Genetic variation in a population describes the existence in that population of different alleles, or alternative forms, for a given gene. The presence of genetic variation implies that individuals of the population vary in the alleles they possess, meaning that individuals differ in genotype. Genetic loci for which there are multiple alleles are described as polymorphic. Humans, for example, are polymorphic for traits such as eye color and blood type.

Genetic variation is one facet of the more general concept of phenotypic variation. Phenotypic variation describes differences in the characteristics of individuals of a population. Phenotypic variation is of interest to biologists because it is what natural selection acts upon: different pheno-types may have different fitnesses, and selection results in fitter phenotypes leaving more descendants.

Phenotypic variation arises from either of two sources: genetic variation and environmental variation. However, only...