To an evolutionary geneticist, the most important property of a new mutation is its effect on fitness. Stress is a reduction in fitness that can also alter the selection on new mutations. Although the effects of environmental and genetic stresses are typically studied separately, it is useful to consider them from the same perspective. Here we evaluate the common perception that stress increases selection. We consider various conceptual paradigms for thinking about selection and stress, and then review the empirical data. We reject the notion that stress typically increases selection. Instead, we find that different types of stresses affect selection differently, though the underlying mechanisms are, as yet, unclear in most cases.
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