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Evolutionary physiology is the emerging physiological discipline. Unlike environmental physiology or ecophysiology, whose definitions have long been made quite clear, evolutionary physiology has a broader scope of objectives, and its definition lacks a concise treatise. This paper presents the argument that the lack of a common definition of evolutionary physiology is retarding the unification of the mechanistic and amechanistic physiological sciences, a multidisciplinary obligation crucial for a holistic understanding of a physiological basis of fitness. The divide between mechanistic "how" questions, devoted primarily to homeostasis, and evolutionary "why" questions, concerned with understanding phenotypic and genotypic physiological variation, remains broad and is currently not conducive to synergy in the physiological disciplines. Unification may be facilitated, however, by embracing a common currency of measurement and analysis. A likely candidate is the cascade of energy from the environment to offspring and the evolution of physiological form and function, including homeostasis, associated with power management. This currency approach seeks to identify an energetic basis of fitness, namely, whether or how the evolution of life-history traits is influenced by energetic constraints and/or trade-offs.

Animals occupy an impressive range of environments, and many approach the limits of conditions that are compatible with life. Such environments are often characterized by low productivity, and survival depends on abilities to maintain levels of hydration sufficient for biochemical reactions and an adequate level of energy turnover. Specific adaptations for acquiring and conserving water affect the intake and utilization of energy that is available for growth and reproduction. Almost any benign, historically stable environment can become hostile if conditions change, and the severity of challenges to animals depends on temporal as well as spatial factors that are related to the intensity and duration of harsh conditions (e.g., drought). Animals have responded to such changes through avoidance, phenotypically plastic adjustments, and evolutionary changes in genomic structure. Here we present an overview of research findings presented at a session of the Third International Conference of Comparative Physiology and Biochemistry. We hope such a synthesis will stimulate new directions in research that address biological responses to environmental change. In this context, we believe that challenging environments in Africa might provide increasingly useful sites for biological investigations relating to the application of comparative and evolutionary approaches toward a deeper understanding of physiological adaptation.