A trait in which the phenotypic expression of the genotype is not adaptive (i.e., the norm of reaction has not been shaped by natural selection). Obligate traits are those that are not facultative. Obligate traits rarely change once they have reached their adult form and generally produce similar phenotypes across a range of “normal” environments. For example, height, eye color, and blood type are all obligate traits. For more details, see our resource on genotype vs. phenotype in Evolution 101.