COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES: PREDICTION AND MEASUREMENT OF NATURAL SELECTION

A classic example of natural selection, that of color-pattern variation in Lake Erie island populations of water snakes, was reexamined to overcome shortcomings resulting from classification of snakes into discrete color-pattern categories and use of cross-sectional data. Four continuously varying color-pattern components (DB, the number of dorsal blotches; LB, the number of lateral blotches; ROWS, the height of lateral blotches measured in scale rows; and VEXT, the extent of ventral pigmentation) were analyzed. Patterns of natural selection were predicted from the relationship between color-pattern scores and independent measures of relative crypsis. Tests for natural selection were carried out using longitudinal data on neonate to juvenile-aged snakes and cross-sectional data on juvenile to adult-aged snakes. As predicted, the form of selection differed between younger and older age classes of snakes: selection resulted in a reduction in DB and LB among neonate and juvenile snakes but had little influence on color-pattern components in older snakes. The correspondence between observed patterns of natural selection and predictions based on the relationship between color-pattern scores and relative crypsis supports the hypothesis that differential predation by visual predators on younger age classes of snakes is the mechanism of selection. Gene flow from mainland populations or the initial lack of an allele necessary for reduced pattern may explain why selection has not resulted in greater differentiation between island and mainland populations.