PHENOTYPIC AND GENETIC-EFFECTS OF HYBRIDIZATION IN DARWINS FINCHES

Morphological consequences of hybridization were studied in a group of three interbreeding species of Darwin's finches on the small Galapago s island of Daphne Major in the inclusive years 1976 to 1992. Geospiza fortis bred with G. scandens and G. fuliginosa. Although interbreedin g was always rare (< 5%), sufficient samples of measurements of hybrid s and backcrosses were accumulated for analysis. Five beak and body di mensions and mass were measured, and from these two synthetic (princip al-component) traits were constructed. All traits were heritable in tw o of the interbreeding species (G. fuliginosa were too rare to be anal yzed) and in the combined samples of F-1 hybrids and backcrosses to G. fortis. In agreement with expectations from a model of polygenic inhe ritance, hybrid and backcross classes were generally phenotypically in termediate between the breeding groups that had produced them. Hybridi zation increased additive genetic and environmental variances, increas ed heritabilities to a moderate extent, and generally strengthened phe notypic and genetic correlations. New additive genetic variance introd uced by hybridization is estimated to be two to three orders of magnit ude greater than that introduced by mutation. Enhanced variation facil itates directional evolutionary change, subject to constraints arising from genetic correlations between characters. The Darwin's finch data suggest that these constraints become stronger when species with simi lar proportions hybridize, but some become weaker when the interbreedi ng species have different allometries. This latter effect of hybridiza tion, together with an enhancement of genetic variation, facilitates e volutionary change in a new direction.