The allopatric phase of speciation: the sharp-beaked ground finch (Geospiza difficilis) on the Galapagos islands

Using microsatellite DNA variation, morphological measurements and sonagram s made from tape-recordings in the field, we examine the allopatric differe ntiation of six populations of the sharp-beaked ground finch, Geospiza diff icilis, in the Galapagos archipelago. We ask how and why the populations be came differentiated, and consider what the differences imply about speciati on. An important factor is dme; G. difficilis is one of the phylogeneticall y oldest species. Populations became differentiated by adapting in beak mor phology to different food supplies. Since beak size and shape also function in conspecific mate recognition and choice, the potential for reproductive isolation from sister and parental taxa arose as a correlated effect of na tural selection on resource-exploiting traits. This conforms to a standard explanation for the origin of pre-mating isolation as a byproduct of adapti ve changes in allopatry without being adaptive itself. However, this explan ation is incomplete. Vocal characteristics also differentiated, partly as a result of natural and sexual selection independent of beak size change in environments with different sound transmitting properties. An additional el ement of chance is indicated by a comparison of closely-related populations on two islands, Wolf and Darwin, that are geographically close, and topogr aphically and floristically similar. The populations have markedly differen t vocalizations. Morphological characters, vocalizations and genetic traits do not vary concordantly among the six populations. One population (Genove sa) is genetically more similar to other congeneric species. Phenotypic sim ilarities with G. difficilis are the result of a uniquely long retention of shared ancestral trails. Arguments under the phylogenetic species concept justify recognizing this population as a separate species, Geospiza acutiro stris. Under the biological species concept it should remain as currently c lassified, G. difficilis. (C) 2000 The Linnean Society of London.