The Evolution and Distribution of Species Body Size

The distribution of species body size within taxonomic groups exhibits a heavy right-tail extending many orders of magnitude, with most species being significantly larger than the smallest species. We present a simple model of cladogenetic diffusion over evolutionary time that omits explicit mechanisms for inter-specific competition and other microevolutionary processes yet fully explains the shape of this distribution. We estimate the model's parameters from fossil data on terrestrial mammals since the K/T boundary and find that it robustly reproduces the distribution of 4002 mammal species from the late Quarternary. The observed fit suggests that the asymmetric distribution arises from a fundamental tradeoff between the short-term selective advantages (Cope's rule) and long-term selective risks of increased species body size, in the presence of a taxon-specific lower limit on body size.