Centres of speciation and ecological differentiation in the Galapagos land bird fauna

Summary

The Hamilton-Rubinoff model of evolution in the avifauna of the Galapagos Islands suggests that speciation occurs on small outlying islands, and that new species invade the central island region, where ecological differentiation takes place. I present an alternative model in which both speciation and ecological differentiation leading to origin of actively colonizing taxa occur on the large islands, with colonization of small and outlying islands being primarily one way. Although forms on outlying islands may differentiate to the level of new species, their fate is postulated to be extinction rather than re-invasion of central islands. Data on species with expanding, differentiating, fragmenting, and relict distributions support this second model. Polytypy and incompleteness of distributions on the large islands indicate that isolation is adequate for differentiation to occur. Distributions of expanding taxa centre on the large islands, and their distributions show sequences leading from large islands to smaller and more outlying islands. Curves of occupancy of large islands versus total islands also agree with the prediction that expansions begin in the large islands.