Repeated evolution of high elevation occupation in tropical lizards

Since Darwin and Humboldt, researchers have tried to understand how species adapt to different elevations. Tropical reptiles are typically diverse in warm, lowland environments, yet many reptile lineages have colonized high elevation environments on both islands and the mainland. Nonetheless, the frequency with which these environments have been colonized remains unclear. The occurrence of high elevation species on multiple mountain ranges could reflect a single lowland-to-highland transition followed by subsequent divergence into multiple species or, alternatively, could result from more than one colonization event with limited within-lineage diversification. Here, we test these alternatives for anole lizards which, though predominantly found in tropical lowlands, also occupy multiple mountain ranges in Central and South America and the Caribbean. Analyzing extensive climatic and distributional data for 303 species and thermal physiology data (cold tolerance or CTmin) for 64 species, we found that colonization of high-elevation regions has occurred many times in anoles, that the frequency of such occupation is lower than the opposite, and that biogeographic dispersal across mountains and shifts in climatic niche are distributed non-randomly across the phylogeny. Additionally, transitions between lowland and highland habitats do not coincide with shifts in the rate of stochastic trait evolution for either cold tolerance or minimum thermal niche. Instead, highland lineages show stronger selection toward a colder optimum for minimum ambient temperature, consistent with climatic niche conservatism in montane environments. Cold tolerance itself shows no such shift, suggesting a decoupling between the climatic environment lineages experience and the evolution of a physiological trait that should, in principle, track it. Together, these results show that repeated mountain colonization has reshaped climatic niche evolution in Anolis, while physiological cold tolerance has followed a comparatively stable, elevation-independent trajectory.