Figure 1. Genetic drift causes random fluctuation in allele frequencies. This can be demonstrated by computer simulations of allele frequencies: (a) In small populations (N = 10), this ‘random walk’ in gene frequencies is more likely to result in fixation of neutral or nearly neutral mutations. (b) In large populations (N = 100), random sampling has less effect on gene frequencies, so the rate of fixation of alleles by drift will be lower.

Figure 2. The selection, neutral and nearly neutral schools of thought differ in their predictions of the proportion of mutations that will be advantageous, deleterious, neutral or nearly neutral. Nearly neutral mutations have small selection coefficients, being only slightly deleterious or advantageous.