, the frequency of which selection is unable to efficiently modify.

Note that by this definition whether or not an allele may described as neutral is a function not just of the fitness associated with that allele but also of . This occurs because the efficiency of selection is dependent on both fitness and the strength of genetic drift relative to that of selection.

All alleles for (or against) which drift is stronger than selection may be described as neutral.

See also balanced polymorphism, neutral variation, , and neutral mutation as well as beneficial allele and deleterious allele.