| Summary: | Evolution proceeds through genetic changes to individuals, which are either propagated or disappear over generations. Adaptation is one of the main mechanisms driving these changes in genetic composition. Speciation can also result from different, and incompatible, genetic changes occurring in different populations. This thesis furthers our knowledge of the genetics of adaptation and speciation using the budding yeast Saccharomyces cerevisiae. My work on the genetic basis of adaptation to high concentrations of copper, when contrasted with a similar experiment using the fungicide nystatin, showed that the environment has a strong influence on both the number of genes that are the targets of selection and the types of potentially beneficial mutations. These results have implications for the repeatability of genetic evolution. In a second study, I found that genetic interactions between individually isolated single-step beneficial mutations from the same selective environment often exhibited the type of epistasis that underlies speciation even though these mutations occurred within a single biosynthetic pathway. These results support the mutation-order model of speciation by adaptation, where the chance order of mutations in separated populations leads to divergence and the build-up of reproductive isolation due to genetic incompatibility. Negative genetic interactions became positive when the level of stress was increased, indicating that genetically-based reproductive isolation can also be environment-dependent. Finally, I found that diploid yeast were generally not able to adapt to a level of fungicide to which haploid yeast can adapt. Diploids have been found to adapt to a lower concentration of the same drug, indicating that the exact environment (type and concentration) and ploidy can have an impact on the likelihood of genetic rescue. Together, these results have implications for our understanding of the genetic basis of adaptation in different types of environments and different levels of the same environmental stressor. === Science, Faculty of === Zoology, Department of === Graduate
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