The stress response systems: Universality and adaptive individual differences

Biological reactivity to psychological stressors comprises a complex, integrated system of central neural and peripheral neuroendocrine responses designed to prepare the organism for challenge or threat. Developmental experience plays a role, along with heritable variation, in calibrating the response dynamics of this system. This calibration occurs through setting of response thresholds in the regulatory mechanisms that coordinate the expression and use of trait-specific gene products and environmental elements that build alternative phenotypes. Whereas natural selection tends to favor developmental plasticity when the fitness of alternative phenotypes can be predicted from observable cues, genetic polymorphisms are most likely to be maintained when the advantages of niche specialization are high and organisms can evaluate and select their niches. Well-developed theories of both adaptive phenotypic plasticity and adaptive genetic variation in the stress-response systems have been advanced in the literature. Taken together, these theories strongly suggest that variation in stress-response phenotypes has been shaped by natural selection, is an adaptation to multiniche environments, and involves an integration of genetic influences and condition-sensitivity.