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. Location: Global. Methods: We collected data on maximum body length for 9805 lepidosaur (squamates and the tuatara) species (99.7% of all species) and converted them to mass using clade-specific allometric equations. Using methods that test for relationships between continuous traits and speciation and extinction rates on a large, dated phylogeny (4155 species), we investigated the relationship between diversification rates and body size. Results: Living squamates span six orders of magnitude in body size, eight when giant extinct snakes and mosasaurs are included. The body size-frequency distributions of snakes and lizards separately, and of all lepidosaur species combined, are unimodal and right skewed. Nonetheless, we find neither linear nor hump-shaped relationships between size and diversification rates, except in snakes, where speciation and diversification are hump shaped. Main conclusions: Despite a clear modality and skew in the body sizes of lepidosaurs, we find little evidence for faster diversification of modal-sized taxa, perhaps implying that larger-sized clades are relatively young.