Master of Science (MS)


Biological Sciences

Document Type



The Western Lyresnake (Trimorphodon biscutatus) is found generally from the desert southwestern United States southward along the pacific lowland versant of Mexico to northwestern Costa Rica. This species exhibits substantial geographic variation in size, squamation, and color pattern across its range, and as such has had an unstable taxonomic history. I investigated populations of T. biscutatus representing currently and formerly recognized forms using both morphological and molecular methods to infer the evolutionary history of this widespread, polytypic taxon. Multivariate statistical analyses (principal components and discriminant analysis) of morphological data (432 specimens, 33 characters) yielded six statistically significantly different groups, consistent with currently or formerly recognized taxa. Molecular phylogenetic analyses using parsimony and Bayesian methods (71 samples, 821 base pairs of the mitochondrial ND4 gene) recovered five clades with strong support, largely concordant with groups identified in multivariate analyses, with one exception. One clade consisted of samples representing two geographically proximate, morphologically distinct forms, Trimorphodon biscutatus lambda of the Sonoran Desert and T. b. vilkinsonii of the Chihuahuan Desert, with very low sequence divergence across large geographic distances. These results suggest very rapid and recent morphological evolution, or, alternatively, recent introgressive hybridization. Nested clade analysis of mitochondrial haplotypes from these two forms was implemented in an attempt to identify putative historical events that may have shaped spatial patterns of diversification. These analyses revealed a pattern consistent with recent population range expansion. However, recent introgressive hybridization is expected to leave the same genetic signature. Based on these results, I conclude that Trimorphodon biscutatus (sensu lato) actually represents at least five, and possibly six independent evolutionary lineages (i.e., species). This hypothesis is tenable in light of the geological and climatic history of arid North and Middle America, and concordant with the evolutionary history of other codistributed organisms. Taxonomic changes are proposed in order to provide a better estimate of the evolutionary history of this species complex.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Jimmy A. McGuire