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.
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.