Figure 2.

Mechanisms through which hybridization can enhance or reduce sexual dimorphism and, in turn, affect local adaptation. (a) Transgressive segregation of sexually antagonistic alleles which have become fixed at different loci in two hybridizing species. These are quantitative trait loci for a trait involved in niche use (e.g. beak shape in birds). After initial hybridization, recombination may lead to different phenotypic outcomes (females above and males below each locus) where sexual dimorphism is either enhanced (left lower panel) or dampened (right lower panel). This may, in turn, have consequences on intersexual niche partitioning and local adaptation. (b) Non-coincident geographical clines between sexes for ecological traits in a hybrid zone. In admixed populations, enhanced sexual dimorphism, due to sex-specific differences in geographical clines (upper panel), may promote the occupation of novel ecological niches. Parent species may be incapable of colonizing this novel ecological niche, not because of morphospace constraints, but simply as a result of decreased mean population fitness due to intersexual competition and costly gender load (lower panel).