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Light-morph Jaguar (typical)
Dark-morph or melanistic Jaguar (about 6% of the South American population)
The adder Viperus berus: normal and melanistic colour patterns

Polymorphism [1] in biology is when two or more clearly different phenotypes exist in the same population of a species.[2] The words forms or morphs are sometimes used.[3]

Polymorphism is common in nature. The most common example is sexual dimorphism, which occurs in many organisms. Another example is sickle-cell anaemia.

In order to be classified as such, morphs must occupy the same habitat at the same time and belong to a population with random mating.[4]

The switch[change | change source]

The mechanism which decides which of several morphs an individual displays is called the switch. This switch may be genetic, or it may be environmental. Taking sex determination as the example, in man the determination is genetic, by the XY sex determination system. In Hymenoptera (ants, bees and wasps), sex determination is by haplo-diploidy: the females are all diploid, the males are haploid.



Genetic polymorphism[change | change source]


  • Genetic polymorphism: two or more distinct forms at the same time and place. The proportion of the rarest form must be above mutation rate (and so it is supported by selection of some kind).[4]:11[5]


Genetic polymorphism is actively and steadily maintained in populations by natural selection. This contrasts with transient polymorphisms where a form is progressively replaced by another.[6]:6-7


Balancing selection[change | change source]


Examples[change | change source]

Humans[change | change source]

Sickle-cell anaemia[change | change source]


An individual homozygous for the recessive sickle haemoglobin, HgbS, has a short expectancy of life.[8] The life expectancy of the standard haemoglobin (HgbA) homozygote and also the heterozygote is normal (though heterozygote individuals will suffer periodic problems).


This is heterozygote advantage, a kind of balance between fierce selection against homozygous sickle-cell sufferers, and selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) so long as malaria exists; and it has existed as a human parasite for a long time. Because the heterozygote survives, so does the HgbS allele survive at a rate much higher than the mutation rate.[9][10]

Lactase persistence[change | change source]

Mammals normally produce lactase only as long as the mother has milk. Then the enzyme lactase is cut off. Modern humans are different.

The human ability to drink milk during adult life is supported by a lactase mutation. Human populations have a high proportion of this mutation wherever milk is important to the diet. The spread of milk tolerance is promoted by natural selection: it helps people survive where milk is available.

Genetic studies suggest that the oldest mutations associated with lactase persistence only reached appreciable levels in human populations in the last ten thousand years.[11][12] Therefore, lactase persistence is often cited as an example of recent human evolution.[13][14] As lactase persistence is genetic, but animal husbandry a cultural trait, this is geneculture coevolution.[15]

Ants[change | change source]


Second, there is differentiation based mostly on feeding of larvae. This determines, for example, whether the adult is capable of reproduction.


Heterostyly[change | change source]

Dissection of thrum and pin flowers of Primula vulgaris


Pin and thrum heterostyly occurs in dimorphic species of Primula, such as P. vulgaris. There are two types of flower. The pin flower has a long style bearing the stigma at the mouth and the stamens half-way down; and the thrum flower has a short style, so the stigma is half-way up the tube and the stamens are at the mouth.


Another most important property of the heterostyly system is physiological. If thrum pollen is placed on a thrum stigma, or pin pollen on a pin stigma, the reproductive cells are incompatible and relatively little seed is set. Effectively, this ensures outcrossing, as described by Darwin. Quite a lot is now known about the underlying genetics; the system is controlled by a set of closely linked genes;[20] these act as a single unit, a so-called super-gene.[2]ch10[7]p86[21]

All sections of the genus Primula have heterostyle species, altogether 354 species out of 419.[22] Since heterostyly is characteristic of nearly all races or species, the system is at least as old as the genus.[23]

Between 1861 and 1863, Darwin found the same kind of structure in other groups, such as flax (Linum) and in Purple Loosestrife and other species of Lythrum.[24]

Heterostyly is known in at least 51 genera of 18 families of Angiosperms.[25][26]

Drosophila[change | change source]

Studies over many years have shown that natural populations of Drosophila are polymorphic for chromosome inversions.[27] The inversions are so common that they must be kept in the population by natural selection.[28][29]

Related pages[change | change source]

References[change | change source]

  1. Greek: πολύ = many, and μορφή = form, figure, silhouette
  2. 2.0 2.1 2.2 2.3 Ford E.B. 1975. Ecological genetics. 4th ed, London: Chapman & Hall.
  4. 4.0 4.1 4.2 4.3 Ford E.B. 1965. Genetic polymorphism. Faber & Faber, London.
  5. Ford E.B. 1940. "Polymorphism and taxonomy". In Julian Huxley (ed) The New Systematics. Oxford: Clarendon Press, 493–513. ISBN 1-930723-72-5
  6. 6.0 6.1 Begon, Townsend, Harper. 2006. Ecology: from individuals to ecosystems. 4th ed, Blackwell, Oxford. ISBN 1-4051-1117-8
  7. 7.0 7.1 7.2 Maynard Smith, John 1998. Evolutionary genetics. 2nd ed, Oxford: Oxford U. Pr.
  8. Platt OS, Brambilla DJ, Rosse WF, et al. (June 1994). "Mortality in sickle cell disease: life expectancy and risk factors for early death". N. Engl. J. Med. 330 (23): 1639–44. doi:10.1056/NEJM199406093302303 . ISSN 0028-4793 . PMID 7993409 . http://content.nejm.org/cgi/content/full/330/23/1639.
  9. Allison A.C. 1956. The sickle-cell and Haemoglobin C genes in some African populations. Ann. Human Genet. 21, 67-89.
  10. Ford E.B. 1973 [1942]. Genetics for medical students. 7th ed, London: Chapman & Hall.
  14. Swaminathan, N. 2006. African adaptation to digesting milk is "strongest signal of selection ever". Scientific American December 11, 2006.
  16. Wilson E.O. 1953. The origin and evolution of polymorphism in ants. Quarterly Review of Biology, 28(2):136–156. DOI:10.1086/399512.
  17. Rossa K.G.; Kriegera M.J. B.; Shoemaker D.D. 2003. Alternative genetic foundations for a key social polymorphism in Fire Ants. Genetics, 165:1853–1867.
  18. Darwin, Charles 1862. On the two forms, or dimorphic condition, in the species of Primula, and on their remarkable sexual relations. Journal of the Proceedings of the Linnaean Society (Botany) 6, 77–96.
  20. 'closely linked' means close on the same chromosome.
  21. Sheppard, Philip M. 1975. Natural selection and heredity. 4th ed, London: Hutchinson.
  22. Bruun H.G. 1938. Studies on heterostyle plants 2. Svensk. Bot. Tidskr. 32, 249-260.
  24. Darwin, Charles. 1977 (collection). Barrett P.H. (ed) The collected papers of Charles Darwin. Chicago: Chicago University Press.
  26. Charlesworth B & D. 1979. The evolutionary genetics of sexual systems in flowering plants. Proc Royal Soc B 205, 513-30.
  27. When a part of a chromosome gets reversed end to end, so the genes run in the opposite direction to before.
  28. Dobzhansky, Theodosius 1981. Dobzhansky's Genetics of natural populations. Lewontin R.C.; Moore J.A.; Provine W.B.; Wallace B. (eds) New York: Columbia University Press.
  29. Krimbas C.B. & Powell J.R. (eds) 1992. Drosophila inversion polymorphism. CRC Press, Boca Raton, FL.