Towards an Evolutionary Theory of Stress Responses.

All organisms have a stress response system to cope with environmental threats, yet its precise form varies hugely within and across individuals, populations, and species. While the physiological mechanisms are increasingly understood, how stress responses have evolved remains elusive. Here, we show that important insights can be gained from models that incorporate physiological mechanisms within an evolutionary optimality analysis (the 'evo-mecho' approach). Our approach reveals environmental predictability and physiological constraints as key factors shaping stress response evolution, generating testable predictions about variation across species and contexts. We call for an integrated research programme combining theory, experimental evolution, and comparative analysis to advance scientific understanding of how this core physiological system has evolved. Extensive experimental and comparative studies provide a solid understanding of the physiological basis of the stress response system and its variation across and within species. However, lagging behind this is a formal theoretical framework to help unify the wealth of existing verbal hypotheses linking stress response mechanisms and fitness, and to explain how such a response system has evolved. We propose an evo-mecho approach, combining optimality models and evolutionary simulations with empirical evidence about the underlying physiology, to show how mechanistic constraints and the predictability of environmental risks shape the stress response. A deeper understanding of stress response evolution will require mechanistically informed evolutionary models, phylogenetically controlled comparative analyses, and experimental evolution studies. [ABSTRACT FROM AUTHOR]