Fig. 6.

No speciation despite disruptive natural selection. If females accept a broad range of mating partners, competition between males does not favour diversification among males. Rather, sexual selection favours the male ornament type (blue) that matches optimally with the most abundant female preference (red). Accordingly, there is no basis for reproductive isolation that could prevent interbreeding between individuals with different bill sizes (green). Although natural selection on bill size is disruptive, we, therefore, do not observe the emergence of two ecological specialists. Rather, disruptive selection leads to the maintenance of a broad unimodal distribution of bill sizes in the population. Adapted from Van Doorn and Weissing (2001)