Is hormesis an evolutionary expectation?

1. This paper approaches the phenomenon of hormesis (i.e. stimulatory effec ts occurring in response to low levels of exposure to agents that are harmf ul at high levels of exposure) from an evolutionary perspective and address es three questions related to its occurrence and consequences: (1) Is the o ccurrence 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 implic ations 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 va rious fitness-related traits. 3. To avoid statistical artefacts, it is essential that the underlying dist ribution of traits that appear to show hormesis be examined, particularly a s many of them may be expected to deviate from normality. 4. The occurrence of hormesis of individual life-history traits can be expl ained as an evolutionary adaptation that acts to maintain fitness in a chan ging environment. 5. As a result of energetic trade-offs among life-history traits, not all t raits are likely to exhibit hormesis simultaneously, and therefore overall fitness is not likely to be enhanced at low levels of exposure to toxic age nts. Because toxic agents affect different traits in different directions a nd to different degrees, interpreting the ecological consequences of hormes is of any single trait is not possible without examining it in relation to overall effects on fitness.