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

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 specialization 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 the fastest in all four clades, and there is a nonlinear relationship in platyrrhines and phalangeriformes, with the 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.