Climatic Adaptation Explains Responses to Pleistocene Oscillations and Diversification in European Vipers

Aim: Allopatric speciation is the primary mode of diversification in the Mediterranean Basin. However, the contribution of climatic adaptation during this process is contradictory. In this work, we investigate the eco-evolutionary processes that drove diversification in this region, using European vipers as a case study. We describe the climatic requirements of different lineages to compare their responses to the Pleistocene climatic oscillations and tackle the evolutionary mechanisms underlying their diversification.Location: Eurasia and North Africa.Taxon: European vipers (genus Vipera).Methods: We used ecological niche modelling (ENM) to identify the climatic requirements of 24 Vipera lineages and infer past range dynamics associated with their diversification during the Pleistocene. To test whether climatic niches varied across lineages, we calculated the phylogenetic signal of different climatic variables and examined the relationship with phylogenetic relatedness. To investigate climatic niche evolution and test for phylogenetic niche conservatism (PNC), we quantified pairwise niche overlap in sister phylogenetic units under a 3D hypervolume approach.Results: ENM identified temperature annual range, precipitation of wettest month and precipitation of driest quarter as the most important climatic variables related to the distribution of most lineages, validating Pelias clade as cold-adapted, and Vipera 1 and Vipera 2 as warm-adapted clades. Projections to past conditions varied among clades, with Pelias and Vipera 1 having more similar responses, while Vipera 2 exhibited greater variability. We found significant phylogenetic signal in one temperature-related and two humidity-related climatic variables and detected high complexity in ecological niche evolution across the phylogeny, both rejecting the hypothesis of PNC.Main Conclusions: Climatic adaptation played a significant role in driving diversification among European vipers. Cold-adapted and warm-adapted lineages presented similar climatic requirements and remarkable responses to Pleistocene stages, resulting in an intricate pattern of niche divergence along the phylogeny that favours local adaptation rather than PNC.