rejecting potential mates—could simultaneously reject one hypothesis and provide support for the alternative.

For species in which parental investment is biased toward one sex, investigators could compete the predictions of the SPT with parental investment theory. Controlling for s, e, n, l, and w distribution for experimental subjects, the predicted behavior of individuals of different sexes would be the same under the SPT. Parental investment theory, by contrast, predicts that in a species with female-biased parental investment, females would reject more and males would accept more potential mates, whereas, in a species with male-biased parental investment, females would accept more and males reject more potential mates. Another valuable test would be of virgins of both sexes, for whom l = 0, in species with female-biased parental investment and in species with male-biased parental investment. As with anisogamy theory, these alternative predictions of the SPT and parental investment theory could be tested with a crucial experiment.

The w distribution has only been characterized for a few populations (unpublished data), and no one to our knowledge has tested the effects of w distributions on individual reproductive decisions. For laboratory populations of flies and other organisms with short generation times and no sex biases in dispersal, it is relatively easy to estimate the shape of the w distribution, measuring fecundity, productivity, and offspring viability from a sample of random pairs breeding under enforced monogamy. An experiment that we plan to do will begin with flies cultured under inbreeding and outbreeding, which may produce w distributions with different shapes, and then to test the predictions (Fig. 11.7) for virgins (l = 0) when e, s, and n are held constant, using pretouching arenas.

The SPT is not a hypothesis for the evolution of fancy male traits, nor does it predict the evolution of traits mediating preferences. Nevertheless, the SPT suggests that selection should favor traits that increase a focal individual’s encounters with potential mates. Enhanced encounters increase reproductive opportunities, thereby reducing the opportunity costs of accepting potential mates who would confer low w. Traits, such as bizarre or easily seen plumage, loud calls, songs, or pheromones that travel over