Species diversification results from biogeographic processes and selective pressures which promote genetic and ecological divergence among populations. Understanding how species functional traits relate to genetic and environmental variability is necessary to infer mechanisms that generate diversity, as well as to anticipate species’ response to anthropogenic change. Adaptions to climatic conditions usually evolve over a long timeframe and multiple generations. However, rapid ongoing climate change is now strongly affecting biodiversity and notably ectotherms, leading to contrasted ecological responses even among closely related species. A major challenge for living organisms is that climate change combines both (1) gradual increase in temperature (seasons, years) and (2) acute weather events (days, weeks) such as heatwaves. The fast pace of current change exacerbates the risks of crossing physiological limits and lethal thresholds and the benefits of plastic responses (physiological and behavioral) at the individual level. In this context, an integrated understanding of the thermal niche is essential to clarify the proximate basis of past climatic adaptation and current species vulnerability. Using the monophyletic group of the snakes of the genus Vipera (15 species) as case study, DIVCLIM deals with this challenge, addressing the diversification of their thermal niche (WP2) and response to climatic stressors (WP3), also focusing at the population level in one species along a climatic gradient in Western Europe (WP4), to provide a novel mechanistic insight on drivers of species vulnerability to climate change (WP5).