Darwin and his finches: The evolution of a legend

First collected by Charles Darwin in the Galapagos Archipelago, the Geospizinae, or "Darwin's finches," have rightly been celebrated as a classic instance of the workings of evolution through natural selection. Among birds, Darwin's finches are rivaled only by the Hawaiian honeycreepers (Drepanididae) as a microcosmic exemplification of the principle of adaptive evolutionary radiation. Although the Drepanididae have undergone more evolution and adaptive radiation than the Geospizinae, the latter are in some ways more valuable to omithologists. "Their special interest today," writes David Lack, "is in providing the best example, in birds, of an adaptive radiation into different ecological niches that is sufficiently recent, geologically speaking, for intermediate and transitional forms to have survived" (1964:178). The Galapagos Archipelago, where Darwin spent five weeks collecting these finches during the voyage of H.M.S. Beagle (1831-1836), comprises sixteen principal islands located on the equator some six hundred miles west of Ecuador (Fig. 1). The islands, most of which are several million years old, are wholly volcanic in origin and have never been connected to the mainland. Darwin's finches were evidently one of the earliest colonists of the Galapagos, since their degree of evolutionary complexity thirteen species distributed among four genera is unmatched by any other avian group in this archipelago. A fourteenth species, belonging to yet another genus, inhabits Cocos Island, four hundred miles to the northeast. Unlike other endemic species of Galapagos birds, the Geospizinae no longer have any close relatives on the American mainland. They are therefore classed in their own separate tribe or subfamily, which is placed with the Emberizidae.1 Being one of the earliest colonists of the Galapagos Islands, the ancestral form of Darwin's finches found an environment in which the types of niches occupied by other, diverse birds on the continent