Ecology Letters (2011) 14: 782–787
One of the oldest ecological hypotheses, proposed by Darwin, suggests that the struggle for existence is stronger between more closely related species. Despite its long history, the validity of this phylogenetic limiting similarity hypothesis has rarely been examined. Here we provided a formal experimental test of the hypothesis using pairs of bacterivorous protist species in a multigenerational experiment. Consistent with the hypothesis, both the frequency and tempo of competitive exclusion, and the reduction in the abundance of inferior competitors, increased with increasing phylogenetic relatedness of the competing species. These results were linked to protist mouth size, a trait potentially related to resource use, exhibiting a significant phylogenetic signal. The likelihood of coexistence, however, was better predicted by phylogenetic relatedness than trait similarity of the competing species. Our results support phylogenetic relatedness as a useful predictor of the outcomes of competitive interactions in ecological communities.