Ecological correlates of morphological evolution in a Darwin's finch, Geospiza difficilis.

Abstract

The morphology of G. difficilis varies on different islands in the Galapagos. Lack (1947) argued that beak and body size are related to feeding, and noted that variation in morphology is associated with inter-island differences in the presence or absence of potential competitor species. Observations support some of Lack's general conclusions concerning the significance of morphological variation in G. difficilis. For example, populations of G. difficilis are similar in diet to G. fuliginosa on islands where this species is absent. Traits have not evolved independently in G. difficilis, however; this complicates diet-morphology comparisons (including those with G. fuliginosa and G. scandens), for example when there are alternative morphological solutions to a single foraging problem. Consideration of these factors helps clarify the significance of variation among G. difficilis populations in certain morphological traits. Beak length and body size appear to be particularly important in relation to food. On the low island of Genovesa, G. difficilis has become smaller; this is correlated with a diet of small seeds and flower nectar. On the low islands of Darwin and Wolf G. difficilis has a relatively longer beak than other populations have. This may be an adaptation to cactus-feeding, but some data indicate that it may have evolved to facilitate blood- and egg-feeding. -from Authors