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 4274. 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 number of individuals are removed from a bottle contain￾ing 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 an￾other 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, de￾spite 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 evolu￾tion 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 fre￾quencies 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 gener￾ation, large deviations in these genotype frequencies can occur by chance. Imagine, for example, that four individu￾als 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 im￾portant 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 popu￾lation at some distance from their place of origin. These pi￾oneers 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 fre￾quency. In some cases, previously rare alleles in the source population may be a significant fraction of the new popula￾tion’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 de￾rive 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 sur￾viving 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 ge￾netically and have probably suffered from a bottleneck ef￾fect 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 nine￾teenth century and was reduced to a single population con￾taining 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 re￾bounded 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