Adaptation in
Darwin's Finches
In Nutcracker
Ground Finches (Geospiza fortis) of the Galapagos
Islands, beak depth is correlated with body size and
provides 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.
