Competition among kin generates balancing selection in a wild population

If an individual’s niche is determined by its genotype, then competition for limiting resources should be most intense among individuals of the same genotype. Theory predicts this will act to maintain genetic variation, but whether this mechanism operates under natural conditions is unclear. Using long-term observations of a population of free-living Trinidadian guppies, we asked (i) whether competition was strongest between kin, and (ii) whether this process maintained genetic variation. Competition between kin was 1.5-1.8 times stronger than that between non-kin. This contributed to balancing selection: after ∼10 generations, variation was 29% higher than expected under drift. Our results show that relatedness can play a major role in structuring ecological competition, with broader consequences for theories of inclusive fitness.