Polymorphism is a condition in which a population possesses more than one allele at a locus. Sometimes it is defined as the condition of having more than one allele with a frequency of over 5% in the population.

There may be several causes of polymorphism:

• polymorphism can be maintained by a balance between variation created by new mutations and natural selection (see mutational load).

• genetic variation may be caused by frequency-dependent selection.

• multiple niche polymorphism exists when different genotypes should have different fitnesses in different niches.

• heterozygous advantage may maintain alleles which would otherwise be selected against.

• if selection is operating, migration can introduce polymorphism into a population.

These are all sources of polymorphism which make use of the mechanisms of natural selection. Genetic drift is also a possible source of genetic variation.

The Heliconius erato butterfly is a species with a high degree of polymorphism for genes encoding wing color.