Darwin's finches in the Galápagos archipelago are an unusual example of adaptive radiation in that the basal split separates two lineages of warbler finches (Certhidea olivacea and Certhidea fusca) believed until recently to be only one species. The large genetic difference between them contrasts with their similarity in plumage, size, shape, and courtship behavior. They differ in song, which is a key factor in premating isolation of other sympatric Darwin's finches. We conducted playback experiments to see whether members of the population of C. olivacea on Santa Cruz Island would respond to songs of C. fusca from two islands, Genovesa and Pinta, and songs of C. olivacea from another island (Isabela). Another set of experiments was performed, using the same playback tapes, with C. fusca on Genovesa. Some members of both populations responded to all playbacks; therefore, the hypothesis of complete premating isolation on the basis of song is rejected. Discrimination between songs of the two lineages was inconsistent. We conclude that premating barriers to interbreeding among the tested populations have not arisen in the 1.5-2.0 m.yr. of their geographical isolation on different islands. This contrasts with strong premating barriers between more recently derived sympatric species. Early learning of song associated with morphology is later used in mate recognition. This explains why sympatric species that are vocally and morphologically distinct yet genetically less differentiated than Certhidea do not interbreed, whereas the Certhidea lineages that are genetically well differentiated but vocally and morphologically similar have no apparent premating barrier. We discuss this unusual situation in terms of the forces that have produced similarities and differences in song, morphology, and ecology and their relevance to phylogenetic and biological species concepts. Neither principles nor details are unique to Darwin's finches, and we conclude by pointing out strong parallels with some continental birds.