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.
Heritability depends on a particular mix of genetic combinations and environments. Measurement of heritability, or of genetic and environmental components for phenotypic traits, gives a description within particular circumstances, not a causal explanation. Heritability is not a fixed property of a trait. Heritability may change if the environment changes or if the gene pool is altered. Heritability can be a counter-intuitive concept.