Performance surfaces and adaptive landscapes

In an earlier characterization of the relationship between morphology, performance and fitness, I focused only on directional selection (Arnold, 1983). The aim of this article is to extend that characterization to include stabilizing and other forms of nonlinear selection. As in the earlier characterization, this more general description of the morphology-performance-fitness relationship splits empirical analysis into two parts: the study of the relationship between morpholgy and performance, and the study of the relationship between performance and fitness. From a conceptual standpoint, my goal is to specify the relationship of performance studies to the adaptive landscape. I begin by reviewing the adaptive landscape concept and its importance in evolutionary biology. A central point emerging from that review is that that key descriptors of the adaptive landscape can be estimated by measuring the impact of selection on the means, variances and covariances of phenotypic traits. Those descriptors can be estimated by making a quadratic (regression) approximation to the selection surface that describes the relationship between the phenotypic traits of individuals and their fitness. Analysis of the effects of morphology on performance follows an analogous procedure: making a quadratic approximation to the individual performance surface and then using that approximation to solve for the descriptors of the performance landscape. I conclude by discussing the evolution of performance and adaptive landscapes. One possibility with biomechanical justification is that the performance landscape evolves along performance lines of least resistance.