The Evolution of Sexual Size Dimorphism in Darwin's Finches

Comparative methods have generally been used in attempts to understand the evolution of sexual dimorphism. Here I outline several hypotheses for the evolution of sexual dimorphism in body size in terms of the types of selection likely to be observed in contemporary studies of populations. I use this to interpret observations of selection in a population of Darwin's medium ground finch, Geospiza fortis, in which males average about 5% larger than females. Results are supplemented with observations on feeding and breeding behavior. There is estimated to be a very high between-sex genetic correlation for homologous characters. This may account for a lack of differentiation in the degree of dimorphism demonstrated among populations and species of Geospiza. There is no evidence that resource partitioning between the sexes has played any significant part in the evolution or maintenance of the dimorphism in this population. Two contemporary selection forces are identified and implicated in the past evolution of the sexual dimorphism. These are: (1) smaller females breed earlier than larger females, and (2) males are under sexual selection for large size.