Adaptation in
Darwin's Finches
In Nutcracker
Ground Finches (Geospiza fortis) of the Galapagos
Islands, beak depth is correlated with 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. Over time,
the graph shows a pattern of stabilizing natural selection,
in which beak depth fluctuates around a mean of about 9.6mm.
Consistent
habitat differences among different islands in the
archipelago, including climate and foods sources
(seeds, cactus, insects, and fruits) promote directional
natural selection of resident finches on different islands
for optimal beak morphology that maximizes survival under local
conditions. This process has produced more than a dozen distinct
species of finches, in different feeding niches, all unique to the
archipelago.
