Parental investment and stabilizing selection

Features of the chemical communication system are also likely to be molded by the probabilities of mate finding under different population densities. In the absence of any other pressures, stabilizing selection (selection for the population norm) should prevail, favoring (in species with a multichemical pheromone) those males most sensitive to the blend emitted by the majority of females, and those females emitting the blend to which males are most sensitive. Individuals varying from the norm should have a reduced probability of finding a mate, especially at moderate to low population densities, even in the absence of competition with other species. At high densities, however, discrimination by females for better males may also mold the system. 

Once males began tuning their receptors to emissions common to most con-specific females, presumably selection should have favored those females producing greater quantities of these volatiles, as such females would be most likely to attract males and succeed in mating, especially at low population densities. However, the amounts of lepidopterous sex pheromone emitted by females, usually of the order of 1-100 billionths of a gram per hour, are miniscule compared to emissions of courtship compounds by males (Birch, 1974) or of defensive compounds by insects of both sexes. 

These examples show that, in addition to the posited interspecific effects of competition for an exclusive communication channel, other factors including energetics, predation, pheromone dispersion, physiochemical constraints, and stabilizing selection, may dictate the design of these signals. We will now consider in detail the sexual communication system of the oriental fruit moth, G. molesta, as an example of the interplay of the chemical and non-chemical signals in sexual communication and the potential role of sexual selection. 

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