Although genetic diversity within a population is suggested to improve population-level fitness and productivity, the existence of these effects is controversial because empirical evidence for an ecological effect of genetic diversity and the underlying mechanisms is scarce and incomplete. Here, we show that the natural single-gene behavioural polymorphism (Rover and sitter) in Drosophila melanogaster has a positive effect on population fitness. Our simple numerical model predicted that the fitness of a polymorphic population would be...