Organismal Performance and Darwinian Fitness: Approaches and Interpretations

Organismal performance lies at the base of ecological and evolutionary processes, and the consequences of individual variation in performance have become a focus of physiological ecology. The study of the relations of performance to Darwinian fitness integrates the traditionally reductionist approach of physiological ecology with the perspectives of genetics and evolution. Four levels of biological organization have been included in this effort: the description of genetic variation, the analysis of biochemical and physiological consequences of that variation, the description of variation in the capacity of individuals to perform various activities, and the analysis of the effect of that variation in performance capacity on individual variation in Darwinian fitness. Mechanisms within each of those levels have been elaborated, and cause-and-effect links have been established among the levels. Two approaches to defining causal links are prominent. The gene-to-performance school begins with the observation of genetic variation (e.g., the presence in a species of allelic isozymes with different responses to temperature) and seeks to trace the performance of different genotypes upward through successive levels of biological organization, ultimately to variation in behavior and the relation of that variation to natural selection. A few studies have been successful in this endeavor, largely because they were based on detailed information about the ecology of the species investigated. These studies have revealed stabilizing selection maintaining genetic polymorphisms. Physiological ecologists have approached the same questions as those addressed by geneticists, but the physiologists have started with the description of individual variation in performance that is almost certainly under the control of multiple genetic loci. From this point they have moved outward in two directions. One line of investigation involves a reductionist analysis of the mechanistic basis of variation in performance, whereas the other seeks associations between performance and fitness. Substantial progress has been made in the reductionist direction, although more information is needed about the heritabilities of particular characteristics. The upward direction has been less traveled, and the chances of success will be maximized by employing the same detailed ecological perspective that has been successful for gene-to-performance studies. The activities to be studied should be selected on the basis of their demonstrable importance to the fitness of individuals under natural conditions, and the behavioral ecology literature contains many examples of activities of that sort.