Sentence examples for beak size from inspiring English sources
A study by Peter R. Grant and B. Rosemary Grant of Princeton University shows the rapid evolution of beak size in one of the finches in response to competition.
For a long time G. fortis ruled the roost, eating both large and small seeds depending on beak size, but in 1982 a breeding population of the large ground finch, G. magnirostris, arrived.
But it was predicted that eventually competition for the larger seeds would put selection pressure on the medium finches toward smaller beak size (because those with smaller beaks would get enough seeds to survive, while those with larger beaks would not).
They find that natural selection happens repeatedly, that finches hybridize and exchange genes rarely, and that they compete for scarce food in times of drought, with the remarkable result that the finch populations today differ significantly in average beak size and shape from those of forty years ago.
Within a population of blue jays, there are differences in beak size.
Fig. 1 Beak size distributions of ground finches (Geospiza) on several Galapagos islands.
-- Olivia Judson, "Stop the Mutants!" Reader Comments: Mutations and evolution may not necessarily be of the slow, incremental sort where beak sizes change over generations.
As one species evolves slightly larger beak sizes, the other species is likely to experience selection favoring birds with smaller beak sizes, which allows them to access a resource with less competition.
Lack proposed that competition for seeds had caused the evolution of exaggerated differences in beak sizes between finch species that coexist.
Beak sizes of the small (G. fuliginosa) and medium (G. fortis) ground finch are more similar where each occurs separately, but are divergent where the species occur together.
Lack (1947) showed that beak sizes of species of Darwin's ground finches tend to be more different from one another when they occur on the same Galápagos island than when they occur on different islands.
