Adaptation in Darwin's Finches

    Beak depth, which is correlated with body size and the ability to crack larger seeds, varies according to drought conditions: plants produce fewer, harder seeds in dry years and more, softer seeds in wet years. Only larger birds with deeper depths survive in drought years. The change is ±5% in extreme years. The graph shows a pattern of stabilizing selection, in which beak depth fluctuates around a mean of about 9.6mm.

    Consistent environmental differences in different habitats on different islands in the Galapagos, as well as the availability of different foods sources (seeds, cactus, insects, and fruit) promotes directional selection on resident finches for optimal beak morphology that maximizes survival under local conditions. This process has produced more than a dozen distinct species of finches, all unique to the archipelago.

Text material © 2018 by Steven M. Carr