Functional trait differences and genetic distance are increasingly used as metrics to predict the outcome of species interactions and the maintenance of diversity. We apply these ideas to intraspecific diversity for the seagrass Zostera marina (eelgrass), by explicitly testing the influence of trait distance and genetic relatedness on the outcome of pairwise interactions among eelgrass genotypes. Increasing trait distance (but not relatedness) between eelgrass genotypes decreased the likelihood that both would persist over a year-long field experiment, contrary to our expectations based on niche partitioning. In plots in which one genotype excluded another, the biomass and growth of the remaining genotype increased with the trait distance and genetic relatedness of the initial pair, presumably due to a legacy of past interactions. Together these results suggest that sustained competition among functionally similar genotypes did not produce a clear winner, but rapid exclusion occurred among genotypes with distinct trait combinations. Borrowing from coexistence theory, we argue that fitness differences between genotypes with distinct traits overwhelmed any stabilizing effects of niche differentiation. Previously observed effects of eelgrass genetic diversity on performance may rely on nonadditive interactions among multiple genotypes or sufficient environmental heterogeneity to increase stabilizing forces and/or interactions.