Multi-trait Selection, Adaptation, and Constraints on the Evolution of Burst Swimming Performance.

Whole organism performance represents the integration of numerous physiological, morphological, and behavioral traits. How adaptive changes in performance evolve therefore requires an understanding of how selection acts on multiple integrated traits. Two approaches that lend themselves to studying the evolution of performance in natural populations are the use of quantitative genetics models for estimating the strength of selection acting on multiple quantitative traits and ecological genetic comparisons of populations exhibiting phenotypic differences correlated with environmental variation. In both cases, the ultimate goal is to understand how suites of traits and trade-offs between competing functions respond to natural selection. Here we consider how these two complimentary approaches can be applied to study the adaptive evolution of escape performance in fish. We first present an extension of Arnold's (1983) quantitative genetic approach that explicitly considers how trade-offs between different components of performance interact with the underlying genetics. We propose that such a model can reveal the conditions under which multiple selection pressures will cause adaptive change in traits that influence more than one component of fitness. We then review work on the Atlantic silversides and Trinidadian guppies as two case studies where an ecological genetics approach has been successfully applied to evaluate how the evolution of escape performance trades-off with other components of fitness. We conclude with the general lesson that whole organism performance is embedded in a complex phenotype, and that the net outcome of selection acting on different aspects of the organism will often result in a compromise among competing influences.