Selection and genetic variation of weaponry in a large mammal

Abstract

Understanding the maintenance of the variation that is typically observed in natural populations has been a central aim of evolutionary biology. In a feral population of Soay sheep on the island of Hirta, St.Kilda there is a phenotypic polymorphism for horns with males growing either normal or reduced (scurred) horns, and females growing either normal, scurred or no (polled) horns, with further variation in horn size within each of the horn types. This thesis examines the potential factors which maintain these polymorphisms. I first present an overview of the literature relating to the factors that potentially maintain variance in traits in natural populations. In chapter two I present an analysis that suggests that polymorphisms in both horn type and horn size may be maintained by trade-offs between allocation to reproductive success and survival in males, and by sexually antagonistic selection between males and females. In chapter three I test the hypothesis that female weaponry may convey an advantage in intrasexual conflicts over resources, rather than just being expressed as a consequence of genetic associations with the male phenotype. Chapter four examines the environmental factors which create variation between individuals in their horn length, revealing that individuals vary in response to the environment. In chapter five I investigate whether the temporally fluctuating environmental conditions of St.Kilda generate fluctuating selection on the horn length of normal-horned males, revealing that this mechanism constrains the evolution of horn length potentially maintaining variance. In chapter six I examine the genetic relationships between morphological traits, revealing that these relationships are dependent upon the environmental conditions experienced during the first year of life. Finally, I discuss the wider implications of these findings for our understanding of the maintenance of trait variation in the wild.