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FIGURE 7.7 Water Snake Differential Survival
(a) Scored color phases A (light) to D (banded). In young born on the islands, most are C or D (dark and banded). But by the time the snakes reach adult stage, most are A or B (light colored). (b) On the islands, predaceous gulls feed on young snakes, usually spotting and eating the more conspicuous banded snakes, producing differential survival of mostly unbanded snakes. Occasional immigrants from the mainland return some of the genes for banded color.