Species that co-occur in nature remain distinct due to multiple reproductive barriers to gene flow. Identifying the chronological order in which these barriers evolve is central to understanding species formation: we seek to identify the first reproductive barrier(s) to evolve to completion during the process of speciation. These initial reproductive barriers represent the first possibility for diverging lineages to remain distinct in a shared community. The extraordinary diversity of conspicuous male sexual ornaments in animals suggests that behavioral differences may often form the first barrier to gene flow.

Male courtship traits appear to evolve both faster and to greater extremes than other traits, such that closely related animal species can differ dramatically in these phenotypes. Importantly, models of sexual selection predict that a rapid evolution of male courtship traits may often be paralleled by a rapid evolution of female preferences, leading directly to behavioral isolation and speciation, as females of one population become less likely to mate with males of another. But does behavioral isolation really evolve faster than other reproductive barriers?

Research in my lab is designed to test this prediction in darters, a diverse genus of North American freshwater fish with striking male nuptial coloration. We do this by comparing the strength of multiple reproductive barriers across multiple stages of evolutionary diversification, testing the prediction that behavioral isolation tends to increase with time faster than other reproductive barriers (see Mendelson 2003a). We are especially interested in

• The relative rates of ecological divergence (resulting from natural selection), and behavioral divergence (resulting from natural and/or sexual selection), and
• The rate of evolution of hybrid sterility, a postzygotic reproductive barrier known to evolve rapidly in several insect groups. These analyses require multiple experiments that take us from the field to the lab, including habitat analysis, mate choice experiments, in vitro fertilization, animal husbandry, and phylogenetic analysis.    Click here to see a movie of darters spawning.