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

A classic example of natural selection, that of color-pattern variatio n in Lake Erie island populations of water snakes, was reexamined to o vercome shortcomings resulting from classification of snakes into disc rete color-pattern categories and use of cross-sectional data. Four co ntinuously varying color-pattern components (DB, the number of dorsal blotches; LB, the number of lateral blotches; ROWS, the height of late ral blotches measured in scale rows; and VEXT, the extent of ventral p igmentation) were analyzed. Patterns of natural selection were predict ed 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 cr oss-sectional data on juvenile to adult-aged snakes. As predicted, the form of selection differed between younger and older age classes of s nakes: selection resulted in a reduction in DB and LB among neonate an d juvenile snakes but had little influence on color-pattern components in older snakes. The correspondence between observed patterns of natu ral selection and predictions based on the relationship between color- pattern scores and relative crypsis supports the hypothesis that diffe rential 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 expla in why selection has not resulted in greater differentiation between i sland and mainland populations.