THE POLYTYPIC SPECIES REVISITED - MORPHOLOGICAL-DIFFERENTIATION AMONGTIGER SALAMANDERS (AMBYSTOMA-TIGRINUM) (AMPHIBIA, CAUDATA)

We examined variation in 11 external morphological characters in 1353 larvae from 60 populations belonging to four subspecies of the polytyp ic salamander Ambystoma tigrinum and two closely related members of th e tiger salamander complex, A. californiense and A. velasci. Phenotypi c differentiation among these taxa is relatively slight, in keeping wi th their relatively shallow levels of differentiation based on mitocho ndrial DNA and allozymes. Significant differences among some taxa were observed in size-corrected gill raker number, with smaller contributi ons from several other variables. Despite this small degree of differe ntiation, the Mexican ambystomatid A. velasci, the eastern tiger salam ander A. t. tigrinum, and to a lesser extent the California tiger sala mander A, californiense form distinguishable morphological groups, whi le the remaining taxa (A. t. mavortium, A. t. melanostictum, and A. t. nebulosum) differ only slightly from one another. By contrast, we fou nd that all size-corrected variables showed large levels of variation among populations within taxa. The causal mechanisms responsible for t his among-pond variation are not obvious, although several lines of in direct evidence suggest that a combination of historical gene now and current natural selection may be required to explain the observed patt erns. Taxonomically, our data provide additional support for the inter pretation that A. t. tigrinum may be a valid taxon at the specific lev el, while A. t. mavortium, A. t. melanostictum, and a. t. nebulosum ar e almost indistinguishable from one another.