. These results established that predation can lead to rapid evolutionary change, but does this laboratory experi￾ments reflect what occurs in nature? To find out, scientists located two streams that had gup￾pies in pools below a waterfall, but not above it. As in other Trinidadian streams, the pike cichlid was present in the lower pools, but only the killifish was found above the wa￾terfalls. The scientists then transplanted guppies to the upper pools and returned at several-year intervals to moni￾tor the populations. Despite originating from populations in which predation levels were high, the transplanted popu￾lations rapidly evolved the traits characteristic of low-pre￾dation guppies: they matured late, attained greater size and brighter colors. Control populations in the lower pools, by contrast, continued to mature early and at smaller size. These results demonstrate that substantial evolutionary change can occur in less than 12 years. To explore this concept further go to our interactive lab at www.mhhe.com/raven6e Evolutionary change in spot number. Populations transported to the low-predation environment quickly increased in number of spots, whereas selection in more dangerous environments, like the predator-filled pool above right, led to less conspicuous fish

20 Genes within populations Concept Outline 20.1 Genes vary in natural populations Gene variation is the raw material of evolution Selection acts on the genetic variation present in populations, favoring variants that increase the likelihood of survival and reproduction Gene Variation in Nature. Natural populations contain considerable amounts of variation, present at the dNA level and expressed in proteins 20.2 Why do allele frequencies change in populations? The Hardy-Weinberg Principle. The proportion of homozygotes and heterozygotes in a population is not altered by meiosis or sexual reproduction. Five Agents of Evolutionary Change. The frequency of alleles in a population can be changed by evolutionary forces like gene flow and selection. Identifying the Evolutionary Forces Maintaining Polymorphism. A number of processes can influence allele frequencies in natural populations, but it is difficult to ascertain their relative importance. Heterozygote Advantage. -In some cases, heterozygotes FIGURE 20.1 are superior to either type of homozygote. The gene fo sickle cell anemia is one particularly well-understood Genetic variation. The range of genetic material in a population example is expressed in a variety of ways--including color 20.3 Selection can act on traits affected by many o other human being is exactly like you(unless you have an identical twin). Often the particular charac- Forms of Selection. Selection can act on traits like height or weight to stabilize or change the level at which teristics of an individual have an important bearing on its the trait is expressed survival, on its chances to reproduce, and on the success of Limits to What Selection Can Accomplish. Selection its offspring. Evolution is driven by such consequences cannot act on traits with little or no genetic variation. Genetic variation that influences these characteristics pro- vides the raw material for natural selection, and natural populations contain a wealth of such variation. In plants (figure 20.1), insects, and vertebrates, practically every gene exhibits some level of var In this ch l e plore genetic variation in natural populations and consider the evolutionary forces that cause allele frequencies in nat ural populations to change. These deceptively simple mat- ters lie at the core of evolutionary biology

421 20 Genes within Populations Concept Outline 20.1 Genes vary in natural populations. Gene Variation Is the Raw Material of Evolution. Selection acts on the genetic variation present in populations, favoring variants that increase the likelihood of survival and reproduction. Gene Variation in Nature. Natural populations contain considerable amounts of variation, present at the DNA level and expressed in proteins. 20.2 Why do allele frequencies change in populations? The Hardy–Weinberg Principle. The proportion of homozygotes and heterozygotes in a population is not altered by meiosis or sexual reproduction. Five Agents of Evolutionary Change. The frequency of alleles in a population can be changed by evolutionary forces like gene flow and selection. Identifying the Evolutionary Forces Maintaining Polymorphism. A number of processes can influence allele frequencies in natural populations, but it is difficult to ascertain their relative importance. Heterozygote Advantage.—In some cases, heterozygotes are superior to either type of homozygote. The gene for sickle cell anemia is one particularly well-understood example. 20.3 Selection can act on traits affected by many genes. Forms of Selection. Selection can act on traits like height or weight to stabilize or change the level at which the trait is expressed. Limits to What Selection Can Accomplish. Selection cannot act on traits with little or no genetic variation. No other human being is exactly like you (unless you have an identical twin). Often the particular charac￾teristics of an individual have an important bearing on its survival, on its chances to reproduce, and on the success of its offspring. Evolution is driven by such consequences. Genetic variation that influences these characteristics pro￾vides the raw material for natural selection, and natural populations contain a wealth of such variation. In plants (figure 20.1), insects, and vertebrates, practically every gene exhibits some level of variation. In this chapter, we will ex￾plore genetic variation in natural populations and consider the evolutionary forces that cause allele frequencies in nat￾ural populations to change. These deceptively simple mat￾ters lie at the core of evolutionary biology. FIGURE 20.1 Genetic variation. The range of genetic material in a population is expressed in a variety of ways—including color

20.1 Genes vary in natural populations. Gene Variation Is the Raw Material of Evolution Evolution Is descent with modification The word"evolution"is widely used in the natural and so- cial sciences. It refers to how an entity-be it a social sys- tem,a gas, or a planet-changes through time. Although can be traced to Darwin's On the Origin of Species, the first modem-day okapi leaves, then passed the never act Rather, Darwin used the phrase "descent with modifica- (a) Lamarck's theory: variation is acquired. tion. "Although many more complicated definitions have en proposed, Darwins phrase probably best captures the essence of biological evolution: all species arise from other, pre-existing species. However, through time, they accumu- late differences such that ancestral and descendant species are not identical Natural Selection Is an Important Mechanism of Evolutionary Change reproduction Rather, he followed a long line of earlier philosophers and bom wuals a Darwin was not the first to propose a theory of evolution. naturalists who deduced that the many kinds of organisms have longer round us were produced by a process of evolution. U ke his predecessors, however, Darwin proposed natural Over many generations, selection as the mechanism of evolution, Natural selec- tion produces evolutionary change when in a population uccessful and pass some individuals, which possess certain inherited charac teristics, produce more surviving offspring than individu to their offspring. als lacking these characteristics. As a result, the (b)Darwins theory: variation is inherited tion will gradually come to include more and more individuals with the advantageous characteristics. In this FIGURE 20.2 way, the population evolves and becomes better adapted How did giraffes evolve a long neck? to its local circumstances Natural selection was by no means the only evolution ary mechanism proposed. A rival theory, championed by the prominent biologist Jean-Baptiste Lamarck, was that in the genetic makeup of populations. Allele frequencies evolution occurred by the inheritance of ace cquired can also change as the result of repeated mutations from characteristics. According to Lamarck, individuals one allele to another and from migrants bringing alleles passed on to offspring body and behavior changes ac- into a population. In addition, when populations are small, quired during their lives. Thus, Lamarck proposed that the frequencies of alleles can change randomly as the result ancestral giraffes with short necks tended to stretch their of chance events. Evolutionary biologists debate the rela necks to feed on tree leaves, and this extension of the tive strengths of these processes. Although no one denies neck was passed on to subsequent generations, leading to that natural selection is a powerful force leading to adaptive the long-necked giraffe(figure 20.2a). In Darwin's the hange, the importance of other processes is less certain by contrast, the variation is not created by experi ence, but is the result of preexisting genetic differences among individuals(figure 20.2b) Darwin proposed that natural selection on variants Although the efficacy of natural selection is now widely within populations leads to the evolution of different accepted, it is not the only process that can lead to changes 422 Part vI Evolution

in the genetic makeup of populations. Allele frequencies can also change as the result of repeated mutations from one allele to another and from migrants bringing alleles into a population. In addition, when populations are small, the frequencies of alleles can change randomly as the result of chance events. Evolutionary biologists debate the rela￾tive strengths of these processes. Although no one denies that natural selection is a powerful force leading to adaptive change, the importance of other processes is less certain. Darwin proposed that natural selection on variants within populations leads to the evolution of different species. 422 Part VI Evolution Gene Variation Is the Raw Material of Evolution Evolution Is Descent with Modification The word “evolution” is widely used in the natural and so￾cial sciences. It refers to how an entity—be it a social sys￾tem, a gas, or a planet—changes through time. Although development of the modern concept of evolution in biology can be traced to Darwin’s On the Origin of Species, the first five editions of this book never actually used the term! Rather, Darwin used the phrase “descent with modifica￾tion.” Although many more complicated definitions have been proposed, Darwin’s phrase probably best captures the essence of biological evolution: all species arise from other, pre-existing species. However, through time, they accumu￾late differences such that ancestral and descendant species are not identical. Natural Selection Is an Important Mechanism of Evolutionary Change Darwin was not the first to propose a theory of evolution. Rather, he followed a long line of earlier philosophers and naturalists who deduced that the many kinds of organisms around us were produced by a process of evolution. Un￾like his predecessors, however, Darwin proposed natural selection as the mechanism of evolution. Natural selec￾tion produces evolutionary change when in a population some individuals, which possess certain inherited charac￾teristics, produce more surviving offspring than individu￾als lacking these characteristics. As a result, the popula￾tion will gradually come to include more and more individuals with the advantageous characteristics. In this way, the population evolves and becomes better adapted to its local circumstances. Natural selection was by no means the only evolution￾ary mechanism proposed. A rival theory, championed by the prominent biologist Jean-Baptiste Lamarck, was that evolution occurred by the inheritance of acquired characteristics. According to Lamarck, individuals passed on to offspring body and behavior changes ac￾quired during their lives. Thus, Lamarck proposed that ancestral giraffes with short necks tended to stretch their necks to feed on tree leaves, and this extension of the neck was passed on to subsequent generations, leading to the long-necked giraffe (figure 20.2a). In Darwin’s the￾ory, by contrast, the variation is not created by experi￾ence, but is the result of preexisting genetic differences among individuals (figure 20.2b). Although the efficacy of natural selection is now widely accepted, it is not the only process that can lead to changes 20.1 Genes vary in natural populations. Proposed ancestor of giraffes has characteristics of modern-day okapi. The giraffe ancestor lengthened its neck by stretching to reach tree leaves, then passed the change to offspring. (a) Lamarck's theory: variation is acquired. stretching stretching reproduction reproduction reproduction reproduction Individuals are born who happen to have longer necks. Over many generations, longer-necked individuals are more successful and pass the long-neck trait on to their offspring. growth to adult growth to adult (b) Darwin's theory: variation is inherited. FIGURE 20.2 How did giraffes evolve a long neck?

Gene Variation in Nature Evolution within a species may result from any process that causes a change in the genetic composition of a population In considering this theory of population genetics, it is best to start by looking at the genetic variation present among individuals within a species. This is the raw material avail able for the selective process Measuring Levels of Genetic Variation As we saw in chapter 13, a natural population can contain a great deal of genetic variation. This is true not only of hu mans, but of all organisms. How much variation usually oc- curs: Biologists have looked at many different genes in an effort to answer this question: 1. Blood groups. Chemical analysis has revealed the ex- istence of more than 30 blood group genes in humans, in addition to the abo locus. At least a third of these genes are routinely found in several alternative allelic forms in human populations. In addition to these, there FIGURE 20.3 are more than 45 variable genes encoding other pro- Polymorphic variation. These Australian snails, all of the species teins in human blood cells and plasma which are not Bankivia fasciata, exhibit considerable variation in pattern and considered blood groups. Thus, there are more than 75 color. Individual variations are heritable and passed on to genetically variable genes in this one system alone. offspring 2. Enzymes. Alternative alleles of genes specifying particular enzymes are easy to distinguish by measur ing how fast the alternative proteins migrate in an than 5%)at more than half of their enzyme-encoding loci electric field(a process called electrophoresis).A although vertebrates are somewhat less polymorphic. Het- eat deal of variation exists at enzyme specifying erozygosity(that is, the probability that a randomly se- loci. About 5% of the enzyme loci of a typical human lected gene will be heterozygous for a randomly selected are heterozygous: if you picked an individual at individual) is about 15% in Drosophila and other inverte- random, and in turn selected one of the enzyme brates. between 5% and 8% in vertebrates and around 8% encoding i in 20(5%)that the gene you selected ity prowide ang plants. These high levels of genetic variabil- nes of that individual at random the chances are mple supplies of raw material for evolution rould be heterozygous in that individual Considering the entire human genome, it is fair to say DNA Sequence polymorphism that almost all people are different from one another. This With the advent of gene technology, it has become possible is also true of other organisms, except for those that repro- to assess genetic var variation even more directly by seq ing the DNA itself. In a pioneering study in 1989, Martin Kreitman sequenced ADH genes isolated from 11 individu Enzyme Polymorphism ls of the fruit fly Drosophila melanogaster. He found 43 var Many loci in a given population have more than one allele able sites, only one of which had been detected by protein at frequencies significantly greater than would occur from electrophoresis! In the following decade, numerous other studies of variation at the dna level have confirmed these mutation alone. Researchers refer to a locus with more findings: abundant variation exists in both the codig variation than can be explained by mutation as polymor phic(pob,“many," morphic,“ forms”)( figure20.3).The gions of genes and in their nontranslated introns--consic tent of such variation within natural populations was not erably more variation than we can detect examining en even suspected a few decades ago, until modern techniques zymes with electrophoresis such as gel electrophoresis made it possible to exa amine en- rmes and other proteins directly. We now know that most Natural populations contain considerable amounts of populations of insects and plants are polymorphic(that is, genetic variation---more than can be accounted for by ave more than one allele occurring at a frequenc Chapter 20 Go ene Populations 423

Gene Variation in Nature Evolution within a species may result from any process that causes a change in the genetic composition of a population. In considering this theory of population genetics, it is best to start by looking at the genetic variation present among individuals within a species. This is the raw material avail￾able for the selective process. Measuring Levels of Genetic Variation As we saw in chapter 13, a natural population can contain a great deal of genetic variation. This is true not only of hu￾mans, but of all organisms. How much variation usually oc￾curs? Biologists have looked at many different genes in an effort to answer this question: 1. Blood groups. Chemical analysis has revealed the ex￾istence of more than 30 blood group genes in humans, in addition to the ABO locus. At least a third of these genes are routinely found in several alternative allelic forms in human populations. In addition to these, there are more than 45 variable genes encoding other pro￾teins in human blood cells and plasma which are not considered blood groups. Thus, there are more than 75 genetically variable genes in this one system alone. 2. Enzymes. Alternative alleles of genes specifying particular enzymes are easy to distinguish by measur￾ing how fast the alternative proteins migrate in an electric field (a process called electrophoresis). A great deal of variation exists at enzyme-specifying loci. About 5% of the enzyme loci of a typical human are heterozygous: if you picked an individual at random, and in turn selected one of the enzyme￾encoding genes of that individual at random, the chances are 1 in 20 (5%) that the gene you selected would be heterozygous in that individual. Considering the entire human genome, it is fair to say that almost all people are different from one another. This is also true of other organisms, except for those that repro￾duce asexually. In nature, genetic variation is the rule. Enzyme Polymorphism Many loci in a given population have more than one allele at frequencies significantly greater than would occur from mutation alone. Researchers refer to a locus with more variation than can be explained by mutation as polymor￾phic (poly, “many,” morphic, “forms”) (figure 20.3). The ex￾tent of such variation within natural populations was not even suspected a few decades ago, until modern techniques such as gel electrophoresis made it possible to examine en￾zymes and other proteins directly. We now know that most populations of insects and plants are polymorphic (that is, have more than one allele occurring at a frequency greater than 5%) at more than half of their enzyme-encoding loci, although vertebrates are somewhat less polymorphic. Het￾erozygosity (that is, the probability that a randomly se￾lected gene will be heterozygous for a randomly selected individual) is about 15% in Drosophila and other inverte￾brates, between 5% and 8% in vertebrates, and around 8% in outcrossing plants. These high levels of genetic variabil￾ity provide ample supplies of raw material for evolution. DNA Sequence Polymorphism With the advent of gene technology, it has become possible to assess genetic variation even more directly by sequenc￾ing the DNA itself. In a pioneering study in 1989, Martin Kreitman sequenced ADH genes isolated from 11 individu￾als of the fruit fly Drosophila melanogaster. He found 43 vari￾able sites, only one of which had been detected by protein electrophoresis! In the following decade, numerous other studies of variation at the DNA level have confirmed these findings: abundant variation exists in both the coding re￾gions of genes and in their nontranslated introns—consid￾erably more variation than we can detect examining en￾zymes with electrophoresis. Natural populations contain considerable amounts of genetic variation—more than can be accounted for by mutation alone. Chapter 20 Genes within Populations 423 FIGURE 20.3 Polymorphic variation. These Australian snails, all of the species Bankivia fasciata, exhibit considerable variation in pattern and color. Individual variations are heritable and passed on to offspring