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.