Plant speciation in the face of recurrent climate changes in the Alps

The main, continuous mountain range of the European Alpine System (i.e., the Alps) hosts a diversified pool of species whose evolution has long been investigated. The legacy of past climate changes on the distribution of high-elevation plants as well as taxa differentially adapted to the mosaic of edaphic conditions (i.e., surmised ecotypes on calcareous, siliceous, serpentine bedrocks) and the origin of new species are here discussed based on available evidence from endemic taxa across the Alps. The integration of main spatial and ecological patterns within and among species supports speciation driven by spatial isolation in main glacial refugia where plant populations survived during cold phases and hindered by intense gene flow along main expansion pathways during warm phases. Despite patterns of genetic differentiation matching environmental heterogeneity, processes underlying the dynamics of distribution ranges likely promoted recurrent homogenization of incipient divergence and generally hindered the completion of speciation (except for cases of hybrid speciation). Even intense selective pressures on toxic bedrocks such as serpentine seemingly fail to support the completion of speciation. Accordingly, typical scenarios of ecological speciation whereby local adaptation to environmental heterogeneity initiates and supports long-term reduction of gene flow may rarely be at the origin of stable species in the Alps. Although consistent with neutral processes whereby spatial isolation driven by past climate changes promoted reproductive isolation and yielded limited diversification, mechanisms at the origin of new species across heterogeneous landscapes of the Alps remain insufficiently known. Necessary advances to reliably understand the evolution of biodiversity in the Alps and identify possible museums or cradles of variation in face of climate changes are discussed.