Is hormesis an evolutionary expectation?

1. This paper approaches the phenomenon of hormesis (i.e. stimulatory effects occurring in response to low levels of exposure to agents that are harmful at high levels of exposure) from an evolutionary perspective and addresses three questions related to its occurrence and consequences: (1) Is the occurrence of hormesis to be expected on the basis of evolutionary arguments? (2) Considering selection as a driving force in the evolution of hormesis, is it likely that certain aspects of organism performance have a greater tendency than others to exhibit hormesis? (3) What are the practical implications of hormesis for ecological risk assessment? 2. Several hypotheses are presented to explain the observations of hormesis, and a literature review is used to assess the evidence for hormesis of various fitness-related traits. 3. To avoid statistical artefacts, it is essential that the underlying distribution of traits that appear to show hormesis be examined, particularly as many of them may be expected to deviate from normality. 4. The occurrence of hormesis of individual life-history traits can be explained as an evolutionary adaptation that acts to maintain fitness in a changing environment. 5. As a result of energetic trade-offs among life-history traits, not all traits are likely to exhibit hormesis simultaneously, and therefore overall fitness is not likely to be enhanced at low levels of exposure to toxic agents. Because toxic agents affect different traits in different directions and to different degrees, interpreting the ecological consequences of hormesis of any single trait is not possible without examining it in relation to overall effects on fitness.