Interactions among Evolutionary orces (Agrostis tenuis) When alleles are not selectively neu- 8 tral,levels of variation retained in a 960mine Non- Mine Non-mine population may be determined by the 3 relative strength of different evolution- ary processes. In theory, for example, if allele B mutates to allele b at a high a enough rate, allele b could be main-%20 tained in the population even if natural selection strongly favored allele B. In nature. however. mutation rates are 020406080100120140160 rarely high enough to counter the ef- Distance in meters fects of natural selection The effect of natural selection also FIGuRE 20.1 may be countered by genetic drift. Degree of copper tolerance in grass plants on and near ancient mine sites. Prevailing Both processes may act to remove vari- winds blow pollen containing nontolerant alleles onto the mine site and tolerant alleles ation from a population. However, beyond the site's borders whereas selection is a deterministic process that operates to increase the representation of alleles that enhance survival and reproductive success, drift is a random process. Thus, in some cases, drift may lead to than in surrounding areas. Heavy metal concentrations are a decrease in the frequency of an allele that is favored by generally toxic to plants, but alleles at certain genes confer loss of a favored allel eme cases, drift may even lead to the resistance. The ability to tolerate heavy metals comes at a selection. In some ext from a population. Remember, how price, however; individuals with the resistance allele exhibit ever, that the magnitude of drift is negatively related to lower growth rates on non-polluted soil. Consequently, we population size; consequently, natural selection is expected would expect the resistance allele to occur with a frequenc to overwhelm drift except when populations are very small. of 100% on mine sites and 0% elsewhere Heavy metal tol- erance has been studied particularly intensively in the slen Gene Flow versus Natural selection der bent grass, Agrostis tenuis, in which researchers have found that the resistance allele occurs at intermediate levels Gene flow can be either a constructive or a constraining in many areas(figure 20. 10). The explanation relates to the force. On one hand, gene flow can increase the adaptedness reproductive system of this grass in which pollen, the male of a species by spreading a beneficial mutation that arises in gamete(that is, the floral equivalent of sperm), is dispersed one population to other populations within a species On by the wind. As a result, pollen-and the alleles it carries- the other hand, gene flow can act to impede adaptation can be blown for great distances, leading to levels of gene within a population by continua ally importing inferior all flow between mine sites and unpolluted areas high enough les from other populations. Consider two populations of to counteract the effects of natural selection species that live in different environments. In this situation general, the extent to which gene flow can hinder the natural selection might favor different alleles-B and b-in effects of natural selection should depend on the relative the different populations. In the absence of gene flow and strengths of the two processes In species in which gene other evolutionary processes, the frequency of B would be flow is generally strong, such as birds and wind-pollinated expected to reach 100% in one population and 0% in the plants, the frequency of the less favored allele may be rela two populations, then the less favor oing on between the tively high, whereas in more sedentary species which ex- other. However, if gene flow were go nu- hibit low levels of gene flow, such as salamanders, the fa ally be reintroduced into each population. As a result, the vored allele should occur at a frequency near 100% frequency of the two alleles in each population would re- flect a balance between the rate at which gene flow brings the inferior allele into a population versus the rate at which Evolutionary processes may act to either natural selection removes it maintain genetic variation within a population. Allele A classic example of gene flow opposing natural selec- equency sometimes may reflect a balance between tion occurs on abandoned mine sites in great britain, Al opposed processes, such as gene flow and natural though mining activities ceased hundreds of years ago, the selection. In such cases, observed frequencies w concentration of metal ions in the soil is still much greater depend on the relative strength of the processe Chapter 20 Go ene hin Populations 431Interactions among Evolutionary Forces When alleles are not selectively neu￾tral, levels of variation retained in a population may be determined by the relative strength of different evolution￾ary processes. In theory, for example, if allele B mutates to allele b at a high enough rate, allele b could be main￾tained in the population even if natural selection strongly favored allele B. In nature, however, mutation rates are rarely high enough to counter the ef￾fects of natural selection. The effect of natural selection also may be countered by genetic drift. Both processes may act to remove vari￾ation from a population. However, whereas selection is a deterministic process that operates to increase the representation of alleles that enhance survival and reproductive success, drift is a random process. Thus, in some cases, drift may lead to a decrease in the frequency of an allele that is favored by selection. In some extreme cases, drift may even lead to the loss of a favored allele from a population. Remember, how￾ever, that the magnitude of drift is negatively related to population size; consequently, natural selection is expected to overwhelm drift except when populations are very small. Gene Flow versus Natural Selection Gene flow can be either a constructive or a constraining force. On one hand, gene flow can increase the adaptedness of a species by spreading a beneficial mutation that arises in one population to other populations within a species. On the other hand, gene flow can act to impede adaptation within a population by continually importing inferior alle￾les from other populations. Consider two populations of a species that live in different environments. In this situation, natural selection might favor different alleles—B and b—in the different populations. In the absence of gene flow and other evolutionary processes, the frequency of B would be expected to reach 100% in one population and 0% in the other. However, if gene flow were going on between the two populations, then the less favored allele would continu￾ally be reintroduced into each population. As a result, the frequency of the two alleles in each population would re￾flect a balance between the rate at which gene flow brings the inferior allele into a population versus the rate at which natural selection removes it. A classic example of gene flow opposing natural selec￾tion occurs on abandoned mine sites in Great Britain. Al￾though mining activities ceased hundreds of years ago, the concentration of metal ions in the soil is still much greater than in surrounding areas. Heavy metal concentrations are generally toxic to plants, but alleles at certain genes confer resistance. The ability to tolerate heavy metals comes at a price, however; individuals with the resistance allele exhibit lower growth rates on non-polluted soil. Consequently, we would expect the resistance allele to occur with a frequency of 100% on mine sites and 0% elsewhere. Heavy metal tol￾erance has been studied particularly intensively in the slen￾der bent grass, Agrostis tenuis, in which researchers have found that the resistance allele occurs at intermediate levels in many areas (figure 20.10). The explanation relates to the reproductive system of this grass in which pollen, the male gamete (that is, the floral equivalent of sperm), is dispersed by the wind. As a result, pollen—and the alleles it carries— can be blown for great distances, leading to levels of gene flow between mine sites and unpolluted areas high enough to counteract the effects of natural selection. In general, the extent to which gene flow can hinder the effects of natural selection should depend on the relative strengths of the two processes. In species in which gene flow is generally strong, such as birds and wind-pollinated plants, the frequency of the less favored allele may be rela￾tively high, whereas in more sedentary species which ex￾hibit low levels of gene flow, such as salamanders, the fa￾vored allele should occur at a frequency near 100%. Evolutionary processes may act to either remove or maintain genetic variation within a population. Allele frequency sometimes may reflect a balance between opposed processes, such as gene flow and natural selection. In such cases, observed frequencies will depend on the relative strength of the processes. Chapter 20 Genes within Populations 431 Index of copper tolerance Distance in meters Non￾mine Mine Non-mine 0 20 40 0 20 40 60 80 100 120 140 160 0 20 40 60 Prevailing wind Bent grass (Agrostis tenuis) FIGURE 20.10 Degree of copper tolerance in grass plants on and near ancient mine sites. Prevailing winds blow pollen containing nontolerant alleles onto the mine site and tolerant alleles beyond the site’s borders