The role of different reproductive barriers during phenotypic divergence of isopod ecotypes.

The question of how diverging populations become separate species by restraining gene flow is a central issue in evolutionary biology. Assortative mating might emerge early during adaptive divergence, but the role of other types of reproductive barriers such as migration modification have recently received increased attention. We demonstrate that two recently diverged ecotypes of a freshwater isopod (Asellus aquaticus) have rapidly developed premating isolation, and this isolation barrier has emerged independently and in parallel in two south Swedish lakes. This is consistent with ecological speciation theory, which predicts that reproductive isolation arises as a byproduct of ecological divergence. We also find that in one of these lakes, habitat choice acts as the main barrier to gene flow. These observations and experimental results suggest that migration modification might be as important as assortative mating in the early stages of ecological speciation. Simulations suggest that the joint action of these two isolating barriers is likely to greatly facilitate adaptive divergence, compared to if each barrier was acting alone.