A trait in which the phenotypic expression of the genotype has been shaped by natural selection such that environmental variation triggers the production of different adaptive phenotypes well-suited to that environment — i.e., a trait with adaptive phenotypic plasticity. For example, melanin level is a facultative trait. Humans that are exposed to higher levels of solar radiation produce more melanin, which provides protection from the sun. This response likely evolved through many generations of natural selection. For more details, see our resource on genotype vs. phenotype in Evolution 101.
→ → facultative trait