Identifying the effects of history on adaptation: origins of different sand‐diving techniques in lizards

The effects of previous history on adaptation can be studied in situations where several taxa have been independently exposed to a well-specified problem posed by their selective regimes. This approach was applied to the ways surface-dwelling dune lizards dive rapidly into loose sand when evading predators. Lacertids of the genus Meroles use all limbs and serpentine body and tail movements, behaviour which arose directly in the context of predator evasion in sandy habitats. The agamids, Phrynocephalus and Agama etoshae employ lateral body oscillations and descend vertically, an aberrant method resulting from previous evolution of a blunt snout. The scincid, Scincus and gerrhosaurid, Angolosaurus, both use high amplitude sinusoidal movements of the body and tail, which apparently evolved in a common ancestor of these taxa long before being co-opted to use in sand. In contrast, the scincid, Mabuya acutilabris, holds the body straight, a method that probably originated in the context of concealment at the end of activity. In the phrynosomatid, Uma, the head oscillates up to 60 times per second, the forelimbs are folded back and hindlimb thrust is supplemented by scales on the upper arms acting like ratchets. Some components of this method originated in the context of slow concealment in firm substrata at the end of activity periods, a basic pattern later co-opted for rapid diving in compact sand with modification of the snout to form a drill. Finally, this elaborated mechanism was transferred to loose sand with further behavioural and morphological change. Rapid sand burial by Phrynosoma species has the same origin as that in Uma but, like Phrynocephalus, includes a strong vertical component. It too is associated with a blunt snout which, however, arose after initial burial behaviour developed, not before. These results suggest that, once related lineages have diverged, differences may still accumulate even in similar situations. Sand diving also exhibits striking cases of convergence and provides an example of how function can be used to polarize characters, suggest the situations in which derived states arose and indicate their likely order of change.