Introduction

Physiologists have long understood that an organism's phenotype is not fixed but rather is partially dependent on the environment experienced during ontogeny. This phenomenon of acclimation, which represents a special type of phenotypic plasticity, offers several research opportunities. Acclimation experiments help elucidate the mechanistic process underlying rapid physiological regulation in response to dynamic environmental fluctuations (Hochachka & Somero, 1984; Prosser, 1986; Cossins & Bowler, 1987). Similarly, acclimation experiments can be used as tools to explore evolutionary adaptation to fluctuating environments (Levins, 1968). Nevertheless, acclimation offers more than a carrot, for it offers a stick as well. The fact of acclimation forces physiologists to control acclimation state as a necessary, if often inconvenient, first step in both mechanistic and evolutionary experiments.

The literature on physiological acclimation is now immense. Inspection, however, suggests that physiologists have been much more rigorous and successful in elucidating the mechanisms underlying acclimation responses than they have in exploring the evolution of these responses. Why have evolutionary explanations in studies of physiological acclimation lacked rigor? (The same question can probably be asked of any field of functional biology (Feder, 1987a; Garland & Carter, 1994; Travis, 1994; Bennett, 1996).) In part, this relative lack of rigor may reflect the fact that functional biologists are often more interested in mechanistic issues than in evolutionary ones; but it may also reflect the fact that a need for greater rigor in evolutionary explanations in biology in general has really been evident only in the past 15 years or so (Gould & Lewontin, 1979; Feder, 1987a).