Quantitative Genetic Variation in Populations of Darwin ‘s Finches

Abstract

Abstract Some populations of organisms are much more variable than others in genetic characteristics and quantitative phenotypic traits. Theories explaining genetic variation focus on a balance between mutation on the one hand and stabilizing selection and drift on the other. Additional factors are needed to explain why some populations are more variable than others. These factors include introgression of genes and nonstabilizing forms of selection (directional and diversifying). A long-term field study of Darwin ‘s Finches on the Galapagos island of Daphne Major shows that differential introgressive hybridization is partly responsible for the higher levels of additive genetic variance in the medium ground finch (Geospiza fortis), an ecological generalist species comprising specialist phenotypes, than in the cactus finch (G. scandens), a specialist species. The two species hybridize, rarely, and in addition, the medium ground finch hybridizes with the small ground finch (G. fuliginosa). There is little or no fitness loss in the hybrids and backcrosses. Additive genetic variances of the medium ground finch and cactus finch are approximately at equilibrium. Equilibrium implies that the medium ground finch is subjected to stronger forces of selection than the cactus finch, balancing the stronger genetic input.