Cultural inheritance of song and its role in the evolution of Darwin's finches

Abstract

Songs of Darwin's finches were studied on the Galapagos Island of Daphne Major from 1976 to 1995. A single, structurally simple, and unvarying song is sung throughout life by each male of the two common species, Geospiza fortis (medium ground finch) and G. scandens (cactus finch). Songs of the two species differ strongly in quantitative features, and individual variation among males is much broader in G. fortis than in G. scandens. Although there are exceptions, songs of sons strongly resemble the songs of their fathers. They also resemble the songs of their paternal grandfathers, but not their maternal grandfathers, indicating that they are culturally inherited and not genetically inherited. Female G. fortis display a tendency to avoid mating with males that sing the same type of song as their father. They also avoid mating with males that sing heterospecific song, with very rare exceptions. Thus song, an evolving, culturally inherited trait, is an important factor in species recognition and mate choice. It constrains the mating of females to conspecifics, even when there is no genetic penalty to interbreeding, and thus may play a crucial role in species formation by promoting genetic isolation on secondary contact. The barrier is leaky in that occasional errors in song transmission result in misimprinting, which leads to a low incidence of hybridization and introgression. Introgression slows the rate of postzygotic isolation, but can produce individuals in novel genetic and morphological space that can provide the starting point of a new evolutionary trajectory.