Genetic Drift

The third requirement of genetic equilibrium is the presence of a large population. The Hardy-Weinberg principle is based on the laws of probability, which are less applicable to smaller populations. Genetic drift is the phenomenon by which allele frequencies in a population change as a result of random events, or chance. In small populations, the failure of even a single organism to reproduce can significantly disrupt the allele frequency of the population, as can greater-than-normal reproduction by an individual, resulting in genetic drift. Because it can result in significant changes within a population, genetic drift is thought to be another possible mechanism for the evolution of new species.

Figure 16-5 shows a graph of genetic drift in populations of three differing sizes. Small populations can undergo abrupt changes in allele frequencies, exhibiting a large degree of genetic drift, whereas large populations retain fairly stable allele frequencies, maintaining a small degree of genetic drift. In the smallest population shown in the graph, the frequency of the example allele reaches zero at about the 45th generation. If we assume that the population started with two alleles for a trait, then only one allele is left, and every individual is homozygous for that trait. Once this change happens, the population is in danger of becoming extinct because there is no variation for natural selection to act on. For example, a natural disaster or a new disease could wipe out the entire population. For this reason, endangered species, such as the northern elephant seal, as shown in Figure 16-6, remain in peril of extinction even as their numbers increase.

figure 16-6

Populations of the once nearly extinct northern elephant seal, Mirounga angustirostris, have lost genetic variability—individuals are homozygous for all of their genes that have been tested. This result of genetic drift could make the species vulnerable to extinction.

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