What factors shape rates of phenotypic evolution? A comparative study of cranial morphology of four mammalian clades

Abstract

Understanding why rates of morphological evolution vary is a major goal in evolutionary biology. Classical work suggests that body size, interspecific competition, geographic range size and specialisation may all be important, and each may increase or decrease rates of evolution. Here we investigate correlates of proportional evolutionary rates in phalangeriform possums, phyllostomid bats, platyrrhine monkeys and marmotine squirrels, using phylogenetic comparative methods. We find that the most important correlate is body size. Large species evolve fastest in all four clades, and there is a non-linear relationship in platyrrhines and phalangeriforms, with slowest evolution in species of intermediate size. We also find significant increases in rate with high environmental temperature in phyllostomids, and low mass-specific metabolic rate in marmotine squirrels. The mechanisms underlying these correlations are uncertain and appear to be size-specific. We conclude that there is significant variation in rates of evolution, but that its meaning is not yet clear.