Reproductive character displacement between the closely related freshwater snails Lymnaea peregra and L. ovata

Theory predicts that enhanced assortative mating, favouring intraspecific mating, should evolve together with reproductive character displacement. Assortative mating may be involved directly in the evolution of species barriers in the case of sympatric speciation, and may strengthen species barriers after secondary contact. If hybrids are at a selective disadvantage, interspecific mating leads to wasted reproductive effort and enhanced assortative mating is predicted to be favoured, while in allopatric populations of the same species, selection for assortative mating does not take place. Here, we assessed whether sympatric and allopatric populations of the two closely related freshwater snails species Lymnaea peregra and L. ovata mate assortatively. More specifically, we tested populations from several allopatric and one sympatric location for discrimination against interspecific versus intraspecific matings in a series of non-choice mating trials. We found, as predicted by the theory, that snails from the sympatric location avoided mating with the opposite species, while allopatric snails showed less discrimination against the opposite species. In a broader perspective, our results support the view that reproductive isolation may commonly be reinforced by selection when two closely related taxa occur in sympatry.