GEOGRAPHIC VARIATION AND PHENOTYPIC PLASTICITY OF NUMBER OF TRUNK VERTEBRAE IN SLENDER SALAMANDERS, BATRACHOSEPS (CAUDATA: PLETHODONTIDAE).

To understand the evolutionary significance of geographic variation, one must identify the factors that generate phenotypic differences among populations. I examined the causes of geographic variation in and evolutionary history of number of trunk vertebrae in slender salamanders, Batrachoseps (Caudata: Plethodontidae). Number of trunk vertebrae varies at many taxonomic levels within Batrachoseps. Parallel clines in number occur along an environmental gradient in three lineages in the Coast Ranges of California. These parallel clines may signal either adaptation or a shared phenotypically plastic response to the environmental gradient. By raising eggs from 10 populations representing four species of Batrachoseps, I demonstrated that number of trunk vertebrae can be altered by the developmental temperature; however, the degree of plasticity is insufficient to account for geographic variation. Thus, the geographic variation results largely from genetic variation. Number of trunk vertebrae covaries with body size and shape in diverse vertebrate taxa, including Batrachoseps. I hypothesize that selection for different degrees of elongation, possibly related to fossoriality, has led to the extensive evolution of number of trunk vertebrae in Batrachoseps. Analysis of intrapopulational variation revealed sexual dimorphism in both body shape and number of trunk vertebrae, but no correlation between these variables in either sex. Females are more elongate than males, a pattern that has been attributed to fecundity selection in other taxa. Patterns of covariation among different classes of vertebrae suggest that some intrapopulational variation in number results from changes in vertebral identity rather than changes in segmentation. © 1997 The Society for the Study of Evolution.