PARENT SESSION
Symposium 20: Evolution on ecological time scales
Organized by: A Hendry, D Réale, and G Fussmann
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 517 B, Level 5, Palais des congrès de Montréal

Evolution by natural selection and hybridization of Darwin's finches.

Grant, Peter^*,1, Grant, Rosemary^*,1, ¹ Princeton University, Princeton, NJ

ABSTRACT- Understanding the evolutionary dynamics of contemporary populations requires careful attention to variation in fitness among members of a population. We studied populations of the medium ground finch (Geospiza fortis) and the cactus finch (G. scandens) on the Galápagos island of Daphne Major from 1973 to 2004, by banding finches with unique combinations of colored bands and following their fates (survival and reproduction) through climatically stressful conditions of droughts and the abundance of rain during El Nino events. We found that both species are subject to natural selection. Selection occurs when many finches die in droughts, and the direction of selection and the particular traits affected depend upon food supply, which in turn depends in part on environmental conditions in previous years. Evolutionary change between generations occurred because beak and body size traits are highly heritable. The populations also evolved as a result of introgressive hybridization. Hybridization occurs when the normal process of imprinting by which young finches learn the song and morphological charcteristics of their parents is perturbed. Hybridization remained rare over most years, but the fitness of hybrids changed after the El Nino year of 1983; hybrids and backcrosses were slightly fitter on average than were members of the parental species, and as a result their frequency increased. Genetic evidence from other islands suggests that introgressive hybridization is not restricted to the small island of Daphne Major.

Key words: Selection, Hybridization, Heritability, Drought