Selection depends on frequency when the adequacy of a phenotype, genotype or gene (or species) varies with its relative abundance in the population (or community) and therefore cannot be detected only when measured at two or more frequencies. In “frequency dependent negative selection”, physical form decreases with frequency and therefore rare genotypes or, in some cases, species, could be kept at a stable equilibrium. As a form of balanced selection, this mechanism could be an explanation for the persistence of higher levels of genetic variation than expected in natural populations, whether measured at the level of molecules, morphology or behavior. In "frequency dependent positive selection", physical fitness increases with frequency so that rare genotypes are eliminated and local genetic diversity reduced.