Mutualisms - cooperative interactions between species - are central to the survival and reproduction of most organisms on earth. Despite their ubiquity, explaining the evolutionary persistence of mutualisms remains one of the greatest challenges for evolutionary biology. The first problem is that selfish individuals can exploit mutualisms, reaping benefits while paying no costs. So, why cooperate at all? A second problem is that the costs and benefits of cooperation depend on a suite of external factors, which vary hugely over time and space. This context-dependence makes it difficult to develop and test predictive theory. Although scientists have developed approaches to test the short-term responses of mutualisms to environmental changes, we lack the tools to predict the long-term evolutionary consequences of context changes. In a series of manipulative, multi-generational experiments, I will test how context modifies evolutionary selection for cooperation. I will utilize the arbuscular mycorrhizal mutualism, arguably the world?s most abundant mutualism and responsible for massive global nutrient transfer, to ask: (i) Partner Context: Do hosts consistently choose the best partner over evolutionary time?(ii) Resource Context: Does high resource availability select for non-cooperative behavior?(iii) Network Context: Can species that eat fungi be co-opted by plant hosts to consume non-cooperative fungal partners? My work is possible through the development and exploitation of novel methodologies: stable isotope probing to track the allocation of host resources to diverse fungal assemblages, root-organ cultures to experimentally evolve fungal genotypes in varying resource conditions, and rhizosphere microcosms to follow population dynamics of non-cooperative mutualists as a food source for fungivores. Mutualisms are evolving in a rapidly changing world - there is a critical need for research linking environmental changes with the evolutionary dynamics of these widespread partnerships. Can mutualisms adapt? My research will provide crucial insights into how ecological context alters the evolution of cooperation..