Introduction: The Evolution of Morphology, Performance, and Fitness

The concept of performance is fundamental to organismal and integrative biology. An organism must perform key tasks or functions—regulate water, ions and temperature; photosynthesize, feed, digest, move, and grow—that are crucial for its survival and reproduction. Understanding how lower-level physiological, morphological, and behavioral traits determine the performance of organisms in various environmental conditions is a central aim of the fields of physiology, functional morphology, and biomechanics (Bartholomew, 1958; Levins, 1968; Bock, 1977, 1980; Huey and Stevenson, 1979). Similarly, the concept of natural selection is fundamental to evolutionary biology. Selection—the relationship between variation in phenotypic traits and variation in fitness among individuals in a population—is the key driving force of adaptive evolution. Understanding the patterns and intensities of selection on phenotypic traits, and how such selection causes adaptive evolution, are central aims of the fields of evolution and ecology (Endler, 1986; Kingsolver et al., 2001). In an ASZ symposium 21 years ago, Steve Arnold outlined a novel framework (‘‘morphology, performance, and fitness’’) that addressed linkages connecting lower-level phenotypic traits, organismal performance, and fitness. Arnold proposed combining laboratory measurements of individual variation in morphology (or physiology) and in performance with field studies of selection and evolution (Arnold, 1983). He also proposed a specific statistical model for quantifying the magnitude of connections among phenotype, performance, and fitness. Arnold illustrated these connections with a simple path diagram (Fig. 1) showing how different morphological traits influenced (directly