Postglacial recolonization in a cold climate specialist in western Europe: patterns of genetic diversity in the adder (; Vipera berus; ) support the central–marginal hypothesis

Understanding the impact of postglacial recolonization on genetic diversity is essential in explaining current patterns of genetic variation. The central–marginal hypothesis (CMH) predicts a reduction in genetic diversity from the core of the distribution to peripheral populations, as well as reduced connectivity between peripheral popula- tions. While the CMH has received considerable empirical support, its broad applica- bility is still debated and alternative hypotheses predict different spatial patterns of genetic diversity. Using microsatellite markers, we analysed the genetic diversity of the adder (Vipera berus) in western Europe to reconstruct postglacial recolonization. Approximate Bayesian Computation (ABC) analyses suggested a postglacial recoloniza- tion from two routes: a western route from the Atlantic Coast up to Belgium and a cen- tral route from the Massif Central to the Alps. This cold-adapted species likely used two isolated glacial refugia in southern France, in permafrost-free areas during the last glacial maximum. Adder populations further from putative glacial refugia had lower genetic diversity and reduced connectivity; therefore, our results support the predic- tions of the CMH. Our study also illustrates the utility of highly variable nuclear markers, such as microsatellites, and ABC to test competing recolonization hypotheses.