Sources of variation in physiological phenotypes and their evolutionary significance

We offer the thesis that environmental physiologists and evolutionary biolo gists can find fertile common ground in the study of how individual variati on in physiological phenotypes originates and develops. The sources of such individual variation are often complex; the consequences affect how natura l selection will act on a suite of traits, of which some may seem, at first glance, far removed from the usual domain of environmental physiology, We illustrate our thesis In two ways. First, we offer two examples drawn from studies of thermal tolerance in the poeciliid fish Heterandria formosa, We show how fitness variation can be a complex function of the gestational tem perature and ther mal tolerance and how these effects can produce environme ntally induced variation among populations in thermal tolerance that mimics a pattern of adaptive variation, Second, we review two case studies that i lluminate how environmental effects on a multivariate phenotype can channel the action of natural selection. The phenotypic plasticity of male life hi story in Poecilia latipinna in response to temperature embraces a spectrum of traits; the effects of each one upon fitness will influence the ability of selection to mold the response of any one of them to temperature. The ph enotypic covariances in thermal tolerance and life-history traits in Hetera ndria formosa differ slightly between populations from different parts of t he species range, apparently because of differences between them in thermal sensitivity; this difference insures that the multivariate nature of selec tion will be correspondingly different in those different populations.