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 EvolutionFive Agents of Evolutionary Change 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 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 (al￾though some genes mutate much more frequently than that). Be￾cause 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 evolu￾tion possible. It is important to remember, however, that the likelihood of a particular mu￾tation 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 ge￾netic material. Sometimes gene flow is obvious, as when an animal moves from one place to another. If the characteris￾tics of the newly arrived animal differ from those of the an￾imals 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 ob￾vious. These subtler movements include the drifting of ga￾metes or immature stages of plants or marine animals from one place to another (figure 20.5). Male gametes of flower￾ing 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 belong￾ing to adjacent populations. However it occurs, gene flow can alter the genetic char￾acteristics of populations and prevent them from maintain￾ing Hardy–Weinberg equilibrium. In addition, even low levels of gene flow tend to homogenize allele frequencies among populations and thus keep the populations from di￾verging 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 al￾lele 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 mat￾ing. Inbreeding (mating with relatives) is a type of nonran￾dom mating that causes the frequencies of particular geno￾types to differ greatly from those predicted by the Hardy–Weinberg principle. Inbreeding does not change the frequency of the alleles, but rather increases the pro￾portion 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 dis￾cussed 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 Self￾fertilization FIGURE 20.5 Five agents of evolutionary change. (a) Mutation, (b) gene flow, (c) nonrandom mating, (d) genetic drift, and (e) selection