The Problem of Environment and Selection

Situations involving an interaction between genotype and environment may be treated by the methods of genetic correlation, if only two different environments are considered. Formulation of the genotype-environment interaction in terms of a genetic correlation leads easily to a solution of problems connected with selection. In this way a precise answer can be given to the question whether it is better to carry out selection in the environment in which the improved breed is required eventually to live, or in some other environment more favorable to the expression of the desired character. Performance in the two environments is regarded as two different characters which are genetically correlated. Selection for one character will then bring about a correlated response of the other character. The magnitude of this correlated response may then be compared with that of the direct response to selection for the desired character itself. The ratio of the correlated to the direct response may be expressed in a simple formula involving the square roots of the two heritabilities and the genetic correlation. It is possible for the correlated response to be greater than the direct response, but it seems probable that this will seldom happen. The expectation of a great increase of heritability would be the only justification for favoring selection in an environment other than the one in which the improved breed is required to live.