Biochemical Genetics And Systematics Of Garter Snakes Of The Thamnophis Elegans-Couchii-Orndinoides Complex

Abstract

Previous immunological studies on transferrins indicate that all natricine snakes of North America are close relatives, having diverged from a common ancestor in the Pliocene. Species formation within the T. elegans-T. couchii-T. ordinoides complex undoubtedly has occurred much more recently. Microcomplement fixation comparisons show that structures of transferrins of members of the complex differ by only 5-15 immunological units, values indicative of divergence times within the past 1-3 million years. The electrophoretic evidence suggests that the complex now consists of at least 3 and possibly as many as 4 spp. The most widely divergent species is T. elegans, which includes 4 very closely related subspecies: T. e. elegans, T. e. terrestris, T. e. vagrans and T. e. biscutatus. The molecular evidence offers no argument against Rossman's (1979) decision to sink T. e. biscutatus; it is recognized here as a matter of convenience in comparing data. T. couchii includes 2 divergent groups of subspecies. T. c. couchii and T. c. hammondii (T. couchii subgroup) are separated from T. c. atratus, T. c. aquaticus, T. c. gigas and T. c. hydrophilus (T. atratus subgroup) by a relatively large Nei genetic distance and fixed alleles at 2 loci. Contradictory findings must be investigated before a definitive decision can be made on whether the 2 subgroups represent 2 spp. T. ordinoides, which appears to be a good species, is close to the T. atratus subgroup, being distinguished from the latter by a genetic distance of similar magnitude to those differentiating subspecies within the T. atratus subgroup. The morphology and ecological preferences of T. ordinoides are more similar to those of T. elegans than to T. couchii.