Geographic Protein Variation and Divergence in the Salamanders of the Plethodon Weller Group (Amphibia, Plethodontidae)

Abstract

Larson, A. and R. Highton (Department of Zoology, University of Maryland, College Park, Maryland 20742) 1978. Geographic protein variation and divergence in the salamanders of the Plethodon welleri group (Amphibia, Plethodontidae). Syst. Zool. 27:431-448.-Variation in 26 loci coding for the production of protein products is analyzed in three populations of Plethodon welleri and in 26 populations of Plethodon dorsalis. Protein variation patterns are not consistent with the variation of morphological characters upon which the taxonomic status of these populations has been evaluated. The taxonomic division of P. welleri into subspecies is inconsistent with the electrophoretic data. P. dorsalis includes two groups of populations whose electrophoretic divergence is greater than that observed in any species previously studied. They do not correspond with previously recognized subspecies. Measures of genetic identity (Nei's I) and of population subdivision (Wright's FST) indicate that P. dorsalis is divided into a number of genetically isolated units. The FST values reported for this species are greater than any such values reported previously. It is suggested that the taxonomic recognition of species denotes the presence of a highly canalized phenotype which has been maintained over genetically diverse populations, rather than the genetically homogeneous units of other vertebrate species. Morphological divergence appears to be unrelated to the amount of electrophoretically detected genetic differentiation occurring among populations of the P. welleri group. [Electrophoresis; geographic variation; Amphibia; Plethodontidae.] The electrophoretic analysis of proteins in vertebrates has revealed widespread genetic similarity, even throughout large geographic ranges, among most conspecific populations (Selander and Johnson, 1973; Ayala, 1975). Electrophoretic differences among vertebrate populations are to a large degree consistent with levels of taxonomic division, with the exception that morphologically different subspecies are often not distinct electrophoretically (Avise, 1974). Adaptive differentiation, however, has produced morphological divergence and speciation in the absence of electrophoretic divergence as demonstrated in fishes by Avise et al. (1975) and by Kornfield and Koehn (1975). Electrophoretic differentiation, when it occurs, is usually accompanied by morphological divergence. The studies upon which these conclusions are based have to a large degree overlooked the caudate Amphibia, which tend to be evolutionarily conser1 Present address: Department of Genetics, University of California, Berkeley, California 94720. vative, having relatively low rates of morphological divergence compared to other vertebrates. Recent evidence shows that in other evolutionarily conservative organisms, populations may diverge genetically without morphological differentiation (Nixon and Taylor, 1977); morphological homogeneity does not necessarily imply genetic continuity among populations. In the present study, we evaluate the electrophoretic divergence among the populations of the welleri group of the caudate genus Plethodon, and present evidence that the widespread genetic homogeneity observed in other vertebrate species may not be a general feature of the genus Plethodon. Our data also suggest that morphological divergence may bear little relationship to electrophoretically detected genetic divergence among some populations of salamanders. The major taxonomic subdivisions of the genus Plethodon reflect differences in geographical distribution (eastern and western) and the eastern species are grouped on the basis of size (large and