Positive genetic correlation between parasite resistance and body size in a free-living ungulate population

Parasite resistance and body size are subject to directional natural select ion in a population of feral Soay sheep (Ovis aries) on the island of St. K ilda, Scotland. Classical evolutionary theory predicts that directional sel ection should erode additive genetic variation and favor the maintenance of alleles that have negative pleiotropic effects on other traits associated with fitness. Contrary to these predictions, in this study we show that the re is considerable additive genetic variation for both parasite resistance, measured as fecal egg count (FEC), and body size, measured as weight and h indleg length, and that there are positive genetic correlations between par asite resistance and body size in both sexes. Body size traits had higher h eritabilities than parasite resistance. This was not due to low levels of a dditive genetic variation for parasite resistance, but was a consequence of high levels of residual variance in FEC. Measured as coefficients of varia tion, levels of additive genetic variation for FEC were actually higher tha n for weight or hindleg length. High levels of additive genetic variation f or parasite resistance may be maintained by a number of mechanisms includin g high mutational input, balancing selection, antagonistic pleiotropy, and host-parasite coevolution. The positive genetic correlation between parasit e resistance and body size, a trait also subject to sexual selection in mal es, suggests that parasite resistance and growth are not traded off in Soay sheep, but rather that genetically resistant individuals also experience s uperior growth.