Date of Award
Doctor of Philosophy (PhD)
Douglas A. Rossman
Character variation was examined in the snake Thamnophis sirtalis . Initially, 187 characters were examined on five populations (OTUs). Only 12% of the characters failed to show geographic variation by ANOVA. A reduced data set of 72 characters remained after eliminating those that were invariant and correlated. The reduced data set was analyzed for interpopulation variation between 37 OTUs, of T. sirtalis. Numbers of significantly differentiated characters between OTUs, determined by ANOVA, were used to generate a UPGMA tree. The tree placed Texas and Chihuahuan populations as sister groups to all other OTUs, and grouped OTUs from west of the Continental Divide, southeastern Coastal Plain, Maritime Provinces, and central Great Plains. A MANOVA produced four components that explained 47% of the model variance. The second component was the best resolved, and separated most of the OTUs by a line running from Ontario to the southwestern Great Plains. A cladistic (parsimony) analysis was performed after ordering all characters and coding them from 0--4. Two most parsimonious trees were generated (C.I. = 0.207), using Thamnophis elegans and T. eques as outgroups. The Texas panhandle OTU was the sister taxon to all other OTUs, and the division discerned by the second component was also resolved by the parsimony analysis. Association of OTUs with classic subspecies of T. sirtalis was not supported. Natural history parameters were correlated with the eastern and western clades. Snakes in the eastern clade fed primarily on earthworms, while western snakes fed primarily on amphibians; used crypsis vs. flight as a means of capture avoidance; produced large litters of small neonates vs. smaller fitters of large neonates. A hypothetical scenario for the history of T. sirtalis suggests the following: (1) T. sirtalis originated in Texas or Mexico; (2) populations reaching the Great Plains evolved red lateral markings, possibly for predator avoidance; (3) populations in forests retained a plesiomorphic color pattern until they invaded woodlands of the southeastern Coastal Plain, where a spotted morph evolved; (4) snakes west of the Continental Divide resulted from a single invasion.
Boundy, John Jeffrey, "Systematics of the Common Garter Snake, Thamnophis Sirtalis." (1999). LSU Historical Dissertations and Theses. 7070.