Swarthmore College Department of Computer Science

Talk by Aaron Clauset, Santa Fe Institute

A quantitative evolutionary theory for the distribution of species body size
Monday, February 25, 2008
4 pm in the Science Center, Room 181

Within most large taxonomic groups (mammals, insects, birds, etc.), the distribution of species body size exhibits both a single prominent mode relatively near but not at the smallest species size and a smooth but heavy right-tail (often described as a right-skew on a log-size scale) that extends for several orders of magnitude. In spite of many years of study, a coherent and mechanistic explanation of the ubiquity of these statistical features remains lacking.

In this talk, I will give just such an explanation, in the form of a simple computer model of cladogenetic diffusion in body sizes. For concreteness, I focus on terrestrial mammals, first estimating most of the model's parameters directly from the available fossil data. Then, through an extensive simulation study, I show that the estimated model can robustly and precisely reproduce the empirical distribution of 4002 terrestrial mammal species from the late Quarternary period. This strong agreement between model and data suggests that the observed asymmetric distribution arises from a fundamental tradeoff between the short-term selective advantages (Cope's rule) and the long-term selective risks of increased species body size, all in the context of a taxon-specific lower limit on species body size.