1. Department of Biology, University of Toronto at Mississauga, Mississauga, Ontario L5L 1C6, Canada
2. Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario M5S 3G5, Canada

Correspondence to: Marla B. Sokolowski¹ Correspondence and requests for materials should be addressed to M.B.S. (Email: msokolow@utm.utoronto.ca).

Accounting for the abundance of genetic variation in the face of natural selection remains a central problem of evolutionary biology^{1, 2}. Genetic polymorphisms are constantly arising through mutation, and although most are promptly eliminated³, polymorphisms in functionally important traits are common. One mechanism that can maintain polymorphisms is negative frequency-dependent selection on alternative alleles, whereby the fitness of each decreases as its frequency increases^{4, 5}. Examples of frequency-dependent selection are rare, especially when attempting to describe the genetic basis of the phenotype under selection. Here we show frequency-dependent selection in a well-known natural genetic polymorphism affecting fruitfly foraging behaviour. When raised in low nutrient conditions, both of the naturally occurring alleles of the foraging gene (for^s and for^R) have their highest fitness when rare—the hallmark of negative frequency-dependent selection. This effect disappears at higher resources levels, demonstrating the role of larval competition. We are able to confirm the involvement of the foraging gene by showing that a sitter-like mutant allele on a rover background has similar frequency-dependent fitness as the natural sitter allele. Our study represents a clear demonstration of frequency-dependent selection, and we are able to attribute this effect to a single, naturally polymorphic gene known to affect behaviour.

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