The importance and consequences of stress in living and fossil populations: from life-history variation to evolutionary change.

Natural populations are subjected to abiotic environmental stresses of varying intensity. Some organisms can increase their resistance to stress by a genetic reduction in metabolic rate. This relationship underlies negative associations of desiccation resistance with many life-history traits but a positive association with longevity in Drosophila melanogaster. The efficacy of stress as an environmental probe exposing such trade-offs follows from the high heritability of stress resistance in these experiments. Generalizations concerning evolutionary change come from the interaction between stress and its metabolic cost. At stressed species borders little available energy may remain beyond maintenance and survival needs so that extinctions become likely. In moderately stressed and narrowly fluctuating environments there should be sufficient genetic variability and metabolic energy to permit genetic adaptation. Phyletic gradualism is an expectation in these environments. In more stressed and widely fluctuating environments a punctuated evolutionary pattern is likely even though stasis may occur most of the time. An interpretation of evolutionary patterns therefore comes from an analysis of the interaction between stress (severity and variability), genetic variability, and energy availability, which can be summarized in an evolutionary templet.