Abstract

Pleistocene climate oscillations influenced the biogeographical history of most species. In the European Alps, mountain plants were restricted to refugial areas during cold phases of glacial cycles and recolonized newly available habitats during warm periods. The current ranges of alpine plants represent a transient stage of a continuous and dynamic recolonization process that started after the last glaciations. Differences in recolonization rate and range filling are observed between different mountain plant species, but the reasons remain insufficiently explored. Here, we investigated hybridization between two related willow species pairs that came into secondary contact to assess contemporary dynamics of selection and the potential effects of hybrid zones on range expansion. RAD sequencing data was used to infer the biogeographical history of each species pair, to characterize individuals across the whole distribution area including the secondary contact zones, and to quantify patterns of genomic clines. Genetic results were supported by morphometric measurements, ecological data, and species distribution modeling. Results suggest that peripheral glacial refugia played a major role in the history of the species. For both species' pairs, the secondary contact zones showed homoploid hybridization between parents, which is also supported by the morphometric analyses. Habitat preferences showed large overlaps of parental species and a broad range of niches for the hybrids. One hybrid zone is geographically broader than expected and characterized by introgression, while the other one is narrow and shows asymmetric patterns. Current projections of species distributions identified suitable habitats beyond the secondary contact zone, suggesting an incomplete range filling.