THE POLYTYPIC SPECIES REVISITED - GENETIC DIFFERENTIATION AND MOLECULAR PHYLOGENETICS OF THE TIGER SALAMANDER AMBYSTOMA-TIGRINUM (AMPHIBIA,CAUDATA) COMPLEX

We present a phylogenetic analysis of the Ambystoma tigrinum complex, based on approximately 840 base pairs of mitochondrial-DNA sequence fr om the rapidly evolving D-loop and an adjacent intron. Our samples inc lude populations of the continentally distributed species, A. tigrinum , plus all described species of Mexican ambystomatids. Sequence diverg ence is low, ranging from 0-8.5%, and most phylogenetic groupings are weakly supported statistically. We identified eight reasonably well-de fined clades from the United States and Mexico, with the geographicall y isolated A. californiense from California as the probable sister gro up to the remaining taxa. Our sequence data are not capable of resolvi ng the relationships among these clades, although the pattern of trans itional-site evolution suggests that these eight lineages diverged dur ing a period of rapid cladogenesis. We roughly calibrate a molecular c lock and identify a few lineages that significantly deviate from the s low, baseline rate of 0.5-0.75% per million years. Our data also sugge st that species boundaries for several U.S. and Mexican species need t o be altered and that the concept of a continentally distributed, poly typic tiger salamander is not valid.