Rapoport's Rule: Do climatic variability gradients shape range extent?

The trend of increasing latitudinal range sizes of species towards higher latitudes, known as Rapoport's Rule, has been highly controversial in the literature since it was first proposed by Stevens in 1989. We contend that the question of interest is not whether general global patterns occur, nor whether they support or refute Rapoport's Rule, but whether the mechanism thought to underlie such patterns, the Climatic Variability Hypothesis, is supported. The Climatic Variability Hypothesis suggests that taxa originating from environmentally variable habitats, such as those at high latitudes and altitudes, should evolve wider environmental tolerances, and consequently establish wider distributions along climate gradients than taxa originating from relatively stable habitats. We applied a novel approach, incorporating measures of temperature variability across habitats within species' ranges into models of range size distributions, to determine whether the Climatic Variability Hypothesis applied to three clades of medium-sized ectotherms (lizards) distributed over Australia. Our results show that the Climatic Variability Hypothesis is supported, even in taxa that do not exhibit a traditional Rapoport Effect, due to complex, non-unidirectional climatic gradients in our study area. The results highlight the strong impact of climatic variability on species' physiological tolerances and their associated geographic distributions.