Figure 1. An illustration of the principles of the breeder's equation (equation (6) in the text). The x- and y-axis represent the measured values of a phenotypic character under selection. Parents with a phenotypic value >90 are selected (blue circles) to produce the next generation of offspring. Individuals with a phenotypic value <90 (red triangles) do not contribute genes to the subsequent generation. The selection differential (S) is the difference in the mean value of all the parents in the population (where the solid line crosses the x-axis) and the mean value of the selected parents (where the dashed line crosses the x-axis). The response to selection (R) is the difference between the mean phenotype of all the parents (where the solid line crosses the y-axis) and the mean phenotype of all the offspring from selected parents (where the dashed line crosses the y-axis).

Figure 2. An example of genotype by environment interaction (E). The plot represents the value of a phenotypic character measured in three different environments (A, B and C). Three genotypes are presented. For example, Genotype 1 produces a phenotype with the lowest value in Environment A but the highest value in Environment C. There is significant genetic variation for the characters in Environments A and C, but no genetically based variation in Environment B.