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Understanding how temperature determines the distribution of life is necessary to assess species’ sensitivities to contemporary climate change. Here we test the importance of temperature in limiting geographic ranges of ectotherms by comparing temperatures across occupied ranges to those species could potentially occupy based on their physiological thermal tolerances. Whereas marine and tropical terrestrial species occupy temperatures that closely match their thermal tolerances, high-latitude terrestrial species under-occupy warm temperatures and are absent from thermally tolerable areas towards the equator. This suggests that on land, temperature less often limits the equatorward range edge of temperate species, supporting the hypothesis that their historic expansion ‘out of the tropics’ was associated with tropical biotic exclusion. Our findings predict more direct responses to climate warming of marine ranges and cool range edges of terrestrial species.

A major challenge in ecology and evolution is to disentangle the mechanisms driving broad-scale variations in biological traits such as body size, colour, thermal physiology, and behaviour. Climate has long been thought to drive trait evolution and abiotic filtering of variation in organisms such as ectotherms, because their thermal performance and fitness are closely related to environmental conditions. However, previous studies investigating climatic variables associated to functional trait variation lacked a mechanistic description of the underpinning processes. Here, we use a mechanistic model to predict how climate affects thermal performance of ectotherms and thereby the direction and strength of the effect of selection on different functional traits. We show that climate drives macroevolutionary patterns in body size, cold tolerance, and preferred body temperatures among lizards, and that trait variation is more constrained in regions where selection is predicted to be stronger. These findings provide a mechanistic explanation for the hypothesis that climate drives trait variations in ectotherms through its effect on thermal performance. By connecting physical, physiological, and macroevolutionary principles, the model and results provide an integrative, mechanistic framework for predicting organismal responses to both present and future climate variations.