Coevolution is prevalent in nature. Mutualisms, or jointly beneficial relationships between species, are one of the most common settings in which coevolution occurs. Despite this prevalence and the widespread medical importance of mutualisms, very little research has been done on the evolutionary dynamics of these relationships. Theory predicts that sets of genes within each interacting species should influence one another and ultimately shape the long-term relationship between the two species. Genetic interactions within a single species are known to have a profound effect on the route that evolution takes for that species. However, in the case of mutualisms, these genetic interactions may affect the route that evolution takes in both species collectively, or even separately. Do these mutualistic relationships alter evolutionary pathways? And if they do, is it for better or worse? I will use experimential evolution, a process of allowing organisms to evolve within a controlled setting but without predetermined results, to answer these questions, I will evolve mutualist partner species either together or apart to determine if the rate of evolution changes depending on their association. Then, I will determine the effects of evolution either with or without a partner on the genome of one partner species. These tests will determine if evolution within a mutualism alters the evolutionary pathway of a population by looking at how a mutualistic relationship changes both the genotypes and phenotypes of a species that can exist either independently or with a mutualist partner. PUBLIC HEALTH RELEVANCE: Humans maintain mutualistic relationships with thousands of microbes. Bacteria in the human gut are known to effect metabolic rates and therefore are linked with many human health issues, like obesity. This project explores the potential limitations and hazards of a mutualistic relationship between a species of bacteria and its animal partner.