DENSITY-DEPENDENT SELECTION IN A FLUCTUATING UNGULATE POPULATION

Despite considerable theoretical interest no direct examples of densit y-dependent natural selection acting on simple polymorphic variation h ave been documented in a natural population. Here we show that the mag nitude of selective differences in survival between phenotypes in two conspicuous polymorphisms of coat colour and horn type in Soay sheep O vis aries living on St Kilda, Scotland are associated with marked chan ges in population density. Selection is strongest in years of high den sity but weak in years of low density. In addition to direct observati ons of density-dependent 'soft' selection in a natural population, the analysis revealed that the level of overcompensatory mortality (respo nsible for promoting population instability) was higher after accounti ng for genetic variation in the coat and horn morph traits. The result s emphasize the importance of understanding the interaction between se lection and population demography for both genetic and ecological stud ies of natural populations.