Direct and indirect selection in moth pheromone evolution: population genetical simulations of asymmetric sexual interactions

Female moths generally use pheromones to attract males. Normally, all females in a population produce a specific chemical blend with only a limited variance, and the local males are highly attracted to this blend. To better understand the direct and indirect selective forces acting on this communication system, where, unusually, it is the reproductively limited sex that signals for matings, a population genetical model has been constructed and numerically analysed. Basic to the model is the assumption that the pheromone attraction system functions asymmetrically, leading to strong sexual selection between males but no direct sexual selection between females. Evolutionary simulations using the model show that sexual selection in males causes an indirect stabilizing selection on the pheromone blends produced by females. Thus, a more narrow range of pheromone variation is selected for, even in the absence of female sexual selection. The strength of the selection is analysed, and it is suggested that this indirect stabilizing selection becomes particularly important in situations where geographically adjacent populations have evolved different pheromone blends.