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 the 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 that can be assessed only by measuring the genotype's fitness in more than one environment. The genotype's tolerance is characterized by the shape of the fitness reaction norm along the environmental gradient whereas the overall height of the function represents its general vigor.