Selection When Traits Have Different Genetic and Phenotypic Variances in Different Environments

Falconer 's concept that performance in environment 2 is a different trait from performance in environment 1 allows calculation of expected response in environment 2 if selection is from environment 1. Response to selection in environment 1 and correlated response in environment 2 depend on heri tabil i ty and phenotypic variance in environment 1, genetic covariance between performance of identical genotypes in the two environments, and selection intensity. If selection is from performance in environment 2, direct response in environment 2 and correlated response in environment 1 also can be calculated. If selection is from animals in both environments and if selected genotypes are expressed randomly in both environments, relative responses in environments 1 and 2 are weighted averages of direct and correlated responses with the weights being Pt and P2, the fractions of animals selected from environments 1 and 2. Fract ion selected from one environment determines the selection intensity factor for the direct and correlated responses in that and the other environment. Other terms determining relative responses are independent of fractions selected. A simple approach to finding the op t imum fractions, Pl and Pz, is to calculate weighted average responses for environments one and two for all combinat ions of Pl + P2 = p, a fixed fraction.