MORPHOLOGICAL VARIATION IN THE NEOTROPICAL SALAMANDER GENUS BOLITOGLOSSA

There are two aspects to any process of evolutionary change. One is the probability of survival of any given trait within a given context (i.e., other traits present in the individual and in the population, physical environment, population structure, etc. . . .). That is, the probability of fixation of a trait under the action of natural selection and stochastic processes. The other aspect is the nature, and origin, of the variation upon which the agents previously mentioned can operate. Mutation and recombination have been the mechanisms traditionally invoked in the generation of variation at the genetic level. However, selection operates at the level of the phenotype and, therefore, it is justified to ask if the properties of the variation at the genetic level can be extrapolated to the phenotypic level, particularly to the level of complex morphologies. Recently, Ho and Saunders (1979), Alberch (1980, 1982a, 1982b), Gould (1980), Rachootin and Thomson (198 1), Oster and Alberch (1982) and Wagner (1982) among others have contended that there are qualitative differences between modes of evolution at the genetic and epigenetic levels. A simple gene-phenotype extrapolation would only be correct if there were a oneto-one correspondence between a given morphological trait and a given region of DNA in the genome. This is obviously not the case. Morphology is the product of complex, and very often regulatory, genetic and epigenetic interactions. By epigenetic I mean interactions among gene products and their environment, not, as often assumed, nongenetic. These developmental interactions constrain the expression of morphological variation upon which natural selection operates. At the morphological level, one can abstract these processes and view phyletic phenotypic change as the result of genetically mediated regulation of a resilient developmental program. The present study addresses the above issue from an empirical perspective. I ask the question: What kind of phenotypic variation do we find in natural populations? However, I purposely do not focus on quantitative variations in metric traits as is often done (e.g., Lande, 1976; Cheverud, 1982). Rather, I analyze the variation encountered in osteological characters used in taxonomy and systematics. Taxonomists have chosen these traits, not because of their adaptive properties, but due to the diagnostic properties of these discrete and highly invariant traits. Furthermore, when paleontologists discuss evolution above the species level in the fossil record, they are basically studying the patterns of change in these species-diagnostic traits. Since the paleontological definition of species is based on purely morphological criteria, analyses of mechanisms of morphological change of such features among extant species can be readily applied to actual macroevolutionary patterns in the fossil record and related to contemporary controversies (e.g., Stanley, 1979). I describe the intra-specific (mostly intra-populational) variation found in the dermal bones of the nasal region, and in the number of tarsal elements and phalanges in various species of the neotropical salamander genus Bolitoglossa. This is a highly diverse genus, characterized by its adaptive radiation into the New World tropics (see Wake and Lynch, 1976, for an introduction to the genus). The main conclusion of this paper is that the variation encountered is characterized by a recurrence of the same variants in