Balanced polymorphism is a situation in which two different versions of a gene are maintained in a population of organisms because individuals carrying both versions are better able to survive than those who have two copies of either version alone. ... Different versions of a gene are called alleles.

Disruptive selection occurs when extreme phenotypes have a fitness advantage over more intermediate phenotypes. ... This can lead to increased phenotypic variation while disruptive selection itself is diminished or eliminated.

A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. The specific case of heterozygote advantage due to a single locus is known as overdominance.

Directional selection occurs when individuals homozygous for one allele have a fitness greater than individuals with other genotypes and individuals homozygous for the other allele that have a fitness less than individuals with other genotypes.

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