Recurrent patterns of natural selection in a population of Darwin's finches.

The adaptive significance of morphological traits can be assessed by measuring and identifying the forces of selection acting on them. Boag and Grant documented directional selection in a small population of Darwin's medium ground finches, Geospiza fortis, on I. Daphne Major, Galápagos, in 1977. Large beak and body size were favoured at a time of diminishing food supply and high adult mortality. We show here that in two subsequent periods of moderate to high adult mortality (1980 and 1982), the population was subject to the same selection. We have used a recently developed technique to ascertain the targets of direct selection. Beak depth and body weight were commonly under direct selection to increase but, surprisingly, beak width was directly selected to decrease, over all three periods of mortality. The results have implications for our understanding of evolutionary change in morphological traits of Darwin's finches.