The evolution and distribution of species body size

TL;DR

This paper presents a simple evolutionary model explaining the heavy-tailed distribution of species body sizes, emphasizing a tradeoff between short-term benefits and long-term risks, validated with fossil data.

Contribution

Introduces a cladogenetic diffusion model that explains species body size distribution without microevolutionary mechanisms, fitting fossil data of mammals.

Findings

Model accurately reproduces mammal body size distribution

Distribution shaped by tradeoff between Cope's rule and size limits

Robust parameter estimation from fossil data

Abstract

The distribution of species body size within taxonomic groups exhibits a heavy right-tail extending over many orders of magnitude, where most species are significantly larger than the smallest species. We provide 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 and find that it robustly reproduces the distribution of 4002 mammal species from the late Quaternary. 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.