Mutational effects and population genetic consequences: A study of two bacteriophages

Mutation is the raw material on which natural selection acts. Determining the shape of the distribution of mutational effects thus has important consequences in understanding how populations change in fitness over time, which is an essential goal of evolutionary biology. Here I present an initial study of the shape of the distribution of mutational effects.; In Chapter I, experimental evolution of the bacteriophage ФX174 was used to infer several important parameters of the distribution, including the general L-shaped form, and the manner in which the distribution changes as fitness increases or decreases.; In Chapter II, a detailed analysis of the molecular changes within these evolved lines was performed to allow a deeper understanding of how each change was affected by selection. This analysis confirmed several of the hypotheses put forth in the first chapter, namely that the distribution of mutational effects has a fairly broad tail, and that the fraction of mutations that are advantageous changes very significantly with fitness.; In the final chapter, I bring the study venue outside of the laboratory into the natural world to investigate the genetic parameters that natural populations of a second bacteriophage, ϕ6. ϕ6 consists of three "chromosomal" segments, and is thus capable of reassortment and sexual reproduction. Surprisingly, I find that natural populations experience high levels of segmental exchange but have very limited recombination within segments. This suggests that while segments usually experience large population sizes, due to tight linkage within segments selective sweeps can drastically cleanse the population of genetic variation. This is in fact observed for one of the segments.; Overall, this produces a parameterized model of bacteriophage evolution for which there are good estimates of both mutational and population genetic parameters. It provides, for the first time, insight into the both distribution of advantageous mutations and the rate of genetic exchange for bacteriophage populations. It is unlikely that these parameters are relevant only to viral evolution. The approach employed here should be extremely useful in further elucidating the shape of the distribution of mutational effects for a wide range of other taxa.