Repeated selection of morphometric traits in the Soay sheep on St Kilda

1. Long-term studies allow the outcomes of repeated selection events to be monitored. Here, we investigate phenotypic selection in successive winter m ortality events in the Soay sheep of St Kilda, Scotland, between 1985 and 1 996. Selection of three quantitative morphometric traits, body weight, hind leg length and incisor arcade breadth, was investigated in different sector s of the population. 2, Evidence from fitness differentials of positive directional selection fo r large size was repeatedly found in lambs and adult females. Selection in the opposing direction was only found in one year in lambs. 3. Selection gradients showed that in most years when significant selection occurred, body weight was the focus of direct selection, whereas selection of hindleg length and incisor breadth was indirect, arising from their cor relation with body weight. 4, Selection was strongest in years of low over-winter survival and almost absent in years when survival was high. Intensity of selection was greatest in lambs, emphasizing the differences in selection pressure experienced by different sectors of the population, in addition to the temporal variation in selection pressure due to population density and environmental conditio ns. 5. Despite repeated positive selection of body weight, no evidence of a cha nge in the population mean was found over the course of the study.