polymorphism

Some traits come in just one flavor across a species; others come in several common varieties at once. Polymorphism is the second situation: when a population contains two or more reasonably common forms of a gene, a trait, or a stretch of DNA, rather than a single uniform version.

In genetic terms, a locus is polymorphic when it has two or more alleles each present above some threshold of frequency in the population (a common convention is at least 1% for the rarer allele). The variation can show up as different phenotypes, like the ABO blood groups, or invisibly at the DNA level, such as a single-nucleotide polymorphism.

Polymorphisms are the raw material that population genetics measures and that selection and drift act upon. They are also the workhorses of genetics in practice: DNA polymorphisms serve as markers for mapping genes, identifying individuals, and tracing ancestry and population history.

Not every polymorphism is neutral. Some persist because balancing selection actively keeps multiple alleles in the population; others are simply variation that drift has not yet removed. Distinguishing the two is a recurring puzzle in the field.