Studying the evolution of whole-organism performance capacity: sex, selection, and haiku - an introduction

474–498; Endler, J., 1986, Natural Selection in the Wild. Princeton, NJ: Princeton University Press; Kingsolver et al., 2001, Am. Nat., 157: 245–261). While we agree that such attention is warranted, we aim to divert your attention towards another topic, namely the microevolution of whole-organism performance capacity. The articles in this special collection present more detail on definitions of performance capacity, but we define it here as the quantitative measure of the ability of an organism to conduct an ecologically relevant task. Classic examples include sprint speed, bite force, and locomotor endurance, among other measures (Irschick, D.J., and Garland, T., Jr., 2001, Annu. Rev. Ecol. Syst., 32: 367–396). The most burning issue that motivated this special collection was how the forces of natural and sexual selection operate on performance traits, in contrast to morphology. Because performance as defined here is measured at the whole-organism level, variation in performance arises from variation from many other traits, such as aspects of physiology and morphological shape. Therefore, visualizing selection on performance is potentially trickier than for morphology because one is immediately drawn to the potential for interrelationships: does selection favour high performance, good morphological attributes, or both? Some of the papers in this special collection address this topic head-on, and suggest a high degree of interrcorrelation between morphology and performance, as well as with behaviour (Le Galliard and Ferriere, this issue). Another paper shows that the strength of selection appears not to differ between morphological and performance traits (Irschick, Meyers, Husak, and Le Galliard, this issue). These findings remind the reader of the complex nature of co-evolved aspects of the phenotype and genotype (Cheverud, J., 1982, Evolution, 36: 499–516), and also imply that selection studies should aim to move towards more comprehensive measures of the entire phenotype and emergent functional and behavioural properties. A review of selection studies (Irschick et al., this issue) reveals another interesting finding – selection on performance, much like morphology, is rarely stabilizing or disruptive. Nearly half of the 23 selection studies reviewed were directional, with a distinct favouring of high performance. One intriguing possibility is that selection may favour different combinations of morphology and performance in different years, as environmental conditions change (Calsbeek, this issue), leading to directional selection within a season and a more static pattern of evolution over ecological time. Sadly, as this special collection highlights, we lack the long-term studies of performance to test this intriguing possibility. One of the most exciting developments over the past 10 years has been an increased appreciation for the dual and potentially conflicting role of sexual and natural selection (Le Galliard and Ferriere, this issue). In some of the papers in this special collection, there is evidence that the two forces act in unison, meaning that high-quality males (at least in the