In natural populations of sexually reproducing organisms, individuals differ phenotypically in a quantifiable way. Most variable phenotypes of interest (e.g., size, fertility, and longevity) can be measured on a continuous scale and are influenced by many genes and a host of environmental factors. Usually, no single gene is a major source of natural variation for a phenotype (major gene), but it is possible to assess the magnitude and nature of the multigenic causes of natural variation for that phenotype, where many genes, each with a tiny effect, act together.
The goal of quantitative genetics is to measure the relative contributions of genotype and environment to phenotypic variation in a population of organisms, breaking down phenotypic variance into genetic and environmental components. The heritability of a trait is the fraction of phenotypic variance due to genes for a particular population of organisms in a particular environment. This genetic variation can be subdivided into components, notably additive effects, dominance, and epistasis. A further component of the environment is genotype-environment interaction. Natural selection can bring about evolutionary change in a trait if and only if a population has additive genetic variation for that trait. Genes that underlie natural quantitative variation for a phenotype can be mapped genetically to quantitative trait loci (QTL).
For any quantitative phenotype, there is usually natural genetic variation. Most of the genes contributing to this variation remain anonymous, as the effects of any one of them on quantitative variation are too small to observe directly, even though the concerted effects of many genes are big enough to measure.