Adaptation in Darwin's Finches

    In the Nutcracker Ground Finches of the Galapagos Islands, beak depth is correlated with body size and the mechanical force necessary to crack seeds. Beak depth varies according to drought conditions: plants in dry years produce fewer, harder seeds, and in wet years greater numbers of softer seeds. Only larger birds with deeper beak depths survive in drought years. The change is ±5% between extreme years. The graph shows a pattern of stabilizing natural 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 natural 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 © 2021 by Steven M. Carr