Tradeoffs and the evolution of thermal reaction norms

Tradeoffs have played a prominent role in the development of theories describing the evolution of reaction norms. Different classes of tradeoffs are known to constrain the evolution of phenotypes, but current theories incorporate only a subset of these tradeoffs. Consequently, these theories cannot account for some of the variation in reaction norms that has been observed within and among species. Empirical studies of thermal reaction norms for physiological and life historical traits have shown that different proximate mechanisms can produce similar reaction norms. As a consequence, certain tradeoffs can be circumvented when the fitness costs imposed by these tradeoffs are severe. We argue that a unified theory that includes all classes of tradeoffs would provide a better understanding of the mechanisms that drive the evolution of reaction norms. The reaction norm (a function relating an environmental variable to the phenotype expressed by a genotype) has become a unifying concept in evolutionary biology [1]. Although the specific reaction norms of interest vary greatly among subdisciplines, any morphological, physiological or behavioral phenotype can be viewed within the context of a reaction norm. Discoveries of heritable variation in reaction norms within and among populations have prompted a proliferation of theories designed to understand their evolution [2][3][4]. As with most evolutionary theories, those constructed to explain the evolution of reaction norms have been based on the assumption that TRADEOFFS (see Glossary) influence the trajectory of evolution. At least three distinct classes of tradeoffs arise from behavioral and physiological processes that operate within the lifetime of an individual (Box 1): (1) tradeoffs that result from the allocation of available resources (ALLOCATION TRADEOFFS); (2) tradeoffs between maximizing the acquisition of resources and minimizing the risk of mortality (ACQUISITION TRADE-OFFS), and (3) tradeoffs that result from specialization for a given environment (SPECIALIST-GENERALIST TRADE-OFFS). These three classes of tradeoffs are fundamental, because they offer mechanisms for the correlated expression of traits among individuals [5] and the correlated evolution of traits among generations [6]. Although all three classes of tradeoffs have been important to the development of evolutionary theory, theorists have focused on particular subsets. Traditionally, evolutionary physiologists have focused primarily on specialist -generalist tradeoffs [7,8], whereas life historians have focused on allocation tradeoffs [9]. More recently, behavioral ecologists have noted the need to expand these foci to include acquisition tradeoffs Here, we argue the need for evolutionary theories that incorporate all classes of tradeoffs, drawing primarily on our knowledge of THERMAL REACTION NORMS of ectotherms. Glossary Acquisition tradeoff: an increase in the probability of being killed or parasitized resulting from an increase in the duration or intensity of foraging and consumption. Allocation tradeoff: a decrement in the resources allocated to one or more functions resulting from an increment in resources allocated to another function. Endurance: the maximum duration of a sustained activity (e.g. running or swimming) measured using a standardized protocol. Often measured on a motorized treadmill (running) or in a flume (swimming). Growth efficiency: the fraction of ingested energy that is used for growth. Often estimated from changes in wet mass, rather than directly determined from energy fluxes. Maximal sprint speed: the fastest speed observed for an animal that is running or swimming under controlled conditions. Can be measured using electronic photocells placed along a linear racetrack or through analysis of high-speed video. Orthologous allozymes: variants of an enzyme that are encoded by multiple alleles at a single locus. Paralogous isozymes: variants of an enzyme that are encoded by multiple loci within an individual. Proximate mechanisms: behavioral or physiological processes that underlie the expression of a phenotype by a genotype. Specialist-generalist tradeoff: a decrement in performance within one range of environmental conditions that results from an increment in performance within another range. Thermal reaction norm: a function relating environmental temperature (or body temperature) to the phenotype expressed by a given genotype. Tradeoff: a linkage between two traits that affects the relative fitness of genotypes and thereby prevents the traits from evolving independently