SEXUAL SELECTION, SPACE, AND SPECIATION

A Fisherian model of sexual selection is combined with a diffusion model of mate dispersal to investigate the evolution of assortative mating in a sympatric population. Females mate with one of two types of polygynous males according to a male's display of one of two sex-limited, autosomal traits; these male traits may be associated with differential phenotypic mortalities. Through a Fisherian runaway process, female preferences and male traits can become associated in linkage disequilibrium, leading to patterns of assortative mating. Dispersing males, whose rate of movement is dependent on mating success, carry female preference genes with them, and displaced males thereby produce daughters with preference genes for their respective traits in locally higher than average frequencies. The reduced diffusion of the more preferred males permits the success of other male types in adjacent areas. Thus, mating-success dependent diffusion, when coupled with the rapid divergence in phenotypes possible under the Fisher process, can lead to the coexistence of two female preferences and two male traits in sympatry. We argue that many existing approaches to sympatric speciation fail to explain observed male polymorphisms because they exclude explicit spatial structure from their speciation models.