High Survival of Darwin's Finch Hybrids: Effects of Beak Morphology and Diets

Abstract

Three species of Darwin's finches (Geospiza fortis, G. scandens, and G. fuliginosa) hybridize rarely on the small Galapagos island of Daphne Major. Following the exceptionally severe El Nino event of 1982—1983, hybrids survived as well as, and in some cases better than, the parental species during dry seasons of potential food limitation. They also backcrossed to two of the parental species. This study was undertaken to compare the diets of hybrids with the diets of the parental species in order to assess possible reasons for the high hybrid survival. Diets of F hybrids and first generation backcrosses to G. fortis were intermediate between the diets of the respective parental species. Distinctiveness of the hybrid diets was most pronounced where the diets of the parental species differed most. A strong determinant was beak morphology; hybrids inherit beak traits from both parents, and, on average, have intermediate beak sizes. Among the combined groups of species and hybrids, and among the hybrids alone, dietary characteristics covaried with beak morphology. Hybrids that differ most from G. fortis in beak morphology, the G. fortis x G. scandens F hybrids, experience a feeding efficiency advantage when feeding on Opuntia echios seeds, commonly consumed in the dry season. These findings are used to interpret the higher survival of hybrids after 1983 than beforehand. The El Nino event that year led to an enduring (10—yr) change in the habitat and plant composition of the island. A decrease in absolute and relative abundance of large and hard seeds apparently caused relatively high mortality among G. scandens and the largest G. fortis individuals. Hybrids were favored by an abundance of small seeds. The high survival of G. fortis x G. scandens F hybrids may have been due, additionally, to a broad diet and to efficient exploitation of Opuntia seeds. The study demonstrates long—term ecological and evolutionary consequences of large—scale fluctuations in climate, and the role of ecological (food) factors in determining hybrid fitness.