Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Variation in alleles at gene loci encoding proteins was used to determine zoogeographic and evolutionary differentiation in thamnophiine snakes. Differentiation among populations within a species, among species within a genus, and among genera was investigated. In the southern banded water snake, Nerodia fasciata, analysis by F-statistics and genetic distances of allele frequencies indicated that extensive differentiation has occurred between those forms adapted to a saltwater environment (N. f. clarkii and N. f. compressicauda) and those adapted to a freshwater environment (N. f. fasciata, N. f. confluens, and N. f. pictiventris). Habitat disturbance is imputed as the causal agent in the breakdown of reproductive barriers between the two differentially adapted groups. The total genetic diversity within Nerodia fasciata is of the same order as found in related taxa. However, the molecular evidence indicates secondary contact with reticulate evolution in some populations of Nerodia fasciata. Genetic distances separating taxa of the saltwater group are no greater than those between infrasubspecific populations. It is proposed that the saltwater-adapted snakes constitute a distinct species, Nerodia clarkii, containing the subspecies N. c. clarkii, N. c. compressicauda, and N. c. taeniata, and that the name Nerodia fasciata be limited to include the subspecies N. f. fasciata, N. f. confluens, and N. f. pictiventris. Both phenetic and cladistic analyses of allelic data from all species of the genus Nerodia and a composite of representatives of the genus Thamnophis indicate that Nerodia is composed of three lineages, the taxispilota lineage, the cyclopion lineage, and the sipedon lineage. Further, it is indicated that the genus Thamnophis is actually a member of the sipedon lineage, thus making the genus Nerodia paraphyletic. Erection of new genera to accommodate the taxispilota and cyclopion lineages is recommended. The retention of the genus Thamnophis is recommended as all members share a unique derived character state and thus form a natural group. Representatives of all thamnophiine genera were analysed for allelic variation and the resulting data were used to create hypotheses of the systematic relationships of these snakes using both phenetic and cladistic methods. A similar phylogeny was indicated using both of these approaches. The genus Storeria was seen to be a highly derived early offshoot of the main thamnophiine line. The molecular data suggest that the genus Regina is polyphyletic and that the four species in this genus are the result of three distinct radiations.