A demographic analysis of the components of selection in a population of Arctic Skuas.

From previously published demographic data of the age distributions and reproductive rates of the pale, intermediate and dark phenotypes of the Arctic Skua, revised estimates are obtained of the intrinsic rates of increase and selective coefficients of the phenotypes in each sex. Two significant components of selection are variation in age of maturity and variation in reproductive success. Sexual selection is a component of the variation in reproductive success. Dark and intermediate birds have a greater reproductive success than pales, partly as a result of their advantage in sexual selection. Pale birds have an overall advantage, however, because they are younger when they first breed. Given the estimated selective coefficients, a computer model predicts that pale birds will gradually replace the others. So far there are not enough data collected over a sufficiently long time to test the model rigorously.