Thesis Title

Comparative Phylogenetic Analysis in Populations of the Western Cottonmouth (Agkistrodon Piscivorus Leucostoma) Distributed in Three North American Drainage Basins


Ryan D. Combs

Date of Graduation

Summer 2007


Master of Science in Biology



Committee Chair

Mark McKnight


vicariance, biogeography, refugia, phylogeny, cytochrome b

Subject Categories



The western cottonmouth is distributed in three major North American drainage basins. Using a molecular phylogenetic analysis, Knights et al. (1992) observed that western cottonmouths were highly divergent from both eastern and Florida cottonmouths yet the degree of divergence among populations of the western cottonmouth remains unexplored. During the Pleistocene ice-ages populations of cottonmouths were confined to southern refugia in both Texas and Florida (Van Devender and Conant 2000). It is thought that post glacial dispersal patterns using stream corridors have resulted in the present biogeographic assemblage of populations in different drainage basins. Here I use the molecular marker mitochondrial cytochrome b in a molecular phylogenetic analysis to test hypotheses: 1) that post- glacial dispersal into confined drainage basins has caused genetic divergence between populations of western cottonmouths, and 2) to determine if the western cottonmouth was restricted to one or more Pleistocene ice-age refugia. Phylogenies produced using Maximum Likelihood (ML) and Bayesian phylogenetic inference show that drainage basins have not caused genetic divergence among populations of western cottonmouths. While the mismatch distribution combined with my phylogenetic data suggests that two Pleistocene refugia may have been present.


© Ryan D. Combs