Date of Award

Spring 2003

Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

Committee Director

Alan H. Savitzky

Committee Member

Richard G. Whittecar

Committee Member

John R. Holsinger

Committee Member

Robert K. Rose


A few traditionally applied diagnostic characters of color pattern were compared with an additional set of morphological and genetic characters to evaluate differences between Nerodia sipedon and N. fasciata and to study the dynamics across their hybrid zone in the Carolinas. Many of the morphological characters exhibited significant interspecific differences, although only the number of dorsally complete crossbands (CBa) was diagnostic by itself. A discriminant function analysis of morphological characters was successful in separating both taxa. Species-specific nuclear markers, identified by the AFLP (Amplified Fragment Length Polymorphism) technique were nearly fixed and served as diagnostic markers. They revealed extensive introgression between the two species not recognizable morphologically. Markers for N. sipedon and F1 hybrids, backcrossed to a parental species exhibiting more than one introgressive marker, were less frequent than expected at Hardy-Weinberg equilibrium, implying selection against N. sipedon and hybrids with an increased introgression, respectively. This is consistent with a pronounced genetic and morphological dominance of N. fasciata in the hybrid zone. The genetic results show that the two taxa are independent entities and qualify for species status under both the Phylogenetic Species Concept and the Evolutionary Species Concept. N. fasciata and N. sipedon segregate ecologically along ecotones of salinity, temperature, and water current. Variation in the width of these ecotones is reflected in the width of hybrid zones.