21.5 MIGRATION, MUTATION, AND GENETIC DRIFT

Selection is evolution’s major driving force, enriching each new generation for the mutations that best fit organisms to their environments. However, as we have seen from the discussion of the Hardy–Weinberg equilibrium, it is not the only evolutionary mechanism. There are other forces that can cause allele frequencies to change. These are migration or gene flow, mutation, and the random effects of finite population size (that is, genetic drift). Like natural selection, these mechanisms can cause allele frequencies to change. Unlike natural selection, they do not lead to adaptations. Therefore, they are often considered non-adaptive evolutionary mechanisms.

21.5.1 Migration reduces genetic variation between populations.

Migration is the movement of individuals from one population to another, resulting in gene flow, the movement of alleles from one population to another. It is relatively simple to see how movements of individuals and alleles can lead to changes in allele frequencies. Consider two isolated island populations of rabbits, one white, the other black. Now imagine that the isolation breaks down—a bridge is built between the islands—and migration occurs. Over time, black alleles enter the white population and vice versa, and the allele frequencies of the two populations gradually become the same.

The consequence of migration is therefore the homogenizing of populations, making them more similar to each other and reducing genetic differences between them. Because populations are often adapted to their particular local conditions (think of dark-skinned humans in regions of high sunlight versus fair-skinned humans in regions of low sunlight), migration may be worse than merely non-adaptive—it may be maladaptive, in that it causes a decrease in a population’s average fitness. Fair-skinned people arriving in an equatorial region are, for example, at risk of sunburn and skin cancer.

21.5.2 Mutation increases genetic variation.

As we saw earlier in this chapter, mutation is a rare event. This means that it is generally not important as an evolutionary mechanism that leads allele frequencies to change. However, as we have also seen, it is the source of new alleles and the raw material on which the other forces act. Without mutation, there would be no genetic variation, and therefore no evolution.

21.5.3 Genetic drift is particularly important in small populations.

Genetic drift is the random change in allele frequencies from generation to generation. By “random” change, we mean that frequencies can either go up or down simply by chance. An extreme case is a population bottleneck, which occurs when a population falls to just a few individuals. Consider a rare allele, A, with frequency of . Now imagine that habitat destruction reduces the population to just one pair of individuals, one of which is carrying A. The frequency of A in this new population is ¼ because each individual has two alleles, giving a total of four alleles. In other words, the bottleneck resulted in a massive change in allele frequencies. Another kind of bottleneck, called a founder event, may be important in the establishment of new populations, for example when just a few individuals arrive to colonize an island.

Figure 21.12: Genetic drift. The fate of neutral mutations is governed by genetic drift, the effect of which is more extreme in small populations than in large populations.

Earlier, we considered the fate of beneficial and harmful mutations under the influence of natural selection. What about neutral mutations? Natural selection, by definition, does not govern the fate of neutral mutations. So what happens to them? Consider a neutral mutation, m, which has no effect on the fitness of its carrier. At first, it is in just a single heterozygous individual. What happens if that individual fails to reproduce (for reasons unrelated to m)? In this case, m will be lost from the population, but not by natural selection (which does not discriminate against m). Alternatively, the m-bearing individual might by chance leave many offspring (again for reasons unrelated to m), in which case the frequency of m will increase. In principle, it is possible over a long period of time for m to take over the population. At the end of the process, every member of the population is homozygous mm.

Like natural selection, genetic drift leads to allele frequency changes and therefore to evolution. Unlike natural selection, however, it does not lead to adaptations, since the alleles whose frequencies are changing do not affect an individual’s ability to survive or reproduce.

The impact of genetic drift depends on population size (Fig. 21.12). If m arises in a very small population, its frequency will change rapidly, as shown in Figs. 21.12a and 21.12b. Imagine m arising in a population of just six individuals (or three pairs). Its initial frequency is 1 in 12, or about 8% (there are a total of 12 alleles because each individual is diploid). If, by chance, one pair fails to breed and the other two (including the one who is an Mm heterozygote) each produce three offspring, and all three of the Mm individual’s offspring happen to inherit the m allele, then the frequency of m will increase to 3 in 12 (25%) in a single generation. In effect, genetic drift is equivalent to a sampling process. In a small sample, extreme departures from the expected outcome are common. Toss a coin 5 times, and you might well end up with zero heads.

On the other hand, if the population is large, as in Figs. 21.12c and 21.12d, then shifts in allele frequency from generation to generation are much smaller, typically less than 1%. A large population is analogous to a large sample size, in which we tend not to see marked departures from expectation. Toss a coin 1000 times, and you will end up with approximately 500 heads. In a small sample of coin tosses, we are much more likely to see marked departures from our 50:50 expectation than in a large sample. The same is true of genetic drift. It is likely to be much more significant in small populations than in large ones.