One of the conditions of the Hardy-Weinberg equilibrium (the set of circumstances under which evolution does not occur) is that the population of breeding individuals is large enough that chance will not alter the allele frequency from one generation to another. When the population is small, however, all but one allele of each gene tends to go extinct. In a population with 20 females, for instance, for each gene with two equally common alleles, there is more than a 75% chance one or the other allele will be lost within 50 generations, independent of any selection. This chance variation in allele frequency in small populations is called genetic drift.
Drift can be enormously important. The most common scenario for speciation involves geographic isolation of a population. This typically occurs when individuals invade a new habitat or are cut off from the rest of the species at one corner of its range. These isolated populations are likely to be small, and they may well by chance be missing some alleles even before genetic drift can take place. The consequence of this kind of biased start in a subpopulation is called the founder effect. The same thing happens when a local population is reduced in size from disease, climate change, or habitat alteration—the so-called genetic bottleneck. Alleles not surviving the bottleneck event, the founding of a population, or subsequent genetic drift are simply lost.
In many cases, the rapid evolution (changes in allele frequency) of a population is caused by genetic drift and selection taking place simultaneously: Isolated populations or populations drastically reduced by external factors are typically subject to unusual selection pressures. The challenges of a new or marginal habitat, different diseases and parasites, or an altered climate select for novel allele combinations. Selection and genetic drift, taken together, seem to account for most of the cases in which an isolated population becomes unable to reproduce with the parent population by the time contact is reestablished. Reproductive isolation is the essential final step in speciation.
- Gould, J. L., & Keeton, W. T. (1996). Biological science (6th ed.). New York: Norton.