Strains of Caenorhabditis elegans obtained from their natural soil environment exhibit one of two forms of foraging behavior. Some strains forage solitarily and disperse evenly on a bacterial lawn. Other strains move rapidly until they encounter groups of conspecifics, and then slow their movement and join the group. Strains expressing these behaviors are globally widespread and have been isolated from the same location, suggesting a foraging polymorphism. We hypothesized that density-dependent selection maintains both foraging alleles in populations. Alternatively, both foraging alleles could be retained in populations through frequency-dependent selection. We tested both of these hypotheses by manipulating strain density and frequency, and observing changes in population density over time. Our results indicated that neither density- nor frequency-dependent selection appears to be responsible for the observed polymorphism. The clumping strain consistently out-competed the solitary strain over all treatment levels. We suggest other potential factors that may maintain both alleles in populations.
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