Fig. 7.

Parameter regime leading to speciation through the interaction of natural and sexual selection. Disruptive selection on ecological characters (bill size) and mating traits (female preference and male ornament), is a necessary but not sufficient condition for speciation in the model by Van Doorn and Weissing (2001). The evolution of reproductively isolated and stably coexisting species requires that the genes responsible for reproductive isolation become associated with the genes responsible for ecological divergence (this occurs in the orange area). Such an association does not easily arise by chance, since recombination tends to rapidly break down any non-random association between genes. However, if mating is partially based on ecological characters (for example, individuals with similar bill sizes may forage in the same areas, and may therefore be more likely to mate), then this can provide a counterforce against the erosion of associations between genes by recombination. A minimal strength of the effect of ecological differences on mating (the horizontal axis reflects the specificity of self-referent phenotype matching based on bill size), is necessary for speciation to occur (orange area) in combination with sufficiently strong disruptive selection (vertical axis)