Balanced polymorphism

ByDr. Girish Chandra

Balanced Polymorphism

(By Dr. Girish Chandra)


            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

Molecular Methods in Ecology (Ecological Methods and Concepts)

Features: Wiley-Blackwell

The incorporation of molecular methods in ecological research has added an exciting new dimension to conventional studies, and opened windows into previously intractable areas of research, at the interface between ecology and genetics. Using these new methods it has now become routine to use genetic markers to study ecological phenomena, from molecular sexing of individuals and parentage of offspring, through to population structure of species and phylogenetic relationships of taxa. These methods have stimulated an explosion of empirical and analytical developments in molecular ecology, which have in turn, increasingly attracted students and professional biologists eager to employ them in their studies. Molecular Methods in Ecology traces the development of molecular ecology by reviewing basic molecular biological techniques and earlier methods such as protein electrophoresis, DNA–DNA hybridisation, restriction analysis of DNA, and DNA fingerprinting. Later chapters review methods using newer classes of markers such as microsatellites, introns, MHC, SSRs and AFLP markers in plants and molecular sexing in animals. The strengths and limitations of methods are discussed and guidance is provided in selecting the most appropriate methods for particular problems in ecology. This book will provide both postgraduates and researchers with a guide to choosing and employing appropriate methodologies for successful research in the field of molecular ecology. Provides up–to–date summaries of the latest molecular approaches in this rapidly expanding field. Gives guidance on the appropriate choice of methods for particular problems in ecology, and their strengths and limitations. Provides brief laboratory protocols for each molecular method and summaries of software available for analysis of data in molecular ecology. Outlines examples of the latest research results from studies of both plants and animals, integrated within the framework of molecular ecology.
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