Individual specialization and the seeds of adaptive radiation in Darwin's finches

Background: Empirical and theoretical studies suggest that individual specialization can be an important force in evolutionary diversification. However, few studies of natural populations have explicitly considered the impact of individual specialization on adaptive divergence. Questions: To what extent do individuals within a bimodal Darwin's finch population specialize on different resources? Is this individual specialization likely to enhance adaptive divergence? Field site: El Garrapatero, Santa Cruz Island, Galapagos, Ecuador. Organism: A population of the medium ground finch, Geospiza fortis, showing large - and bimodal - morphological and genetic variation resulting from ecologically based adaptive divergence. Methods: We described the diets of individual G. fortis through feeding observations in the wild. We calculated several indices of individual specialization. We then examined the relationship between individual specialization, adaptive morphological traits (beak and head dimensions), and neutral genetic variation (microsatellites). We also performed a cluster analysis on the basis of individual foraging observations and asked whether the clusters were morphologically and genetically divergent. Results: We found significant levels of individual specialization and expected, but weak, associations between individual diet differences, morphological traits, and neutral genetic variation. The cluster analysis yielded two distinct diet-clusters of individuals that differed in morphological traits but not in neutral genetic markers. In the early stages of adaptive radiation, individual specialization appears to be associated with morphological divergence but not neutral genetic divergence.