Bimodal hybrid zones and speciation.

Contact zones exemplify a series of stages in speciation. In unimodal hybrid zones intermediates predominate; in bimodal zones hybrids are rare and parental forms predominate; and finally, species might overlap, but never hybridize. Recent studies show bimodality to be associated strongly with assortative mating or fertilization, and only weakly with overall levels of genetic divergence or intrinsic genomic incompatibility. Ecological divergence across most bimodal hybrid zones suggests that ecology contributes more to speciation than genomic incompatibility. This continuum of stable contact zones provides empirical evidence for a route to speciation, which does not require allopatry.