THE MAINTENANCE OF GENETIC-POLYMORPHISM IN SMALL ISLAND POPULATIONS -LARGE MAMMALS IN THE HEBRIDES

Conventionally, small populations living on islands are expected to lo se genetic variation by drift. Fluctuations in population size, combin ed with polygynous mating systems, are expected to contribute to the p rocess by increasing sampling effects on genetic variation. However, i n individually monitored populations of Red deer on Rum and Soay sheep on St. Kilda, which experience fluctuations in population size, two p rocesses have been identified which mitigate loss of genetic variation . First, in a number of examples, population reductions are associated with selection. Selection may be in favour of heterozygotes, or, as w e have documented in several cases, it may fluctuate in direction temp orally. Second, in Soay sheep, in which mortality over population cras hes is male-biased, ostensibly leading to low effective numbers of mal es, molecular studies show that there are systematic changes in the re productive success of young males, and in variance in male success, th at broaden genetic representation compared with expectation.