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, increasing the genetic diversity of mutants contending for fixation. Despite genetic diversity, most contending mutants reinforce the mutualism, which makes evolution more repeatable at the community level. Thus, ecological interactions not only alter the trajectory of evolution but also dictates its repeatability at multiple levels of biological organization.