What drives diversification? Range expansion tops climate, life history, habitat and size in lizards and snakes

Aim: A major challenge in ecology and evolutionary biology is to explain the dramatic differences in species richness among clades. Much variation in richness is explained by the differences in diversification rates among clades, and variation in diversification rates is often linked to various traits. But what types of traits are most important for explaining diversification? Here, we compared the impacts of different types of traits on diversification rates among lizard and snake families and tested predictions about the relative importance of ecology vs. morphology, static vs. dynamic traits and alpha vs. beta niche traits. Location: Global. Taxon: Squamata. Methods: We compared the relative impacts of traits related to biogeography (range size, range expansion), climate, life history (viviparity), microhabitat and morphology (body-size) on diversification rates among all 72 family-level clades of squamates. We compiled data on traits and tested for relationships between traits and diversification rates using phylogenetic multiple regression models. Results: The best-fitting model explained ~60% of the variation in diversification rates across squamate families. This model included only microhabitat (proportion of arboreal species) and a novel, dynamic, ecological/biogeographic beta-niche trait (rate of range expansion), which explained most variance. Other variables had more variable or non-significant contributions, including rates of climatic-niche change. Rates of range expansion were related to species richness, larger body size and faster rates of climatic-niche change. Main conclusions: Overall, we provide possibly the most comprehensive comparison of the types of traits that can drive diversification. We also help explain diversity patterns in one of the largest vertebrate clades. We show that the rate of range expansion is the most important variable for explaining diversification rates and richness patterns in squamates. We also identify traits that help explain variation in rates of range expansion among clades.