SELECTION ON BILL CHARACTERS IN A POPULATION OF DARWIN'S FINCHES: GEOSPIZA CONIROSTRIS ON ISLA GENOVESA, GALÁPAGOS.

Lande and Arnold's (1983) technique for measuring selection on correlated quantitative traits was used to identify the targets of selection and to reveal the direction of selection on three bill dimensions, during different stages of the life cycle in a population of Darwin's finches, Geospiza conirostris, on Isla Genovesa, Galápagos. There was a tendency towards disruptive selection during dry conditions, arising from differential survival. In terms of longevity and breeding success of females, the direction of selection was to increase bill length. For males competing for territories, selection acted to increase bill depth and bill length. The effects of male-male interactions were separated from those of female choice. Male-male interactions selected for deep and long bills, whereas females chose their mates on the basis of a male's territory position and plumage coloration. The results reveal three factors constraining changes in bill dimensions: a tendency for the mean of a dimension to shift in one direction is counteracted by selection in the opposite direction on 1) another, positively correlated, bill dimension, 2) the same dimension in the other sex, and 3) the same dimension at another stage of the life cycle. If these factors are overcome by strong directional selection at one stage of the life cycle and relaxation at another, there can be an evolutionary response because the bill dimensions in this population are known to be heritable. The results complement those found in studies of G. fortis on another island and strengthen the view that these populations of Darwin's finches are frequently subjected to natural selection.