Figure 3.—

Evolutionarily stable level of assortative mating in the preference-based model without costs. Assortative mating initially evolves when , but this process is self-limiting if heterozygotes more readily find mates. Once the strength of sexual selection generated by assortment causes heterozygotes and homozygotes to become equally fit (; curves), further increases in the level of assortative mating are no longer favored (Equation 17). Whether or not a stable internal polymorphism exists along these curves depends on the form of frequency-dependent natural selection. The solid circles show stable equilibrium points, , using a specific model of frequency dependence: with , , , , and . Frequency-dependent natural selection becomes weaker as is reduced from 1 to 0.1 in increments of −0.1 (solid circles from left to right, without assortment along the x-axis or with assortment along the curves). Assortative mating drives the frequency of allele A away from (arrow with light shading, ; arrow with dark shading, ; solid arrow, ). A stable internal equilibrium exists along the curve only if frequency-dependent natural selection is sufficiently strong ( when ; when ; when ). Otherwise, the frequency of allele A rises as assortment evolves, until the polymorphism is lost. Note that the y-axis is measured relative to in the current population.