Sexual selection at the protein level drives the extraordinary divergence of sex-related genes during sympatric speciation

An increasing number of molecular studies are indicating that, in a wide va riety of species, genes directly related to fertilization evolve at extraor dinarily high rates. We try to gain insight into the dynamics of this rapid evolution and its underlying mechanisms by means of a simple theoretical m odel. In the model, sexual selection and sympatric speciation act together in order to drive rapid divergence of gamete recognition proteins. In this process, intraspecific competition for fertilizations enlarges male gamete protein variation by means of evolutionary branching, which initiates sympa tric speciation. In addition, avoidance of competition for fertilizations b etween the incipient species drives the rapid evolution of gamete recogniti on proteins. This mechanism can account for both strong stabilizing selecti on on gamete recognition proteins within species and rapid divergence betwe en species. Moreover, it can explain the empirical finding that the rate of divergence of fertilization genes is not constant, but highest between clo sely related species.