Aaron Clauset, Santa Fe Institute

The evolution and distribution of species body size

Friday March 21st, 4pm, Phillips 332
(unfortunately, no refreshments will be served at the usual 3:30 time, because of the university holiday)

Abstract: Within most large taxonomic groups (mammals, insects, birds, etc.), the distribution of species body size exhibits a single prominent mode relatively near but not at the smallest species size and a smooth but heavy right-tail (often described as a strong right-skew on a log-scale) that extends for several orders of magnitude (for mammals, it extends over 6 decades). 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 describe the results of modeling species body size evolution as a cladogenetic process where body size diffuses in a multiplicative fashion against a fundamental lower limit. After briefly describing the model, I show how we estimate almost all of its parameters directly from the available fossil data. Through an extensive simulation study, I'll show that the estimated model robustly and precisely reproduces the entire empirical distribution of 4002 terrestrial mammal species from the late Quarternary period. This strong agreement between model and data suggests that the particular shape of the distribution for mammals is the result of a fundamental tradeoff between the short-term selective advantages (Cope's rule), and the long-term selective risks, associated with increased species body size.