On the Evolution of Reliable Indicators of Fitness

We study the evolution of traits that have conflicting effects on two components of fitness in a population in which there are prior heritable viability differences. Examples of such traits may include the conspicuous male traits, or displays, that increase a male's mating success but decrease his viability. We consider the simple case of two viability classes controlled by a single haploid locus and a second modifier locus, which modifies the expression of the trait in one or both of the viability classes. Variation at the viability-class locus is maintained by a balance between selection and a force, like mutation or migration, that restores the less fit class. As a result of the prior differences in viability and the effects of the trait, overall fitness in each viability class is maximized at different values (optima) of the trait. In the absence of modifiers that can differentially express the trait in the different viability classes, the evolutionarily stable value of the trait is a compromise between the optimum values for the two fitness classes. Modifiers that can act independently on the trait in the two viability classes allow the trait to take on its optimum values in the two viability classes. Since the optimum of the trait for the high-viability class is greater than that for the low-viability class, evolutionary modification should allow such traits to be used as reliable indicators of genetic fitness. Differential cost of the trait in the two viability classes is not required for the trait to be maintained as a reliable indicator of fitness in an additive model of viability effects considered here. However, in a multiplicative model of viability effects, differential cost would be necessary. What seems to be required in all cases is simply that the overall fitness of the two viability classes be maximized at different values of the trait. Constraints on the effects of the modifiers in the two viability classes can interfere with this optimization process. However, the trait still evolves to have different values in the two viability classes. We apply our results to the good-genes theory of sexual selection, which postulates that the male display is a reliable indicator of male fitness.