Figure 1. A two-locus genotypic fitness surface in which the fitness of the genotypes is determined by formulae first published by Crow et al. (1990), and configured as used in Moore and Tonsor (1994). In this kind of epistatic interaction, there are nine possible two-locus, two-allele combinations, involving 0, 1 or 2 copies of mutant ‘A’ allele at one locus, and of the mutant ‘B’ allele at a second locus (the alternatives are the wild type ‘a’ and ‘b’ alleles, respectively). Three fitness phenotypes are produced by the nine genotypes: 1) Individuals with at least one copy of A and one copy of B. These have a relative fitness of 1 + ks. 2) Individuals homozygous for both a and b alleles. These individuals have a relative fitness of 1-s. 3) Individuals heterozygous at one locus but homozygous wild type (aa or bb) for the other locus. These have an intermediate relative fitness of 1. In this figure, k = 4 and s = 0.025.

Figure 2. An ‘adaptive topography’ for the two-locus two-allele interaction whose genotypic fitnesses are illustrated in Figure 1. Based on Moore and Tonsor (1994).

Figure 3. The frequency of SBP through Phase III for the epistatic interactions used Figures 1 and 2. Ten thousand demes with average N = 30, simulations run for 12 000 generations.