ON THE EVOLUTION OF RELIABLE INDICATORS OF FITNESS

Studied the evolution of traits that have conflicting effects on 2 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. The authors consider the case of 2 viability classes controlled by a single haploid locus and a 2nd 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 2 fitness classes. Modifiers that can act independently on the trait in the 2 viability classes allow the trait to take on its optimum values in the 2 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. -from Authors