Evolution can favour more than one reproductive tactic among conspecifics of the same sex(1,2). Under the conditional evolutionarily stable strategy, individuals adopt the tactic that generates the highest fitness return for their status: large males guard females, whereas small males sneak copulations(3,4). Tactics change at the status at which fitness benefits switch from favouring one tactic to favouring the alternative(1,5). This 'switchpoint' is expressed in many species as a threshold between divergent morphologies(3). Environmental and demographic parameters that influence the relative fitness of male tactics are predicted to determine a population's switchpoint(1,5) and consequently whether the population is monomorphic or dimorphic. Here we show threshold evolution in the forceps dimorphism of the European earwig Forficula auricularia and document the transition from completely monomorphic to classical male-dimorphic populations over a distance of only 40 km. Because the superior fighting ability of the dominant morph(6) will be more frequently rewarded at high encounter rates, population density is likely to be a key determinant of the relative fitness of the alternative tactics, and consequently the threshold. We show that, as predicted, population density correlates strongly with the shift in threshold, and that this factor drives the local evolution of the male dimorphism in these island populations. Our data provide evidence for the origin of phenotypic diversity within populations(7-9), through the evolution of a switchpoint in a conditional strategy that has responded to local population density.