Defining tolerance as a norm of reaction

Tolerance of an environmental factor is the ability to maintain fitness in the face of stress imposed by that factor. A tolerant genotype minimizes th e decline in fitness from that achieved in a relatively benign environment to that produced in environments with more stressful levels of the factor. Hence, tolerance is a phenotypically plastic characteristic of a genotype t hat can be assessed only by measuring the genotype's fitness in more than o ne environment. The genotype's tolerance is characterized by the shape of t he fitness reaction norm along the environmental gradient whereas the overa ll height of the function represents its general vigor.