Highly fecund mothers sacrifice offspring survival to maximize fitness

Why do highly fecund organisms apparently sacrifice offspring size for incr eased numbers when offspring survival generally increases with size(1-3)? T he theoretical tools for understanding this evolutionary trade-off between number and size of offspring have developed over the past 25 years(1,4-10); however, the absence of data on the relation between offspring size and fi tness in highly fecund species, which would control for potentially confoun ding variables, has caused such models to remain largely hypothetical(11,12 ). Here we manipulate egg size, controlling for maternal trait interactions , and determine the causal consequences of offspring size in a wild populat ion of Atlantic salmon. The joint effect of egg size on egg number and offs pring survival resulted in stabilizing phenotypic selection for an optimal size. The optimal egg size differed only marginally from the mean value obs erved in the population, suggesting that it had evolved mainly in response to selection on maternal rather than offspring fitness. We conclude that ma ximization of maternal fitness by sacrificing offspring survival may well b e a general phenomenon among highly fecund organisms.