Analysis of area of intergradation between described subspecies of the common garter snake, Thamnophis sirtalis, in Canada
Two area of colour pattern intergradation have been described between subspecies of the garter snake, 'Thamnophis sirtalis': one along the Manitoba/Ontario border; the other through Quebec and the New England states. Two competing hypotheses have been proposed for the origins of these areas. The first is that these areas represent regions of postglacial secondary contact between two dispersing lineages of snake. The other is that these regions represent 'in situ' differentiation of colour pattern, perhaps in response to some form of natural selection. The strict consensus tree constructed from a 364 bp portion of cytochrome ' b' sequenced from 52 snakes (24 trees, 85 steps, CI = 0.76, RI = 0.93, RC = 0.71) shows that three distinct mtDNA lineages of 'T. sirtalis ' recolonized central and eastern Canada. A "western" lineage is found in Manitoba and northeastern Ontario, an "eastern" lineage is found in northeastern and southern Ontario and in parts of Quebec and New York, and a "maritime" lineage is found in the maritime provinces and parts of the northeastern United States. The geographical regions where the various lineages meet do not correspond to the geographical position of the area of colour pattern intergradation (p < 10-17). Analysis of allozyme variation reveals considerable population substructuring (overall loci FST = 0.259) which is concordant with the distribution of mtDNA lineages. A meristic character, the number of ventral scales, also varies significantly with sex and haplotype. There is no evidence of male-biased gene flow between any of the lineages, therefore the areas of colour pattern intergradation were likely formed 'in situ'. One area of postglacial secondary contact was identified along the northern shore of Lake Superior, just east of Terrace Bay, Ontario. Neutral introgression can not be rejected as an explanation for the pattern of genetic variation observed at this area of secondary contact.