Hybridization of Darwin’s finches on Isla Daphne Major, Galápagos

Abstract

There has been much debate in the past about whether Darwin’s finches hybridize in nature, and if they do whether hybridization could account for the intermediate appearance of certain forms. To resolve these issues the breeding of all finches on the small Galápagos island of Daphne Major was studied in every year from 1976 to 1992. The island supported breeding populations of Geospiza fortis (harmonic mean of 198 breeding individuals), G. scandens ( H = 80), G. fuliginosa ( H = 3) and, in the past 10 years, G. magnirostris ( H = 6). Morphological criteria for defining species were developed in a study of the finches on the neighboring large island of Santa Cruz. These were then used with modification on Daphne to classify members of the first few generations to species. Observations of breeding birds showed that in a few cases species interbred. G. fortis hybridized with G. fuliginosa in 11 out of the 13 years in which both species bred. G. fortis and G. scandens hybridized in six of the years. Hybridization was always rare. Hybridizing individuals constituted 1.8% of breeding G. fortis , on average, 0.8% of G. scandens , but 73.0% of the much rarer G. fuliginosa . F ₁ hybrids were viable and fertile. They rarely bred with each other to produce an F ₂ generation. Much more frequently they backcrossed to the common species, G. fortis and G. scandens . In all these cases hatching and fledging success were high, giving scarcely any indication of genetic incompatibilities in the F ₁ , F ₂ or backcross generations. The demonstration of natural hybridization answers some questions and raises others. It shows that introgression of genes could be a small factor contributing to the interm ediate appearance of G. fortis on Daphne Major: that is between 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 . Hybridization and introgression sometimes complement the effects of natural selection, sometimes they are opposed by it. Introgression also contributes to the large morphological variation displayed by this and several other populations in the archipelago. Hybridization raises questions about how species of Darwin’s finches (and other organisms) should be defined and recognized. In terms of the broad biological species concept there are four species of finches on Daphne Major, neither completely independent evolutionarily on the one hand (except for G. magnirostris , nor approaching panmixia on the other hand.