What Constrains Adaptive Radiation? Documentation and Explanation of Under-evolved Morphologies in Lizards

Adaptive radiations fill ecological and morphological space during evolutionary diversification. Why do some trait combinations evolve during such radiations, whereas others do not? 'Required' constraints of pleiotropy and developmental interaction frequently are implicated in explanations for such patterns, but selective forces also may discourage particular trait combinations. Here, we use a dataset of 351 species to demonstrate the dearth of some theoretically plausible trait combinations of limb, toe and tail length in lizards. For example, disproportionately few species display long limbs and short toes. We evaluate recovered patterns within three species of , and find that cladewide patterns are not evident at intraspecific levels. For example, within species, the combination of long limbs and short toes is not significantly rarer than long limbs and long toes. Differences in scale complicate inter- and intraspecific comparisons and disallow concrete conclusions of cause. However, the absence of the interspecific pattern at the intraspecific level is more compatible with selection favouring particular trait combinations than with 'required' forces dictating which trait combinations are available for selection. We also demonstrate the isometry of toe, tail and hindlimb length relative to body length between species but allometry in four of nine trait-body comparisons within species.