Background/Question/Methods The distribution of species body sizes within many extant and extinct taxonomic groups of species, such as mammals, birds, fish, insects, and possibly dinosaurs and trilobites, exhibits a canonical form, which is unimodal with a long right- but a short left-tail. Among terrestrial mammals, for instance, the smallest and largest masses are 2 g and 10^7 g respectively, while the typical mass is about 40 g. Explanations of the origin of this immense morphological diversity have wide implications due to the strong correlation between species body size and other species-level characteristics such as habitat, life history, life span, metabolism and extinction risk.

Results/Conclusions Here we present an evolutionary explanation of species body size diversity that is not based on optimal body-size theory or allometry. Instead, the model is based on constrained cladogenetic diffusion in morphospace, and omits explicit mechanisms for inter-specific competition and other ecological processes. We present new empirical results supporting this explanation of body size diversity, and discuss the model's macroecological and evolutionary implications. For instance: species body size distributions are shaped by a fundamental tradeoff between the short-term selective advantages for (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.