The pattern of expression of a given phenotype across a range of values of some environmental variable. An example is the variation in yield of wheat with varying levels of nitrogen fertilizer applied to the soil. An accurate picture of a reaction norm is only attainable when using organisms of the same genotype, to exclude the influence of genetic variation. This can be accomplished in horticultural research, for example, by using clones of genetically identical plants. The reaction norms of different clones can then be compared. A typical pattern is a series of overlapping bell-shaped curves when plotting a graph of, say, grain yield versus nitrogen level. In natural populations, which are generally heterogeneous genetically, the interactions between genotype and environment are complex and make reaction norms much harder to discern. Compare phenotypic plasticity.
Subjects: Biological Sciences.