Balanced polymorphism was first defined by E. B. Ford (1964) in relation with populations of melanic moth (Biston betularia) in England. Natural Selection forces tend to maintain two or more alleles of a gene in appreciable frequencies in a population in a mosaic environment. When none of the alleles is eliminated and all genetic variants are maintained in a population in more or less stable equilibrium for longer duration, the phenomenon is called Balanced Polymorphism, which is created by balancing selection.
Variants have neutral or equal fitness and loss of deleterious recessive genes by way of death of homozygotes is balanced by the gain resulting from large number of offspring produced by the favoured heterozygotes. Heterozygote superiority or Heterosis, therefore, promotes balancing selection and deleterious mutation cannot be completely eliminated because fitness of heterozygotes is greater than homozygote non-carriers.
Sickle cell anaemia is an abnormality of haemoglobin first reported by an American physician, James B. Herrick (1949) in Africa. In sufferers RBCs become crescentic under oxygen deficiency conditions and clog blood capillaries causing death. The incidence of sickle cell anaemia is more widespread in regions malaria caused by Plasmodium falciparum and earlier it was believed that it was the only well-established case of balanced polymorphism. When forests were cleared to provide land for agriculture in Africa, it produced more breeding areas for mosquito, Anopheles gambiae, and caused spread of malaria.
In this changed situation heterozygotes having both normal and sickle genes got selective advantage as their sickled RBCs could not support schizogony of malarial parasite but still carried enough oxygen under hypoxia conditions through normal RBCs. Dominant homozygotes suffered from malaria and died while recessive homozygotes carrying sickle gene died before reaching sexual maturity because of the inability of blood to carry enough oxygen and clogging of blood capillaries by sickled cells. Therefore, about 25% superiority of heterozygotes prevented elimination of the sickle gene and its frequency increased in malaria prone areas.
The outcome of natural selection will therefore be a balanced equilibrium, conserving the polymorphism in the population.
One of the first instances of balanced polymorphism was studies in industrial melanic moth in England by R.A. Fisher (1929), E.B. Ford (1940) and later by H.B.D. Kettlewell (1956). Before industrialisation, grey coloured carbonaria form was found in England. First black melanic form was collected in 1848. As the industrial pollution killed lichens covering tree trunks exposing the dark brown bark underneath, melanic forms were favoured by natural selection. By 1895, 98% of the moths became melanic in the industrial areas but light coloured carbonaria form continued to flourish in countryside. Now there are over a dozen forms of different shades being maintained in England by balancing selection