Body sizes and diversification rates of lizards, snakes, amphisbaenians and the tuatara

Aim Size is one of the most important and obvious traits of an organism.Both small and large sizes have adaptive advantages and disadvantages. Body size–frequency distributions of most large clades are unimodal and right skewed. Species larger than the mean or range midpoint of body sizes are relatively scarce. Theoretical models suggest evolutionary rates are higher in small organisms with short generation times. Therefore diversification rates are usually thought to be maximal at relatively small body sizes. Empirical studies of the rates of molecular evolution and clade diversification, however, have usually indicated that both are unrelated to body size. Furthermore, it has been claimed that because snakes are longer than lizards, the size–frequency distribution of all squamate species is bimodal overall. We examined the shape of the size–frequency distribution of nearly all Squamata and Rhynchocephalia species, and investigated how size affected diversification rates.