Many traits, such as height, body size, and physiological tolerance, are influenced by many genes and the environment. Quantitative genetics studies how these traits vary continuously and respond to selection.

A simple model for response to selection is $$R = h^2 S$$, where $R$ is the response, $h^2$ is narrow-sense heritability, and $S$ is the selection differential. This equation shows that strong selection produces little evolutionary change if heritability is low.

When the effect of a genotype depends on the environment, the same genotype can perform differently in different conditions. This is especially important in agriculture, medicine, and climate-change biology.