There is also the complete sequence of environments in which the organism develops. Genetics is often studied as if genes, all by themselves, determined traits. Descriptions of Mendelian genetics tend to concentrate on traits that vary sharply with genotype but are relatively unaffected by ordinary environmental variation. So much of biological organization is so highly canalized, we often forget how much the environment necessarily contributes to each and every phenotypic trait. However, in our consideration of the quantitative genetics of quantitative traits, we must pay attention to the fact that phenotypic variation is influenced by both genetic variation and by environmental variation.
Selection acts on variation, but only on that portion of variation which is heritable.
There are several issues of major significance associated with quantitative genetics . Quantitative genetics deals only with variance , with the quantitative differences among individuals which appear when genotypes and environments vary. Highly canalized traits which show no variation among individuals are not the subject of quantitative genetics. Most selection does not act on genotypes, but on phenotypes. All selection coefficients are thus dependent on many factors besides the genotype itself.