Investigating the evolutionary effects of one feature on another: does muscle spread suppress caudal autotomy in lizards?

Hypotheses of secondary evolutionary change, where alteration in a particular feature is thought to result in change in another, can be tested in two main ways. First, phylogenies can be used to identify separate cases where one of the features changes and each case can then be examined to see whether the other change also actually takes place and if the perceived sequence of the alterations is appropriate. Secondly, the mechanism by which change in the second feature is supposed to be effected can be scrutinized and, in some cases, subjected to experimental investigation. This approach was applied to a recent hypothesis, that backward spread of the caudifemoralis longus muscle in the tail base of lizards was the primary cause of loss of capacity to autotomize the tail. Some 23 to 25 independent cases of total autotomy loss in adult lizards were identified. In all but six of these there was no substantial spread of the muscle. In two of the remainder, the muscle appears to have spread ufiev autotomy loss, and another case cannot be tested properly as information about relationships is equivocal. The final three cases exhibit extension of the caudifemoralis longus before autotomy loss, but the latter is not found in related species that also inherit muscle extension, which suggests that other causal factors may be involved. In about 15 other cases, where autotomy becomes restricted to the tail base, there is no marked spread of the caudifemoralis longus. The proposed functional link between muscle extension and autotomy loss is also discussed and discounted