Post-glacial demographic and migration history of the alpine flora for an enhanced assessment of its fate under ongoing climate change

Current biodiversity patterns in the northern hemisphere have primarily been shaped by climatic fluctuations that took place during the Pleistocene. While the identification of glacial refugia and post-glacial migration routes has long been a major focus in historical biogeography, the question of where species currently restricted to Alpine areas persisted during glaciations has long appeared as a striking puzzle. This question is particularly timely as it impacts our understanding of the speed at which organisms migrated to track areas of suitable climate, with substantial implications regarding their ability to face the current velocity of climate change. Using RAD sequencing data for selected land plant species (bryophytes, pteridophytes, spermatophytes), we determine whether current distribution patterns result from the in-situ persistence of populations under the ice or migrations from ice-free refugia. We further assess the rate at which migrations effectively occurred, and how these rates differ within and among biota characterized by contrasting dispersal syndromes using coalescence simulations. Bryophytes and spermatophytes exhibited contrasted patterns of genetic structure, consistent with their dispersal capacities. Although the analyses revealed different biogeographic origins for species currently distributed across the Alps, observed genetic structure and diversity were globally largely consistent across taxa with simulations under a scenario of persistence within the Alps. Spatially explicit models of coalescence were finally implemented, considering movements of individuals and genes while attempting to connect current patterns of genetic variation with the evolution of the species range over time. We aim to determine whether back-colonisation took place from ice-free refugia at the southern limit of the mountain range or from ice-covered areas within the core of it, and at which rate. This spatially explicit framework points to different recolonization scenario accross species, evidencing that some species could have survived under the ice sheet and back colonized the Alps lower than thought.