. The Canadian field-naturalist. 1958 Bleakney: Variation in Northern Water Snakes 129 of the bar is separate, to a condition in which the separate spots form a lateral stripe (Figure 1,C). When the 42 specimens are arranged in a series, this pattern change is most gradual, but it is interesting to consider that if only the extremes had been at hand, one might have been tempted to describe a new fl" /r\ O ,i^'*^s'^*,. 3* gMM* HfJjWIilfSgMf^^ i *, Figure 1. Male Matrix s. sipedon from the Ottawa litter selected to show range of variation in color pattern. Males and females are
. The Canadian field-naturalist. 1958 Bleakney: Variation in Northern Water Snakes 129 of the bar is separate, to a condition in which the separate spots form a lateral stripe (Figure 1,C). When the 42 specimens are arranged in a series, this pattern change is most gradual, but it is interesting to consider that if only the extremes had been at hand, one might have been tempted to describe a new fl" /r\ O ,i^'*^s'^*,. 3* gMM* HfJjWIilfSgMf^^ i *, Figure 1. Male Matrix s. sipedon from the Ottawa litter selected to show range of variation in color pattern. Males and females are distributed rather evenly through this color pattern series except in the 11 most aberrant examples (that is, those tending toward a lateral stripe) of which 9 are males. Examination of five other broods of Natrix s. sipedon from eastern Ontario and western Quebec, totaling 134 young, resulted in the discovery of 12 with a lateral row of spots (seven from Quebec and five from Ontario) and only 2 specimens (one from Quebec and one from Ontario) that had a lateral stripe. In contrast, only one adult with short lateral stripes, a small male, could be found in the collections of the National Museum of Canada. Dunn (1942) and Inger (1942, 1943) have both shown that the frequency of occurrence of a character in a series of juveniles and adults of the same species may be strikingly different. Their explanation for this is that only the better adapted individuals survive the rigors of natural selection. Their work dealt with the variations in the number of scales and they demonstrated that specimens with atypical scale arrangement were selected against. In the case of these Ontario and Quebec Northern Water Snakes, it is the aberrant pigment pattern of lateral stripes which apparently has a negative survival value. It may be that the normal barred pattern serves to break up the linear body form of young Northern Water Snakes making them inconspicuous whereas the dark lateral stripe ad
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