Figure 2.—

Strong costs prohibit the evolution of assortative mating. In the grouping model, assortative mating will evolve if homozygotes are, on average, more fit (, Equation 9) and if the costs of assortative mating are sufficiently weak (below curves, from Equation 14). Here we assume that Aa individuals form their own group () and consider costs to be either fixed (dashed curve) or relative to the difficulty of finding a mate (solid curve). Given that , assortative mating can evolve only if costs lie below the appropriate curve and only as long as the A locus remains polymorphic. Note that the y-axis is measured relative to (Equation 9b), the degree to which homozygotes are more fit, on average, than heterozygotes given the current genotype frequencies.