Secondary contact in Erebia butterflies

Secondary contact zones are ideal systems to study the processes that govern the evolution of reproductive barriers, especially at advanced stages of the speciation process. An increase in reproductive isolation resulting from selection against maladaptive hybrids (i.e. reinforcement) is thought to contribute to reproductive barrier buildup in secondary contact zones. While reinforcement has been invoked for many systems, it remains unclear to which extent it influences contact zone dynamics in nature. Here, we study a very narrow contact zone between the butterfly species Erebia cassioides and E. tyndarus in the Swiss Alps. We quantified phenotypic traits related to wing shape and reproduction as well as ecology in order to compare the degree of intra- and interspecific differentiation. Even though only very few first-generation hybrids occur, we find no strong indications for current reinforcing selection, suggesting that if reinforcement occurred in our system, it likely operated in the past. Additionally, we show that both species differ less in their ecological niche at the contact zone than elsewhere, which could explain why co-existence between these butterflies may currently not be possible.