Theoretical studies of the interaction between deleterious and beneficial mutations

• Main Author: Johnson, Toby
• Published: University of Edinburgh 2000
• Subjects: 576.58
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.653038

Mutation, broadly defined, provides the raw material for evolution. Most mutations are deleterious, that is they confer a disadvantage and the tendency of natural selection is to eliminate them. Populations suffer a reduction in mean fitness, or load, as a consequence of deleterious mutation. One question of interest in evolutionary biology is: How large is the deleterious mutational load? On the other hand, a few mutations are beneficial, conferring an advantage. The establishment within a population, or fixation, of such mutations is the basis of adaptive evolution, but the probability that any one such mutation becomes fixed is small. A second question is: At what rate are beneficial mutants fixed? Some insight into the answers to these questions can be gained from analysis of theoretical population genetic models. In this thesis I concentrate on cases in which, to answer either question alone, it is necessary to understand the interaction between both deleterious and beneficial mutations. The first part of the thesis concentrates on models of asexual populations. Specifically, I explore how beneficial mutations sweeping through the population influences the deleterious mutational load, and how the load influences the probability that a given beneficial mutation becomes fixed. Clearly, the underlying mutation rate has great bearing on the questions addressed here, and in the second part of the thesis a number of modifier models for the evolution of mutation rates are developed. Separate models are studied for sexual and for asexual populations, but in both cases I examine how the mutation rate is expected to evolve under two conflicting pressures. Higher mutation rates are favoured because they are associated with more frequent beneficial mutations, but lower mutation rates are favoured because they are associated with reduced deleterious mutational load.