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Galápagos finches—natural selection on beak shape  

Finch beaks are their tools for opening seeds. Different seeds open best with different beak size and features, which are inherited characteristics. When all seed sizes were easily available in normal seasons, ground finches (Geospiza fortis), shown here, with large and small beaks survived about equally well. But in dry seasons, small, easy-to-open seeds were scarce. Large, hard-to-open seeds were more plentiful. Predominantly, finches with large, strong beaks could open the large seeds and survive (red), but those with small beaks did less well (green).

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