Fig. 9.

Sexual selection facilitates speciation through disruptive natural selection even in the absence of diverging preferences (after Van Doorn et al. 2009). In the absence of sexual selection, disruptive selection for ecological specialisation will typically not lead to speciation, but to a broad distribution of ecological characters (bill sizes). The variation in ecological characters, many of which not well adapted to local circumstances, creates favourable conditions for the spread of costly female preferences for a costly condition-dependent male ornament (plumage colour). Once this preference has evolved, natural and sexual selection mutually reinforce each other, eventually leading to reproductively isolated ecological specialist species