STABILIZING SELECTION AND THE COMPARATIVE-ANALYSIS OF ADAPTATION

Comparative studies tend to differ from optimality and functionality s tudies in how they treat adaptation. While the comparative approach fo cuses on the origin and change of traits, optimality studies assume th at adaptations are maintained at an optimum by stabilizing selection. This paper presents a model of adaptive evolution on a macroevolutiona ry time scale that includes the maintenance of traits at adaptive opti ma by stabilizing selection as the dominant evolutionary force. Inters pecific variation is treated as variation in the position of adaptive optima. The model illustrates how phylogenetic constraints nor only le ad to correlations between phylogenetically related species, but also to imperfect adaptations. From this model, a statistical comparative m ethod is derived that can be used to estimate the effect of a selectiv e factor on adaptive optima in a way that would be consistent with an optimality study of adaptation to this factor. The method is illustrat ed with an analysis of dental evolution in fossil horses. The use of c omparative methods to study evolutionary trends is also discussed.