Balancing Selection

Balancing selection refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies above that of gene mutation. This usually happens when the heterozygotes for the alleles under consideration have a higher adaptive value than the homozygote. In this way genetic polymorphism is conserved.

There are three main types of natural selection: In directional selection the allele frequency for a trait continuously shifts in one direction. In stabilizing selection the frequency of the alleles of lower fitness decreases until they vanish. Balancing selection is similar but not identical to disruptive selection where individuals of extreme trait values are favored against those with average trait values. These terms are used for quantitative characters controlled by a number of genes.

Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is ubiquitous in panmictic populations. It is a genetic expression of the field experience of Darwin, Wallace and others, that natural populations in the wild are extraordinarily varied (though not all such variation is of genetic origin).

There are several mechanisms (which are not exclusive within any given population) by which balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection.