A GENETIC-POLYMORPHISM IN THE SWORDTAIL XIPHOPHORUS-NIGRENSIS - TESTING THE PREDICTION OF EQUAL FITNESSES

When a genetically determined polymorphism is selectively maintained in a population, the different morphs should have equal fitnesses at equilibrium. We empirically examined this prediction for the size polymorphism of the swordtail Xiphophorus nigrensis, in which a single locus on the Y chromosome controls male size. Small males mature earlier and chase females, whereas large males mature later and court females. We analyze our data with a model that uses the differential mating success and the ages at sexual maturity of the two morphs to calculate the per capita death rate necessary for them to have equal fitness. We demonstrate how female fecundity data can be used to determine whether the estimated death rate is biologically realistic. Our data support the hypothesis that morph fitnesses are equal, and the model is fairly robust to changes in population growth rate and differential death rates of morphs. However, the confidence intervals for our estimates are large, which suggests that the null hypothesis only be accepted with caution. We show that in many circumstances very large sample sizes will be needed to distinguish between alternative hypotheses concerning the relative fitnesses of the two morphs. We emphasize that despite the popularity of alternative mating behaviors, specifically, and mixed evolutionarily stable strategies, in general, there is almost no empirical evidence that alternative behavioral morphs have equal fitnesses. Also, the conclusion that morph fitnesses are equal does not address the hypothesis that frequency-dependent mating success is the mechanism maintaining the equilibrium of fitnesses. This requires additional evidence directly demonstrating the fitness effect of changes in morph frequency.