Similarly, the response to selection is defined as the difference between the offspring mean and the mean of the entire population from which their parents were selected. This equation tells us how the response to selection depends on h 2 . But, since R and S can be directly measured in artificial selection experiments, this equation also gives us another way to determine h 2 . The relation between genotype and phenotype may be non-linear, producing remarkable responses to selection. This section is not critical for basic understanding, but should be interesting.
Selection reduces the genetic variability of a character.
This section translates the basic ideas from the previous sections into a qualitative description of how selection on quantitative traits is expected to work in nature. One standard expectation is for genetic variability to be reduced by consistent directional or stabilizing selection for quantitative polygenic traits. Note that selection reduces heritability. As less fit genotypes are eliminated by selection, the variance which defines heritability is eliminated. Characters in natural populations subject to stabilizing selection show genetic variation.