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 controversy, evolution for the first time in this century disappeared 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 become the central operating concept of the science of biology, organic evolution being one of the most solidly validated 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 central 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 biologist who has examined it seriously. In this section, we will review the evidence supporting Darwin’s theory. Evolutionary 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 astronomy 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 observational 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. However, in recent years many case studies of natural populations have demonstrated that in some circumstances evolutionary 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 necessary to identify a population in nature upon which strong selection might be operating (see above). Then, by manipulating the strength of the selection, an investigator can predict 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 northeastern 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 reproduce 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 females. 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 possible. 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 killifish (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 indistinguishable, brightly colored and large. In contrast, the guppies in the pike cichlid pools were smaller and drab in coloration. These results established that predation can lead to rapid evolutionary change, but does this laboratory experiments reflect what occurs in nature? To find out, scientists located two streams that had guppies 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 waterfalls. The scientists then transplanted guppies to the upper pools and returned at several-year intervals to monitor the populations. Despite originating from populations in which predation levels were high, the transplanted populations rapidly evolved the traits characteristic of low-predation 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 characteristics 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 provides 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 explore genetic variation in natural populations and consider the evolutionary forces that cause allele frequencies in natural populations to change. These deceptively simple matters 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 relative 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 social sciences. It refers to how an entity—be it a social system, 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 modification.” 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 accumulate 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. Unlike his predecessors, however, Darwin proposed natural selection as the mechanism of evolution. Natural selection produces evolutionary change when in a population some individuals, which possess certain inherited characteristics, produce more surviving offspring than individuals lacking these characteristics. As a result, the population 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 evolutionary 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 acquired 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 theory, by contrast, the variation is not created by experience, 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 available 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 humans, but of all organisms. How much variation usually occurs? Biologists have looked at many different genes in an effort to answer this question: 1. Blood groups. Chemical analysis has revealed the existence 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 proteins 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 measuring 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 enzymeencoding 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 reproduce 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 polymorphic (poly, “many,” morphic, “forms”) (figure 20.3). The extent 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 enzymes 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. Heterozygosity (that is, the probability that a randomly selected gene will be heterozygous for a randomly selected individual) is about 15% in Drosophila and other invertebrates, between 5% and 8% in vertebrates, and around 8% in outcrossing plants. These high levels of genetic variability 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 sequencing the DNA itself. In a pioneering study in 1989, Martin Kreitman sequenced ADH genes isolated from 11 individuals of the fruit fly Drosophila melanogaster. He found 43 variable 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 regions of genes and in their nontranslated introns—considerably more variation than we can detect examining enzymes 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
20.2 Why do allele frequencies change in populations? Population genetics is the study of the properties of genes In algebraic terms, the Hardy-Weinberg principle is in populations. Genetic variation within natural popula written as an equation. Consider a population of 100 cats, tions was a puzzle to Darwin and his contemporaries. The with 84 black and 16 white cats. In statistics, frequency ray in which meiosis produces genetic segregation among is defined as the proportion of individuals falling within the progeny of a hybrid had not yet been discovered Selec certain category in relation to the total number of indi- tion,scientists then thought, should always favor an opti- viduals under consideration. In this case, the respective mal form, and so tend to eliminate variation. Moreover, the frequencies would be 0.84(or 84%)and 0. 16(or 16%) theory of blending inheritance--in which offspring were Based on these phenotypic frequencies, can we deduce expected to be phenotypically intermediate relative to their the underlying frequency of genotypes? If we assume that rents-was widely accepted. If blending inheritance were the white cats are homozygous recessive for an allele we orrect,then the effect of any new genetic variant would designate b, and the black cats are therefore either ho- quickly be diluted to the point of disappearance in subse- mozygous dominant BB or heterozygous Bb, we can cal- quent generations culate the allele frequencies of the two alleles in the population from the proportion of black and white indi- The Hardy-Weinberg Principle viduals. Let the letter p designate the frequency of one al- lele and the letter g the frequency of the alternative al Following the rediscovery of Mendels research, two people lele. Because there are only two alleles, p plus g must 1908 independently solved the puzzle of why genetic variation persists--G. H. Hardy, an English mathemati The Hardy-Weinberg equation can now be expressed in cian, and G. Weinberg, a German physician. They pointed the form of what is known as a binomial expansion out that the original proportions of the genotypes in a pop- p+q)2=p2 2pq ulation will remain constant from generation to generation as long as the following assumptions are met: Individuals (Individuals (Individuals azygous heterozygous monozygous 1. The population size is very large allele B) with alleles B+b) for allele b) 2. Random mating is occurring 3. No mutation takes place If g2= 0. 16(the frequency of white cats), then g=0.4 4. No genes are input from other sources(no immig Therefore, P, the frequency of allele B, would be 0.6(1.0 tion takes place) 0. 4=0.6). We can now easily calculate the genotype fre- 5. No selection occurs quencies: there are p=(0.6)x 100(the number of cats in the total population), or 36 homozygous dominant BB indi Dominant alleles do not, in fact, replace recessive ones. viduals. The heterozygous individuals have the Bb gen Because their proportions do not change, the genotypes are type, and there would be 2pg, or(2 X 0.6 0.4)X100,or said to be in Hardy-Weinberg equilibrium. 48 heterozygous Bb individuals Sperr p=0.6 Phenotypes p2=0.36 q=0.4 Genotypes BB bb pq=0.24 pq=0.24 genotype in population 0.36 0.48 0.16 q2=0.16 Frequency of gametes 0.36+0.24=0.6B 024+0.16=0.4b FIGURE 20.4 The Hardy-Weinberg equilibrium. In the absence of factors that alter them, the frequencies of gametes, genotypes, and phenotypes remain constant generation after generation 424 Part vi Evolution
Population genetics is the study of the properties of genes in populations. Genetic variation within natural populations was a puzzle to Darwin and his contemporaries. The way in which meiosis produces genetic segregation among the progeny of a hybrid had not yet been discovered. Selection, scientists then thought, should always favor an optimal form, and so tend to eliminate variation. Moreover, the theory of blending inheritance—in which offspring were expected to be phenotypically intermediate relative to their parents—was widely accepted. If blending inheritance were correct, then the effect of any new genetic variant would quickly be diluted to the point of disappearance in subsequent generations. The Hardy–Weinberg Principle Following the rediscovery of Mendel’s research, two people in 1908 independently solved the puzzle of why genetic variation persists—G. H. Hardy, an English mathematician, and G. Weinberg, a German physician. They pointed out that the original proportions of the genotypes in a population will remain constant from generation to generation, as long as the following assumptions are met: 1. The population size is very large. 2. Random mating is occurring. 3. No mutation takes place. 4. No genes are input from other sources (no immigration takes place). 5. No selection occurs. Dominant alleles do not, in fact, replace recessive ones. Because their proportions do not change, the genotypes are said to be in Hardy–Weinberg equilibrium. In algebraic terms, the Hardy–Weinberg principle is written as an equation. Consider a population of 100 cats, with 84 black and 16 white cats. In statistics, frequency is defined as the proportion of individuals falling within a certain category in relation to the total number of individuals under consideration. In this case, the respective frequencies would be 0.84 (or 84%) and 0.16 (or 16%). Based on these phenotypic frequencies, can we deduce the underlying frequency of genotypes? If we assume that the white cats are homozygous recessive for an allele we designate b, and the black cats are therefore either homozygous dominant BB or heterozygous Bb, we can calculate the allele frequencies of the two alleles in the population from the proportion of black and white individuals. Let the letter p designate the frequency of one allele and the letter q the frequency of the alternative allele. Because there are only two alleles, p plus q must always equal 1. The Hardy-Weinberg equation can now be expressed in the form of what is known as a binomial expansion: (p + q)2 = p2 + 2pq + q2 (Individuals (Individuals (Individuals homozygous heterozygous homozygous for allele B) with alleles B + b) for allele b) If q2 = 0.16 (the frequency of white cats), then q = 0.4. Therefore, p, the frequency of allele B, would be 0.6 (1.0 – 0.4 = 0.6). We can now easily calculate the genotype frequencies: there are p2 = (0.6)2 100 (the number of cats in the total population), or 36 homozygous dominant BB individuals. The heterozygous individuals have the Bb genotype, and there would be 2pq, or (2 0.6 0.4) 100, or 48 heterozygous Bb individuals. 424 Part VI Evolution 20.2 Why do allele frequencies change in populations? Sperm Eggs Phenotypes Genotypes BB Bb bb 0.36 0.48 0.16 0.36 + 0.24 = 0.6B 0.24 + 0.16 = 0.4b Frequency of genotype in population Frequency of gametes b B BB Bb Bb bb q2 = 0.16 pq = 0.24 pq = 0.24 p2 = 0.36 p = 0.6 q = 0.4 p = 0.6 q = 0.4 b B FIGURE 20.4 The Hardy–Weinberg equilibrium. In the absence of factors that alter them, the frequencies of gametes, genotypes, and phenotypes remain constant generation after generation.
Using the Hardy-Weinberg Equation Table 20.1 Agents of Evolutionary Change The Hardy-Weinberg equation is a simple extension of the Facto Punnett square described in chapter 13, with two alleles as- signed frequencies p and g. Figure 20.4 allows you to trace Mutation The ultimate source of variation. Individual mutations occur so rarely that mutation genetic reassortment during sexual reproduction and se alone does not change allele frequency how it affects the frequencies of the B and b alleles durin the next generation. In constructing this dhar hese cats is Gene flow A very potent agent of change. Populations assumed that the union of sperm and egg in random. so that all combinations of b and b alleles occur. For this reason, the alleles are mixed randomly and repre- sented in the next generation in proportion to their original eration has a 0.6 chance of receiving a B allele(p=0.6)and g om Inbreeding is the most common form. It representation. Each individual egg or sperm in each gen Nonrandom does not alter allele frequency but a 0.4 chance of receiving a b allele(=0.4 decreases the proportion of heterozygotes. In the next generation, therefore, the chance of combin ing two B alleles is p2, or 0.36(that is, 0.6 X0.6),and proximately 36% of the individuals in the population will Genetic drift Statistical accidents Usually occurs only in continue to have the BB genotype. The frequency of bb in dividuals is g2(0.4 X 0.4)and so will continue to be about 16%, and the frequency of Bb individuals will be 2pg(2X Selection The only form that produces adaptive 0.6 X 0.4), or approximately 48%. Phenotypically, if the evolutionary changes population size remains at 100 cats, we will still see approx- imately 84 black individuals(with either BB or Bb geno- types)and 16 white individuals(with the bb genotype)in ment are important. In fact, they are the key to the im the population. Allele, genotype, and phenotype frequen ortance of the Hardy-Weinberg principle, because indi cies have remained unchanged from one generation to the vidual allele frequencies often change in natural popula ions with some alleles becoming more common and This simple relationship has proved extraordinarily others decreasing in frequency. The Hardy-Weinberg useful in assessing actual situations. Consider the recessive principle establishes a convenient baseline against which allele responsible for the serious human disease cystic fi- to measure such changes. By looking at how various fac- brosis. This allele is present in North Americans of Cau- tors alter the proportions of homozygotes and heterozy- casian descent at a frequency g of about 22 per 1000 indi- gotes, we can identify the forces affecting particular situa viduals, or 0.022. What proportion of North American tions we observe Caucasians, therefore, is expected to express this trait Many factors can alter allele frequencies. Only five The frequency of double recessive individuals(o)is ex- however, alter the proportions of homozygotes and het pected to be 0.022 0.022, or 1 in every 2000 individu- erozygotes enough to produce significant deviations from als. What proportion is expected to be heterozygous car- the proportions predicted by the Hardy-Weinberg princi riers? If the frequency of the recessive allele q is 0.022, ple: mutation, gene flow(including both immigration into then the frequency of the dominant allele p must be and emigration out of a given population), nonrandom is(2pq) is thus expected to be 2 0.978 X 0.022, or 43 which is more likely in small populations), and selection in every 1000 individuals (table 20.1). Of these, only selection produces adaptive evo- How valid are these calculated predictions? For many lutionary change because only in selection does the result genes, they prove to be very accurate. As we will see, fo depend on the nature of the environment. The other some genes the calculated predictions do not match the ac- tors operate relatively independently of the environment, tual values. The reasons they do not tell us a great deal so the changes they produce are not shaped by environ about evolution mental demands Why Do Allele Frequencies Change The Hardy-Weinberg principle states that in a large According to the Hardy-Weinberg principle, both the al population mating at random and in the absence of lele and genotype frequencies in a large, random-mating other forces that would change the proportions of the cOpulation will remain constant from generation to gen- different alleles at a given locus, the process of sexual ation if no mutation, no gene flow, and no selection reproduction(meiosis and fertilization) alone will not occur. The stipulations tacked onto the end of the state hange these proportions. Chapter 20 Genes within Populations 425
Using the Hardy–Weinberg Equation The Hardy–Weinberg equation is a simple extension of the Punnett square described in chapter 13, with two alleles assigned frequencies p and q. Figure 20.4 allows you to trace genetic reassortment during sexual reproduction and see how it affects the frequencies of the B and b alleles during the next generation. In constructing this diagram, we have assumed that the union of sperm and egg in these cats is random, so that all combinations of b and B alleles occur. For this reason, the alleles are mixed randomly and represented in the next generation in proportion to their original representation. Each individual egg or sperm in each generation has a 0.6 chance of receiving a B allele (p = 0.6) and a 0.4 chance of receiving a b allele (q = 0.4). In the next generation, therefore, the chance of combining two B alleles is p2, or 0.36 (that is, 0.6 0.6), and approximately 36% of the individuals in the population will continue to have the BB genotype. The frequency of bb individuals is q2 (0.4 0.4) and so will continue to be about 16%, and the frequency of Bb individuals will be 2pq (2 0.6 0.4), or approximately 48%. Phenotypically, if the population size remains at 100 cats, we will still see approximately 84 black individuals (with either BB or Bb genotypes) and 16 white individuals (with the bb genotype) in the population. Allele, genotype, and phenotype frequencies have remained unchanged from one generation to the next. This simple relationship has proved extraordinarily useful in assessing actual situations. Consider the recessive allele responsible for the serious human disease cystic fibrosis. This allele is present in North Americans of Caucasian descent at a frequency q of about 22 per 1000 individuals, or 0.022. What proportion of North American Caucasians, therefore, is expected to express this trait? The frequency of double recessive individuals (q2) is expected to be 0.022 0.022, or 1 in every 2000 individuals. What proportion is expected to be heterozygous carriers? If the frequency of the recessive allele q is 0.022, then the frequency of the dominant allele p must be 1 – 0.022, or 0.978. The frequency of heterozygous individuals (2pq) is thus expected to be 2 0.978 0.022, or 43 in every 1000 individuals. How valid are these calculated predictions? For many genes, they prove to be very accurate. As we will see, for some genes the calculated predictions do not match the actual values. The reasons they do not tell us a great deal about evolution. Why Do Allele Frequencies Change? According to the Hardy–Weinberg principle, both the allele and genotype frequencies in a large, random-mating population will remain constant from generation to generation if no mutation, no gene flow, and no selection occur. The stipulations tacked onto the end of the statement are important. In fact, they are the key to the importance of the Hardy–Weinberg principle, because individual allele frequencies often change in natural populations, with some alleles becoming more common and others decreasing in frequency. The Hardy–Weinberg principle establishes a convenient baseline against which to measure such changes. By looking at how various factors alter the proportions of homozygotes and heterozygotes, we can identify the forces affecting particular situations we observe. Many factors can alter allele frequencies. Only five, however, alter the proportions of homozygotes and heterozygotes enough to produce significant deviations from the proportions predicted by the Hardy–Weinberg principle: mutation, gene flow (including both immigration into and emigration out of a given population), nonrandom mating, genetic drift (random change in allele frequencies, which is more likely in small populations), and selection (table 20.1). Of these, only selection produces adaptive evolutionary change because only in selection does the result depend on the nature of the environment. The other factors operate relatively independently of the environment, so the changes they produce are not shaped by environmental demands. The Hardy–Weinberg principle states that in a large population mating at random and in the absence of other forces that would change the proportions of the different alleles at a given locus, the process of sexual reproduction (meiosis and fertilization) alone will not change these proportions. Chapter 20 Genes within Populations 425 Table 20.1 Agents of Evolutionary Change Factor Description Mutation The ultimate source of variation. Individual mutations occur so rarely that mutation alone does not change allele frequency much. Gene flow A very potent agent of change. Populations exchange members. Nonrandom Inbreeding is the most common form. It mating does not alter allele frequency but decreases the proportion of heterozygotes. Genetic drift Statistical accidents. Usually occurs only in very small populations. Selection The only form that produces adaptive evolutionary changes.
Five Agents of UV light DNA Evolutionary Change fertilization 1. Mutation Mutation from one allele to an- other can obviously change the proportions of particular alleles in a population. Mutation rates are generally so low that they ffect on the a)Mutation b)gene flow c)Nonrandom mating Hardy-Weinberg proportions of ommon alleles. A single gene ay mutate about 1 to 10 times FIGURE 20.5 per 100,000 cell divisions(al- agents of though some genes mutate much evolutionary change. more frequently than that). Be- (a) Mutation, (b)gene flow, cause most environments are (C)nonrandom mating. constantly changing, it is rare for (a) genetic drift, and (e)selection. a population to be stable enough to accumulate changes in allele fr produced by a prod this slow. Nonetheless, mutation is the ultimate source of genetic (d)Genetic drift variation and thus makes evolu tion possible. It is important to remember, however, that the likelihood of a particular mu- among populations and thus keep the populations from di tation occurring is not affected by natural selection; that is, verging genetically. In some situations, gene flow can mutations do not occur more frequently in situations in counter the effect of natural selection by bringing an allele which they would be favored by natural selection. into a population at a rate greater than that at which the al- lele is removed by selection 2. Gene flow Gene flow is the movement of alleles from one population 3. Nonrandom mating to another. It can be a powerful agent of change because Individuals with certain genotypes sometimes mate with members of two different populations may exchange ge- one another more commonly than would be expected on a netic material. Sometimes gene flow is obvious, as when an random basis, a phenomenon known as nonrandom mat- ties of the newly arrived animal differ from those of the an- dom mating that causes the frequencies of particular gela animal moves from one place to another. If the characteris- ing Inbreeding(mating with relatives) is a type of nonrar imals already there, and if the newcomer is adapted well types to differ greatly from those predicted by the enough to the new area to survive and mate successfully, Hardy-Weinberg principle Inbreeding does not change the genetic composition of the receiving population may be the frequency of the alleles, but rather increases the pro- altered. Other important kinds of gene flow are not as ob- portion of homozygous individuals because relatives are vious. These subtler movements include the drifting of ga- likely be genetically similar and thus produce offsprin metes or immature stages of plants or marine animals from with two copies of the same allele. This is why populations one place to another(figure 20.5). Male gametes of flower- of self-fertilizing plants consist primarily of homozygous ing plants are often carried great distances by insects and individuals, whereas outcrossing plants, which interbreed other animals that visit their flowers. Seeds may also blow with individuals different from themselves, have a higher in the wind or be carried by animals or other agents to new proportion of heterozygous individuals far from their place of origin. In addition, gene By increasing homozygosity in a population, inbreeding flow may also result from the mating of individuals belong- increases the expression of recessive alleles. It is for this ing to adjacent populations reason that marriage between close relatives is discouraged However it occurs, gene flow can alter the genetic char- and to some degree outlawed-it increases the possibility acteristics of populations and prevent them from maintain- of producing children homozygous for an allele associated ing Hardy-Weinberg equilibrium. In addition, even low with one or more of the recessive genetic disorders dis- levels of gene flow tend to homogenize allele frequencies cussed in chapter 13 426 Part vI Evolution
Five Agents of Evolutionary Change 1. Mutation Mutation from one allele to another can obviously change the proportions of particular alleles in a population. Mutation rates are generally so low that they have little effect on the Hardy–Weinberg proportions of common alleles. A single gene may mutate about 1 to 10 times per 100,000 cell divisions (although some genes mutate much more frequently than that). Because most environments are constantly changing, it is rare for a population to be stable enough to accumulate changes in allele frequency produced by a process this slow. Nonetheless, mutation is the ultimate source of genetic variation and thus makes evolution possible. It is important to remember, however, that the likelihood of a particular mutation occurring is not affected by natural selection; that is, mutations do not occur more frequently in situations in which they would be favored by natural selection. 2. Gene Flow Gene flow is the movement of alleles from one population to another. It can be a powerful agent of change because members of two different populations may exchange genetic material. Sometimes gene flow is obvious, as when an animal moves from one place to another. If the characteristics of the newly arrived animal differ from those of the animals already there, and if the newcomer is adapted well enough to the new area to survive and mate successfully, the genetic composition of the receiving population may be altered. Other important kinds of gene flow are not as obvious. These subtler movements include the drifting of gametes or immature stages of plants or marine animals from one place to another (figure 20.5). Male gametes of flowering plants are often carried great distances by insects and other animals that visit their flowers. Seeds may also blow in the wind or be carried by animals or other agents to new populations far from their place of origin. In addition, gene flow may also result from the mating of individuals belonging to adjacent populations. However it occurs, gene flow can alter the genetic characteristics of populations and prevent them from maintaining Hardy–Weinberg equilibrium. In addition, even low levels of gene flow tend to homogenize allele frequencies among populations and thus keep the populations from diverging genetically. In some situations, gene flow can counter the effect of natural selection by bringing an allele into a population at a rate greater than that at which the allele is removed by selection. 3. Nonrandom Mating Individuals with certain genotypes sometimes mate with one another more commonly than would be expected on a random basis, a phenomenon known as nonrandom mating. Inbreeding (mating with relatives) is a type of nonrandom mating that causes the frequencies of particular genotypes to differ greatly from those predicted by the Hardy–Weinberg principle. Inbreeding does not change the frequency of the alleles, but rather increases the proportion of homozygous individuals because relatives are likely be genetically similar and thus produce offspring with two copies of the same allele. This is why populations of self-fertilizing plants consist primarily of homozygous individuals, whereas outcrossing plants, which interbreed with individuals different from themselves, have a higher proportion of heterozygous individuals. By increasing homozygosity in a population, inbreeding increases the expression of recessive alleles. It is for this reason that marriage between close relatives is discouraged and to some degree outlawed—it increases the possibility of producing children homozygous for an allele associated with one or more of the recessive genetic disorders discussed in chapter 13. 426 Part VI Evolution (a) Mutation UV light DNA T A G G G G C C (b) Gene flow (c) Nonrandom mating (d) Genetic drift (e) Selection Selffertilization FIGURE 20.5 Five agents of evolutionary change. (a) Mutation, (b) gene flow, (c) nonrandom mating, (d) genetic drift, and (e) selection
4. Genetic Drift In small populations, frequencies of particular alleles may change drastically by chance alone. Such changes in allele frequencies occur randomly, as if the frequencies were drifting, and are thus known as genetic drift. For this rea- son,a population must be large to be in Hardy-Weinberg equilibrium. If the gametes of only a few individuals form the next generation, the alleles they carry may by chance not be representative of the parent population from which they were drawn, as illustrated in figure 20.6, where a small f number of individuals are removed from a bottle contain ing many. By chance, most of the individuals removed are 8 blue, so the new population has a much higher population of blue individuals than the parent one had population (drastic reduction individuals generation A set of small populations that are isolated from one an- in population) other may come to differ strongly as a result of genetic drift even if the forces of natural selection do not differ between FIGURE 20.6 the populations. Indeed, because of genetic drift, harmful Genetic drift: The bottleneck effect. The parent population ontains roughly equal numbers of blue and yellow individuals. By spite selective disadvantage, and favorable alleles may be chance, the few remaining individuals that comprise the next generation are mostly blue. The bottleneck occurs because so fer lost even though selectively advantageous. It is Interestin dividuals form the next generation, as might happen after an to realize that humans have lived in small groups for much epidemic or catastrophic storm. of the course of their evolution; consequently, genetic drift may have been a particularly important factor in the evolu of Even large populations may feel the effect of genetic Many self-pollinating plants start new populations from a drift. Large populations may have been much smaller in the le seed past, and genetic drift may have greatly altered allele fre- Founder effects have been particularly important in the quencies at that time. Imagine a population containing only evolution of organisms on distant oceanic islands, such as two alleles of a gene, B and b, in equal frequency(that is, p the Hawaiian Islands and the Galapagos Islands visited by 9=0.5). In a large Hardy-Weinberg population, the Darwin. Most of the organisms in such areas probably de genotype frequencies are expected to be 0.25 BB, 0.50 Bb, rive from one or a few initial"founders. In a similar way, and 0.25 bb. If only a small sample produces the next gener- isolated human populations are often dominated by genetic ation, large deviations in these genotype frequencies can features characteristic of their particular founders occur by chance. Imagine, for example, that four individu als form the next generation, and that by chance they are The Bottleneck Effect. Even if organisms do not move two Bb heterozygotes and two BB homozygotes--the allele from place to place, occasionally their populations may be requencies in the next generation are p= 0.75 and g=0.25! drastically reduced in size. This may result from flooding, If you were to replicate this experiment 1000 times, each drought, epidemic disease, and other natural forces, or time randomly drawing four individuals from the parental from progressive changes in the environment. The few sur population, one of the two alleles would be missing entirely viving individuals may constitute a random genetic sample from about 8 of the 1000 populations. This leads to an im- of the original population(unless some individuals survive portant conclusion: genetic drift leads to the loss of alleles specifically because of their genetic makeup). The resultant in isolated populations. Two related causes of decreases in alterations and loss of genetic variability has been termed a population s size are founder effects and bottlenecks the bottleneck effect Some living species appear to be severely depleted ge Founder Effects. Sometimes one or a few individuals netically and have probably suffered from a bottleneck ef- disperse and become the founders of a new, isolated popi fect in the past. For example, the northern elephant seal lation at some distance from their place of origin. These pi which breeds on the western coast of north America and oneers are not likely to have all the alleles present in the nearby islands, was nearly hunted to extinction in the nine- source population. Thus, some alleles may be lost from the teenth century and was reduced to a single population con new population and others may change drastically in fre- taining perhaps no more than 20 individuals on the island quency. In some cases, previously rare alleles in the source of Guadalupe off the coast of Baja, California. As a result of population may be a significant fraction of the new popula- this bottleneck, even though the seal populations have re- ions genetic endowment. This phenomenon is called the bounded and now number in the tens of thousands, this founder effect. Founder effects are not rare in natur species has lost almost all of its genetic variation Chapter 20 Genes within Populations 427
4. Genetic Drift In small populations, frequencies of particular alleles may change drastically by chance alone. Such changes in allele frequencies occur randomly, as if the frequencies were drifting, and are thus known as genetic drift. For this reason, a population must be large to be in Hardy–Weinberg equilibrium. If the gametes of only a few individuals form the next generation, the alleles they carry may by chance not be representative of the parent population from which they were drawn, as illustrated in figure 20.6, where a small number of individuals are removed from a bottle containing many. By chance, most of the individuals removed are blue, so the new population has a much higher population of blue individuals than the parent one had. A set of small populations that are isolated from one another may come to differ strongly as a result of genetic drift even if the forces of natural selection do not differ between the populations. Indeed, because of genetic drift, harmful alleles may increase in frequency in small populations, despite selective disadvantage, and favorable alleles may be lost even though selectively advantageous. It is interesting to realize that humans have lived in small groups for much of the course of their evolution; consequently, genetic drift may have been a particularly important factor in the evolution of our species. Even large populations may feel the effect of genetic drift. Large populations may have been much smaller in the past, and genetic drift may have greatly altered allele frequencies at that time. Imagine a population containing only two alleles of a gene, B and b, in equal frequency (that is, p = q = 0.5). In a large Hardy–Weinberg population, the genotype frequencies are expected to be 0.25 BB, 0.50 Bb, and 0.25 bb. If only a small sample produces the next generation, large deviations in these genotype frequencies can occur by chance. Imagine, for example, that four individuals form the next generation, and that by chance they are two Bb heterozygotes and two BB homozygotes—the allele frequencies in the next generation are p = 0.75 and q = 0.25! If you were to replicate this experiment 1000 times, each time randomly drawing four individuals from the parental population, one of the two alleles would be missing entirely from about 8 of the 1000 populations. This leads to an important conclusion: genetic drift leads to the loss of alleles in isolated populations. Two related causes of decreases in a population’s size are founder effects and bottlenecks. Founder Effects. Sometimes one or a few individuals disperse and become the founders of a new, isolated population at some distance from their place of origin. These pioneers are not likely to have all the alleles present in the source population. Thus, some alleles may be lost from the new population and others may change drastically in frequency. In some cases, previously rare alleles in the source population may be a significant fraction of the new population’s genetic endowment. This phenomenon is called the founder effect. Founder effects are not rare in nature. Many self-pollinating plants start new populations from a single seed. Founder effects have been particularly important in the evolution of organisms on distant oceanic islands, such as the Hawaiian Islands and the Galápagos Islands visited by Darwin. Most of the organisms in such areas probably derive from one or a few initial “founders.” In a similar way, isolated human populations are often dominated by genetic features characteristic of their particular founders. The Bottleneck Effect. Even if organisms do not move from place to place, occasionally their populations may be drastically reduced in size. This may result from flooding, drought, epidemic disease, and other natural forces, or from progressive changes in the environment. The few surviving individuals may constitute a random genetic sample of the original population (unless some individuals survive specifically because of their genetic makeup). The resultant alterations and loss of genetic variability has been termed the bottleneck effect. Some living species appear to be severely depleted genetically and have probably suffered from a bottleneck effect in the past. For example, the northern elephant seal, which breeds on the western coast of North America and nearby islands, was nearly hunted to extinction in the nineteenth century and was reduced to a single population containing perhaps no more than 20 individuals on the island of Guadalupe off the coast of Baja, California. As a result of this bottleneck, even though the seal populations have rebounded and now number in the tens of thousands, this species has lost almost all of its genetic variation. Chapter 20 Genes within Populations 427 Parent population Bottleneck (drastic reduction in population) Surviving individuals Next generation FIGURE 20.6 Genetic drift: The bottleneck effect. The parent population contains roughly equal numbers of blue and yellow individuals. By chance, the few remaining individuals that comprise the next generation are mostly blue. The bottleneck occurs because so few individuals form the next generation, as might happen after an epidemic or catastrophic storm
5. Selection Selection to Avoid Predators. Many of the most dra- As Darwin pointed out, some individuals leave behind matic documented instances of a daptauon involve gene more progeny than others, and the rate at which they do so hanges which decrease the probability of capture by a is affected by phenotype and behavior. We describe the re- predator. The caterpillar larvae of the common sulphur butterfly Colias eurytheme usually exhibit a dull Kelly green Its of this process as selection and speak of both artifi- color, providing excellent camouflage on the alfalfa plants the breeder selects for the desired characteristics. In natural selection. environmental conditions determine which indi- is kept at very low frequency because this color renders the larvae highly visible on the food plant, making it easier for viduals in a population produce the most offspring. For bird predators to see them. In a similar fashion, the way the natural selection to occur and result in evolutionary change three conditions must be met shell markings in the land snail Cepaea nemoralis match its background habitat reflects the same pattern of avoiding 1. Variation must exist among individuals in a popu- predation by camouflage lation. Natural selection works by favoring individ- of the most dramatic examples of background uals with some traits over individuals with alternative matching involves ancient lava flows in the middle of traits. If no variation exists, natural selection cannot deserts in the American southwest. In these areas, the black oPerate rock formations produced when the lava cooled contrasts 2. Variation among individuals results in differences starkly to the surrounding bright glare of the desert sand in number of offspring surviving in the next gen- Populations of many species of animals--including lizards, eration. This is the essence of natural selection Be- rodents, and a variety of insects--occurring on these rocks cause of their phenotype or behavior, some individu- are dark in color, whereas sand-dwelling populations in als are more successful than others in producing surrounding areas are much lighter(figure 20.7). Predation offspring and thus passing their genes on to the next is the likely cause selecting for these differences in color Laboratory studies have confirmed that predatory birds are 3. Variation must be geneticall lly inherited. For adept at picking out individuals occurring on backgrounds natural selection to result in evolutionary change, ch the the selected differences must have a genetic basis However, not all variation has a genetic basis--even genetically identical individuals may be phenotype cally quite distinctive if they grow up in different environments. Such environmental effects are com mon in nature. In many turtles, for example, indi viduals that hatch from eggs laid in moist soil are heavier, with longer and wider shells, than individu Is from nests in drier areas. As a result of these en vironmental effects, variation within a population does not always indicate the existence of underlyin genetic variation. When phenotypically different individuals do not differ genetically, then differ- ences in the number of their offspring will not alter the genetic composition of the population in the next generation and, thus, no evolutionary change It is important to remember that natural selection and evolution are not the same-the two concepts often are incorrectly equated. Natural selection is a process, whereas evolution is the historical record of change through time. Evolution is an outcome, not a process Natural selection(the process) can lead to evolution(the outcome), but natural selection is only one of several processes that can produce evolutionary change. More- FIGURE 20.7 over, natural selection can occur without producing evo- Pocket mice from the Tularosa Basin of New Mexico whose tionary change; only if variation is genetically based will color matches their background. (a) The rock pocket mouse natural selection lead to evolution lives on lava, (b)while the Apache pocket mouse lives on white 428 Part vI Evolution
5. Selection As Darwin pointed out, some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior. We describe the results of this process as selection and speak of both artificial selection and natural selection. In artificial selection, the breeder selects for the desired characteristics. In natural selection, environmental conditions determine which individuals in a population produce the most offspring. For natural selection to occur and result in evolutionary change, three conditions must be met: 1. Variation must exist among individuals in a population. Natural selection works by favoring individuals with some traits over individuals with alternative traits. If no variation exists, natural selection cannot operate. 2. Variation among individuals results in differences in number of offspring surviving in the next generation. This is the essence of natural selection. Because of their phenotype or behavior, some individuals are more successful than others in producing offspring and thus passing their genes on to the next generation. 3. Variation must be genetically inherited. For natural selection to result in evolutionary change, the selected differences must have a genetic basis. However, not all variation has a genetic basis—even genetically identical individuals may be phenotypically quite distinctive if they grow up in different environments. Such environmental effects are common in nature. In many turtles, for example, individuals that hatch from eggs laid in moist soil are heavier, with longer and wider shells, than individuals from nests in drier areas. As a result of these environmental effects, variation within a population does not always indicate the existence of underlying genetic variation. When phenotypically different individuals do not differ genetically, then differences in the number of their offspring will not alter the genetic composition of the population in the next generation and, thus, no evolutionary change will have occurred. It is important to remember that natural selection and evolution are not the same—the two concepts often are incorrectly equated. Natural selection is a process, whereas evolution is the historical record of change through time. Evolution is an outcome, not a process. Natural selection (the process) can lead to evolution (the outcome), but natural selection is only one of several processes that can produce evolutionary change. Moreover, natural selection can occur without producing evolutionary change; only if variation is genetically based will natural selection lead to evolution. Selection to Avoid Predators. Many of the most dramatic documented instances of adaptation involve genetic changes which decrease the probability of capture by a predator. The caterpillar larvae of the common sulphur butterfly Colias eurytheme usually exhibit a dull Kelly green color, providing excellent camouflage on the alfalfa plants on which they feed. An alternative bright blue color morph is kept at very low frequency because this color renders the larvae highly visible on the food plant, making it easier for bird predators to see them. In a similar fashion, the way the shell markings in the land snail Cepaea nemoralis match its background habitat reflects the same pattern of avoiding predation by camouflage. One of the most dramatic examples of background matching involves ancient lava flows in the middle of deserts in the American southwest. In these areas, the black rock formations produced when the lava cooled contrasts starkly to the surrounding bright glare of the desert sand. Populations of many species of animals—including lizards, rodents, and a variety of insects—occurring on these rocks are dark in color, whereas sand-dwelling populations in surrounding areas are much lighter (figure 20.7). Predation is the likely cause selecting for these differences in color. Laboratory studies have confirmed that predatory birds are adept at picking out individuals occurring on backgrounds to which they are not adapted. 428 Part VI Evolution (b) (a) FIGURE 20.7 Pocket mice from the Tularosa Basin of New Mexico whose color matches their background. (a) The rock pocket mouse lives on lava, (b) while the Apache pocket mouse lives on white sand
