1. How evolutionary relatedness influences the strength of competitive interactions among genetically isolated populations has been a long-standing interest in ecology. Darwin's “Competition Relatedness Hypothesis (CRH)” states that, since closely related species should compete more strongly, they are less likely to coexist, while Herbert’s “Bimodal Competition Hypothesis (BCH)” predicts that competitive exclusion is less likely to occur when the competing species are genetically close or distant.

2. To test these hypotheses, we experimentally examined the difference in the competitive ability and life tables of fecundity and survivorship among four different genotypes of asexual Daphnia cf. pulex that diverged from a single ancestral genotype.

3. The experiments showed that the competitive consequences differed depending on the pairing of the competing genotypes, and that the degree of the competitive exclusion was lower when the competing genotypes were genetically closer to each other. These results partially supported the BCH but not the CRH at all. More importantly, the degree of competitive exclusion was better predicted by the phenotypic rather than genetic distances between the competing genotypes.

4. The life table experiments revealed that competitively inferior genotypes had higher early reproduction rates, but survival rates decreased with age and thus body size, probably a result of selection by predation pressures found in nature.

5. These results indicate that competitive superiority is highly dependent on selection pressures that given organisms have been evolutionally subjected to, and, that genetic similarity is not necessarily an appropriate measure for predicting the completive exclusion on an ecological time scale. To predict competitive relationships among the organisms, it is essential to comprehend their phenotypic differences rather than simply knowing their genetic or phylogenetic relationships.