Early adaptation in a microbial community is dominated by mutualism-enhancing mutations

Evolutionary dynamics in ecological communities are often repeatable, but how species interactions affect the distribution of evolutionary outcomes at different levels of biological organization is unclear. Here, we use barcode lineage tracking to experimentally address this gap in a facultatively mutualistic community formed by the alga Chlamydomonas reinhardtii and the yeast Saccharomyces cerevisiae. We find that interactions with the alga alter the magnitude but not the sign of the fitness effects of adaptive mutations in yeast, changing the distribution of mutants contending for fixation. In the presence of alga, most contending mutants reinforce the mutualism and make evolution more repeatable at the community level. Thus, ecological interactions not only alter the trajectory of evolution but also dictate its repeatability at multiple levels of biological organization. One-sentence summaryThe mutualistic interaction between two species drives evolution to increase both species yields.