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.
