MOLECULAR-GENETIC VARIATION AND INDIVIDUAL SURVIVAL DURING POPULATIONCRASHES OF AN UNMANAGED UNGULATE POPULATION

Theoretical models of the effect of population bottlenecks on genetic variation assume that individuals are removed at random from the popul ation. We investigated this assumption in a naturally regulated, unsta ble population of Soay sheep (Ovis aries). During rapid population dec lines or 'crashes', individuals were not removed at random with respec t to genotype: we found associations between individual survival and c ertain genotypes at five polymorphic protein or microsatellite DNA loc i (Ada, Got, Tf, MAF18 and OPACAP). Some loci appeared to show simple associations with survival whereas others had more complex interaction s with crash year or age: all displayed different patterns of associat ion between the sexes. Simple overdominance was not a general feature of our data; it seems likely that fluctuating selection, countervailin g selection in different fitness components or frequency-dependent sel ection may explain the pattern and complexity of the associations show n at different loci. Our study cannot distinguish between selection ac ting at these loci or at other, closely linked loci. However, our empi rical study implies that the molecular genetic outcome of population b ottlenecks in natural populations does not always follow theoretical e xpectations based on the random removal of genotypes. Bottlenecks in w hich individuals are removed at random are distinct from bottlenecks i n which there is scope for selection via non-random survival of indivi duals.