DIRECTIONAL SELECTION AND CLUTCH SIZE IN BIRDS

Price and Liou (1989; see 90L/11771) suggested that directional selection operates on a correlated, environmentally determined trait (nutritional state). In their model, the genetic component of clutch size could also be subject to selection, but, if clutch size is not evolving, this selection must be nondirectional (eg normalizing). The authors here point out an alternative explanation for the lack of apparent response to selection acting on clutch size. Components of the genotype may, in fact, be evolving, but the phenotypic expression of the genotype may not change because it depends not only on the individual's genotype but on the deviation of that genotype from the mean of the population. If the population mean is changing with time, this can be considered a change in the individual's environment, which necessarily affects the response to selection. Such a situation could arise not only for clutch size but for many other traits closely associated with fitness. -from Authors