A context for molecular studies of the thermal phenotype

The thermal phenotype of an organism comprises hundreds if not thousands of traits. Some of these traits determine the tolerance limits, others determine the thermal sensitivity of physiological performance within the zone of tolerance, and still others underlie responses to changes in temperature such as acclimation, behavioural thermoregulation and physiological thermoregulation. The descriptions or explanations of these traits now constitute an enormous but still-growing literature (Fig. 1a).

The traits underlying the thermal phenotype might behave as an ensemble in at least two ways. On the one hand, each trait could play an essential role such that variation in any trait would have discernible if not major effects on physiological performance and evolutionary fitness. (Fig. 1b analogises this alternative to a ‘house of cards’, in which removal of any one card causes the entire structure to collapse.) On the other hand, numerous and redundant traits could underlie each aspect of the thermal phenotype such that variation in any given trait might have negligible consequences for the thermal phenotype as a whole; i.e. variation in numerous traits is necessary to affect the thermal phenotype. (Fig. 1c analogises this alternative to a ‘roller coaster’, in which many struts must be removed before the structure will fail.) These are obviously extreme alternatives, and intermediate states of ensemble behaviour clearly exist in organisms.