HYBRIDIZATION OF DARWIN FINCHES ON ISLA-DAPHNE-MAJOR, GALAPAGOS

There has been much debate in the past about whether Darwin's finches hybridize in nature, and if they do whether hybridization could accoun t for the intermediate appearance of certain forms. To resolve these i ssues the breeding of all finches on the small Galapagos island of Dap hne Major was studied in every year from 1976 to 1992. The island supp orted breeding populations of Geospiza fortis (harmonic mean of 198 br eeding individuals), G. scandens (H = 80), G. fuliginosa (H = 3) and, in the past 10 years, G. magnirostris (H=6). Morphological criteria fo r defining species were developed in a study of the finches on the nei ghboring large island of Santa Cruz. These were then used with modific ation on Daphne to classify members of the first few generations to sp ecies. Observations of breeding birds showed that in a few cases speci es interbred. G.fortis hybridized with G.fuliginosa in 11 out of the 1 3 years in which both species bred. G. fortis and G. scandens hybridiz ed in six of the years. Hybridization was always rare. Hybridizing ind ividuals constituted 1.8% of breeding G.fortis, on average, 0.8% of G. scandens, but 73.0% of the much rarer fuliginosa. F1 hybrids were via ble and fertile. They rarely bred with each other to produce an F2 gen eration. Much more frequently they backcrossed to the common species, G. fortis and G. scandens. In all these cases hatching and fledging su ccess were high, giving scarcely any indication of genetic incompatibi lities in the Fl, F2 or backcross generations. The demonstration of na tural hybridization answers some questions and raises others. It shows that introgression of genes could be a small factor contributing to t he intermediate appearance of G. fortis on Daphne Major: that is betwe en typically larger forms of this species elsewhere in the archipelago and the smaller G.fuliginosa. However hybridization with the larger G . scandens has the opposite directional effect on G. fortis. Hybridiza tion and introgression sometimes complement the effects of natural sel ection, sometimes they are opposed by it. Introgression also contribut es to the large morphological variation displayed by this and several other populations in the archipelago. Hybridization raises questions a bout how species of Darwin's finches (and other organisms) should be d efined and recognized. In terms of the broad biological species concep t there are four species of finches on Daphne Major, neither completel y independent evolutionarily on the one hand (except for G. magnirostr is), nor approaching panmixia on the other hand.