Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. Balancing selection is in contrast to irectional selection which favors a single allele. A balanced polymorphism is a situation in which balancing selection within a population is able to maintain stable frequencies of two or more phenotypic forms. Evidence for balancing selection can be found by increased levels of genetic variation between alleles or haplotypes in a species. Note that balancing selection will not always result in an observable phenotypic difference because the genotype may not be one-to-one with the phenotype.