For a given gene locus, the frequency of a particular allele in a polymorphic population provides useful information, in that it describes the genetic composition of a given population at a particular point of time during evolution (Fig. 1). In principle, the allele frequency will never remain the same; it will always change with time because of mutation, selection, and nonrandom mating. Even when random mating occurs in a population, the allele frequency will change in a random manner over time. The random change of an allele frequency over time is called "genetic drift". In general, the effect of genetic drift is greater the smaller the population size, and the effect of selection on allele frequencies dominates the effect of genetic drift when the population size is larger.
Figure 1. Scheme demonstrating genetic drift of an allele frequency in a population by chance over time.
Genetic drift is an important mechanism for gene evolution because it can change the genetic composition drastically over a very short period of time, particularly when the population size is extremely small. This was initially recognized by Sewall Wright (1931) and therefore known as the Wright effect (1). The Wright effect can be seen when a small number of individuals from the mother population immigrate to a new place (the founder effect) and when the population size suddenly reduces, for reasons such as climatic change or volcanic eruption (the "bottleneck effect").
For identifying genes responsible for disease at the DNA level, it is necessary to assess linkage disequilibrium or genetic association with certain marker genes. For this practice, the effect of genetic drift should be evaluated correctly in order to discriminate between the effects of selection and of genetic drift.
