Abstract
The female-biased sexual size dimorphism (SSD) in Drosophila cannot be explained by the fecundity-selection hypothesis, because fecundity decreases with increasing body size within the interspecific comparison. Because the offspring of single females compete for the same resource, however, and size decreases have larger consequences for the fitness of females than for that of males, genes favouring female-biased SSD can easily invade the system. Allometry for SSD was consistent with Rensch's rule; e.g. larger species showed less female-biased SSD. Specialist species are larger, and larger flies have longer potential dispersal distances increasing the chance that they will find new breeding patches. Females emerge from the pupae before the males, so males must find new breeding locations before they can mate. The increase in travel distance therefore limits the potential size decrease of the males, and the larger specialist species indeed show less female-biased SSD.