Figure 1. The effects of genetic drift on 107 laboratory populations of the fruit fly, Drosophila melanogaster. The populations were maintained at a size of 16 individuals, giving a total of 32 possible different alleles (two per individual). The two horizontal axes respectively give the number of generations since the populations were all started (initial allele frequency of 0.5) and the number of mutant brown eye alleles in the population. The vertical axis reports how many of the populations possess that many alleles at a given time period. Looking across all 107 populations shows that genetic drift generates variation among the populations that increases with time, eventually ending in fixation of one or the other of the two alleles. Data from Buri (1956); figure from Hartl and Clark (1997).

Figure 2. The effects of time and population size on populations undergoing genetic drift. Each line shows the allele frequency from generation to generation of a separate population of size either 10 (a) or 100 (b). Each population was started at the same initial allele frequency (0.4). Note that the populations diverge from one another over time as genetic drift takes them in different directions. Also note that the rate of divergence is much less in the larger population. Given enough time, however, even these larger populations will diverge as much as the smaller populations. Thus, population size affects the rate of drift but not necessarily its eventual outcome.