THE DEVELOPMENT OF A PHYLOGENY OF FROGS.

Abstract

A phylogeny is an outline of the history of a taxon. As such it must take into account not only genetic relationships among the members of the group, but also sequence of evolution of the various branches. Knowledge of sequence implies knowledge of direction of change; primitive character states precede derived ones. Knowledge of relationships implies knowledge of the number of evolutionary steps that occurred between an ancestral form and the various descendents. A recent proposal for determination of phylogenies chooses among alternative interpretations on the basis of the minimum number of changes or evolutionary steps required by each phylogeny (Camin and Sokal, 1965). This criterion does not adequately take into account the numerous parallelisms that may occur within a taxon. Neither does it allow for alteration in the conditions of selection that may lead a population to head first in one genetic direction and then in another (Lewontin, 1965). In order to define the sequence of branching, it is not sufficient to know direction of change. One must also know whether or not a character state is unique, that is, has arisen in just one phylum (Wilson, 1965) (See Appendix: Note A). Ideally phylogenies are worked out on the basis of reasonably good fossil sequences. Numerous fossil frogs have been recovered and described, but they contribute relatively little to our understanding of the phylogeny of frogs. The basic reason for this inadequacy of the fossil record is that the Salientia constitute an unusually invariable, narrow, isolated group morphologically. As an order frogs are so specialized that a tremendous morphological gap separates them from all other amphibians. Yet the variation within the order, despite adaptations to fossorial, aquatic, arboreal, or terrestrial life, is slight. To paraphrase Gertrude Stein-a frog is a frog, is a frog. The simplification of the skeleton, which is the significant result of this specialization of the order, makes parallelism within the order or convergence between families likely (Appendix: Note B). Indeed, we can be certain that parallelism has occurred a number of times (Parker, 1931b). The median element of the anuran pectoral girdle consists of paired epicoracoidal cartilages, which may be fused (firmisternal) along the mid-ventral line or overlapping (arciferal). In general, families of frogs are characterized by one or the other condition. In a scattering of genera in otherwise arciferal families the epicoracoidal cartilages have fused for greater or lesser portions of their length; for example, Sminthillus-Leptodactylidae (Noble, 1931); Cacophryne-Bufonidae (Griffiths, 1959). These cases of incomplete firmisterny constitute an example of parallelism within the order and raise the possibility that the major firmisternal families (Microhylidae and Ranidae) share this resemblance as a result of independent evolution. Because of the simplicity of the skeleton, some of the characteristic features of certain families could be modified by relatively minor changes in developmental patterns. The shape of the vertebral centra provides an example. The centra of most families are concave anteriorly though in a few families they are concave posteriorly. Mookerjee (1931) and Griffiths (1960) have shown that during ontogeny the car-