Evolution is the process by which species change over time and adapt to their environment. However, understanding and predicting evolution is difficult because its course is neither completely random nor completely pre-determined. In other words, if we “rewound the tape of life” and “replayed” it, the outcomes of evolution would likely be different, but not totally random. How repeatable evolution is depends in part on how an organism interacts with other species through competition for resources, nutrient exchange, predation, etc. This project uses laboratory experiments to study how interactions between two microbial species, a yeast and an alga, affect how repeatable their evolution is. As such, this project will advance our basic understanding of evolution in the context of species interactions. It will also develop a new curriculum to help college students in California and Mississippi strengthen their scientific reasoning skills and participate firsthand in scientific research. The repeatability of evolution has been described both theoretically and experimentally across many individual species and some ecological communities, but how it depends on the type and strength of ecological interactions between species remains unclear. To address this question, the investigators will conduct a long-term evolution experiment in a tunable two-species mutualism formed by the yeast Saccharomyces cerevisiae and the alga Chlamydomonas reinhardtii under two environmental conditions. In one environment, each species can survive without the aid of the other. In the other environment, the alga can survive alone, but the yeast cannot. The first aim of the project is to characterize the evolutionary changes of this community at the genomic and ecological levels and compare repeatability across the two environments. To probe possible mechanisms underlying differences in repeatability, the second aim will measure the range of adaptive mutations available to both partners across