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In biology, 'polymorphism' (from Greek: ''poly'' 'many', ''morph'' 'form') is defined as discontinuous^[1] variation in a single population Ecological Genetics, , E. B., Ford, Chapman & Hall, 1964, —in other words, the occurrence of more than one form or ''morph''. The morphs are visible or, if cryptic, are made visible by a test.
Polymorphism is extremely common; it retains variety of form in a population which lives in a varied environment. The most common example of polymorphism is the sexual dimorphism of most higher organisms; this retains diversity by the process of genetic recombination. Other examples are mimetic forms of butterflies (see mimicry) and certain cryptic forms of moths, the banding pattern on snail shells, human blood groups and many other cases.
''All polymorphism is heritable'', and so may be both caused and influenced by selection (either artificial or in the wild). In polyphenism, an individual's genetic make-up allows for different morphs, and the switch mechanism that determines which morph is shown is environmental. In genetic polymorphism or balanced polymorphism, the genetic make-up determines the morph.
Polymorphism as described here involves morphs of the phenotype. The term is also used by molecular biologists to describe certain point mutations in the genotype, such as SNPs and RFLPs. This usage is not discussed in this article.

Contents

What polymorphism is not

Nomenclature

Ecology

The switch

Investigative methods

Genetic polymorphism

Sexual dimorphism

Other human polymorphisms

The Cuckoo

Other examples

See also

References

External links

What polymorphism is not

★ The term omits continuous variation (such as weight) even though this has a heritable component. Polymorphism deals with forms in which the variation is discrete (discontinuous) or strongly bimodal or polymodal.

★ Morphs must occupy the same habitat at the same time: this excludes geographical races and seasonal forms.^[2]

★ Rare variations are not classified as polymorphisms; and mutations by themselves do not constitute polymorphisms. To qualify as a polymorphism there has to be some kind of balance between morphs underpinned by inheritance. The criterion is that the frequency of the ''least'' common morph is too high simply to be the result of new mutations The New Systematics, , E.B., Ford, Clarendon Press, 1940, or, as a rough guide, that it is greater than 1 percent (though that is far higher than any normal mutation rate for a single allele).^[3]

Nomenclature

In zoology, the different forms can be called morphs or morphotypes; trait (biology) and characters are also possible descriptions, though they imply just a limited aspect of the body. ''Morph'' or ''form'' is better, since many of the examples greatly change the appearance of an individual (e.g. sex; mimicry).
The word "morpha" plus a Latin name for the morph can be added to a binomial or trinomial name. However, morphs have no formal standing in the ICZN, and the practice invites confusion with various kinds of geographic variation.
In botany, morphs may be named with the terms "variety", "subvariety", and "forms" which are formally regulated by the ICBN. Again, the practice invites confusion with geographic variation, especially the term ''variety''.

Ecology

Selection, whether natural or artificial, changes the frequency of morphs within a population; this occurs when morphs reproduce with different degrees of success. A genetic (or ''balanced'') polymorphism usually persists over many generations, maintained by two or more opposed and powerful selection pressures. Diver (1929) found banding in ''Cepaea nemoralis'' could be seen in pre-fossil shells going back to the Upper Pleistocene.^[4]
The relative proportions of the morphs may vary; the actual values are determined by the effective fitness of the morphs at a particular time and place. The mechanism of heterozygote advantage assures the population of some alternative alleles at the locus or loci involved. Only if competing selection disappears will an allele disappear. However, heterozygote advantage is not the only way a polymorphism can be maintained. Apostatic selection, whereby a predator consumes a common morph whilst overlooking rarer morphs is possible and does occur. This would tend to preserve rarer morphs from extinction.
A polymorphic population does not initiate speciation; it has little or nothing to do with species splitting. However, ''it has a lot to do with the adaptation of a species to its environment'', which may vary in colour, food supply, predation and in many other ways. Polymorphism is one good way the opportunities get to be used; it has survival value, and the selection of modifier genes may reinforce the polymorphism.

The switch

The decision 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. However, in some animals an environmental trigger determines the sex: alligators are a famous case in point. In ants the distinction between workers and guards is environmental, by the feeding of the grubs. Polymorphism with an environmental trigger is called polyphenism.
The polyphenic system does have a degree of environmental flexibility not present in the genetic polymorphism. However, such environmental triggers are the less common of the two methods.

Investigative methods

Investigation of polymorphism requires a coming together of field and laboratory technique. In the field:

★ detailed survey of occurrence, habits and predation

★ estimation of population sizes

★ relative numbers and distribution of morphs

★ capture, mark, release, recapture data.
And in the laboratory:

★ genetic data from crosses

★ population cages

★ chromosome cytology if possible

★ use of chromatography or similar techniques if morphs are biochemical in nature
Both types of work are equally important. Without proper field-work the significance of the polymorphism to the species is uncertain; without laboratory breeding the genetic basis is obscure. Even with insects the work may take many years; examples of Batesian mimicry noted in the nineteenth century are still being researched.

Genetic polymorphism

Since all polymorphism has a genetic basis, ''genetic polymorphism'' has a particular meaning:

★ Genetic polymorphism is the occurrence together in the same locality of two or more discontinuous forms of a species in such proportions that the rarest of them cannot be maintained just by recurrent mutation The New Systematics, , E. B., Ford, Clarendon Press, 1940, . It is sometimes called balancing selection, and is intimately connected with the idea of heterozygote advantage.
The definition has three parts: a) sympatry: one interbreeding population b) discrete forms, and c) not maintained just by mutation.

Sexual dimorphism

We meet genetic polymorphism daily, since our species (like most other eukaryotes) uses sexual reproduction, and of course, the sexes are differentiated. The system is relatively stable (with about half of the population of each sex) and heritable by means of sex chromosomes. Every aspect of this everyday phenomenon bristles with questions for the theoretical biologist. Why is the ratio ~50/50? How could it arise from an original situation of asexual reproduction, which has the advantage that every member of a species could reproduce? Why the visible differences? These questions have engaged the attentions of biologists such as Ronald Fisher, John Maynard Smith and W.D. Hamilton, with some success. ^{[5] [6] [7]}

Other human polymorphisms

There are a large number of less spectacular examples of human genetic polymorphisms. All the common blood types, such as the ABO system, are genetic polymorphisms. Here we see a system where there are more than two morphs: the phenotypes are A, B, AB and O are present in all human populations, but vary in proportion in different parts of the world. The phenotypes are controlled by multiple alleles at one locus. These polymorphisms are seemingly never eliminated by natural selection; why should this be?
Statistical research has shown that the various phenotypes are more, or less, likely to suffer a variety of diseases. For example, an individual's susceptibility to cholera (and other diarrheal infections) is correlated with their blood type: those with type O blood are the most susceptible, while those with type AB are the most resistant. Between these two extremes are the A and B blood types, with type A being more resistant than type B. This suggests that the pleiotropic effects of the genes set up opposing selective forces, thus maintaining a balance. ^{[8] [9]}
Such a balance is seen more simply in sickle-cell anaemia, which is found mostly in tropical populations in Africa and India. An individual homozygous for the recessive sickle haemoglobin, HgbS, has a short expectancy of life, whereas the life expectancy of the standard haemoglobin (HgbA) homozygote and also the heterozygote is normal (though heterozygote individuals will suffer periodic problems).
So why does the sickle-cell variant survive in the population? ''Because the heterozygote is resistant to malaria and the malarial parasite kills a huge number of people each year.'' This is balancing selection or genetic polymorphism, balanced 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. ^[10] and refs in Sickle-cell disease. Other human polymorphisms are discussed in Ford (1942, 7th ed 1973)^[11]

The Cuckoo

Over fifty species in this family of birds practise brood parasitism; the details are best studied in the British or European Cuckoo (''Cuculus canorus''). The female lays 15-20 eggs in a season, but only one in each nest of another bird. She removes some or all of the host's clutch of eggs, and lays an egg which closely matches the host eggs. Although, in Britain, the hosts are always smaller than the Cuckoo itself, the eggs she lays are small, and coloured to match the host clutch but thick-shelled. This latter is a defence which protects the egg if the host detects the fraud.
The intruded egg develops exceptionally quickly; when the newly-hatched Cuckoo is only ten hours old, and still blind, it exhibits an urge to eject the other eggs or nestlings. It rolls them into a special depression on its back and heaves them out of the nest. The Cuckoo nestling is apparently able to pressure the host adults for feeding by mimicking the cries of the host nestlings. The diversity of the Cuckoo's eggs is extraordinary, the forms resembling those of its most usual hosts. In Britain these are:

★ Meadow Pipit (''Anthus pratensis''): brown eggs speckled with darker brown.

★ Robin (''Erithacus rubecula''): whitish-grey eggs speckled with bright red.

★ Reed warbler (''Acrocephalus scirpensis''): light dull green eggs blotched with olive.

★ Redstart (''Phoenicurus phoenicurus''): clear blue eggs.

★ Hedge Sparrow (''Prunella modularis''): clear blue eggs, unmarked, not mimicked. This bird is an uncritical fosterer; it tolerates in its nest eggs that do not resemble its own.
Each female Cuckoo lays one type only; the same type laid by her mother. In this way female Cuckoos are divided into groups (known as gentes), each parasitises the host to which it is adapted. The male Cuckoo has its own territory, and mates with females from any gente; thus the population (all gentes) is interbreeding. In birds the mechanism of sex determination is XX/XY, but unlike mammals, the XY is the female. The determining gene (or super-gene) is carried on the Y chromosome, and is apparently backed up by others on the autosomes which act only in the presence of the controller.
Ecologically, the system of multiple hosts protects host species from a critical reduction in numbers, and maximises the egg-laying capacity of the population of Cuckoos. There are some other advantages, too: it extends the range of habitats where the Cuckoo eggs may be raised successfully. Detailed work on the Cuckoo started with Chance in 1922^{[12] [13] [14]} and research continues to the present day.

Peppered Moth

The Peppered Moth, Biston betularia, is justly famous as an example of a population responding in a heritable way to a significant change in their ecological circumstances.
Although the moths are cryptically camouflaged and rest during the day in unexposed positions on trees, they are predated by birds hunting by sight. The original camouflage (or crypsis) seems near-perfect against a background of lichen growing on trees. The sudden growth of industrial pollution in the nineteenth century changed the effectiveness of the moths' camouflage: the trees became blackened by soot, and the lichen died off. In 1848 a dark version of this moth was found in the Manchester area. By 1895 98% of the Peppered Moths in this area were black (see photos at Peppered moth). This was a rapid change for a species that has only one generation a year.
In Europe, there are three morphs: the typical white morph (''betularia''), and ''carbonaria'', the melanic black morph. They are controlled by alleles at one locus, with the carbonaria being dominant. There is also an intermediate or semi-melanic morph ''medionigra'', controlled by other alleles (see Majerus 1998)^[15].
A key fact, not realised initially, is the advantage of the heterozygotes, which survive better than either of the homozygotes. This affects the caterpillars as well as the moths, in spite of the caterpillars being monomorphic in appearance (they are twig mimics). In practice heterozygote advantage puts a limit to the effect of selection, since neither homozygote can reach 100% of the population. For this reason, it is likely that the carbonaria allele was in the population originally, pre-industrialisation, at a low level. With the recent reduction in pollution, the balance between the forms has already shifted back significantly.
This type of industrial melanism has only affected such moths as obtain protection from insect-eating birds by resting on trees where they are concealed by an accurate resemblance to their background (about 200 examples are known). No species which hide during the day, for instance, among dead leaves, is affected, nor has the melanic change been observed among butterflies. ^{[16] [17] [15]}
This is, as advertised, 'evolution in action', though perhaps what it really illustrates is the benefits of sexual reproduction, which keeps the variability of a population much higher than would be the case with asexual reproduction.

Other examples

An example of genetic polymorphism from botany is heterostyly, in which flowers occur in different forms with different arrangements of the pistil and the stamens. Pin and thrum heterostyly in ''Primula vulgaris'' is an example, where there are two types of flower. The pin flower has a long style bearing the stigma at the mouth and the anthers half-way down, and the thrum has a short style, so the stigma is half-way up the tube and the anthers are at the mouth. Effectively, this ensures cross-breeding between the two forms as described by Darwin in 1877.^[19] Quite a lot is now known about the underlying genetics; the system is controlled by a set of closely linked genes which act as a single unit, a super-gene.^[20][21]
Polymorphisms in the scarlet tiger moth and in the African swallowtail ''Papilio dardanus'' (in which one of the eight races has 13 morphs)^[22] have been the subjects of considerable ecological and genetic study.
The predatory mosquito ''Chaoborus americanus'' has two larval morphs, one large, yellow, and quick to develop; the other small, pale, and slow to develop. The larvae overwinter, and the polymorphism is an adaptation to unpredictable spring weather. The fast-developing larvae do well in warm springs, but die off in springtime freezes, whereas the slow-developing ones do well in springs with freezes. Many other insects with polymorphisms in development and dormancy are known.^[23]
A possible example of behavioral polymorphism is that anadromous individuals of brown trout (''Salmo trutta'' morpha ''trutta'') may occur sympatrically with a stream-resident ones (''Salmo trutta'' morpha ''fario''). The two morphs are part of the same species and ''may'' interbreed [1]; are they polymorphic?. To be accepted as polymorphic it is essential that they are part of the same interbreeding population; otherwise they must be considered as parapatric variants. Frequently, the likelihood of interbreeding is reduced because the morphs also have distinct reproductive behaviours such as variance in timing or in the selection of sites for reproduction; this argues for parapatric geographical variation.
The segregation of a species into sympatric morphs often can be thought of as a mechanism for the partitioning of available resources. An example of this might be certain lacustrine Arctic char populations which segregate into planktivorous and piscivorous morphs within the same lake. However, this example is probably also parapatric variation rather than true polymorphism.
Other polymorphisms are detectable only in the laboratory. An example is that several enzymes (e.g., phosphoglucomutase) taken from different individuals move with different speeds when exposed to an electric field (electrophoresis).

See also

★ CEPH

References

1. The environment and the genotype in polymorphism, , W. C., Clark, Zool. J. Linn. Soc., 1976
2. Sheppard P.M. 1975. ''Natural selection and heredity''. 4th ed, Hutchinson, London.
3. Atlas of Human Chromosome Heteromorphisms, , Herman E., Wyandt, Kluwer Academic Publishers, 2004, ISBN 1-4020-1303-5
4. Diver C. 1929. Fossil records of Mendelian mutants. ''Nature'' '124', 183.
5. Fisher R. 1930. ''The Genetical Theory of Natural Selection''
6. Hamilton W.D. 2002. ''Narrow Roads of Gene Land vol. 2: Evolution of Sex''. Oxford
7. Maynard Smith J. 1978. ''The evolution of sex''. Cambridge
8. Clarke C.A. 1964. ''Genetics for the clinician''. Blackwell, Oxford
9. Crow J. 1993. Felix Bernstein and the first human marker locus. ''Genetics'' '133', 1, 4-7
10. Allison A.C. 1956. The sickle-cell and Haemoglobin C genes in some African populations. ''Ann. Human Genet.'' '21', 67-89.
11. Ford E.B. 1942; 7th ed 1973. ''Genetics for medical students''. Chapman & Hall, London.
12. Chance E. 1922. ''The Cuckoo's secret''. London.
13. Ford E.B. 1975. ''Ecological genetics'', 4th ed. Chapman & Hall, London.
14. Ford E.B. 1981. ''Taking genetics into the countryside''. Weidenfeld & Nicolson, London
15. Majerus, Michael 1998. ''Melanism: evolution in action''. Blackwell, Oxford.
16. Ford E.B. ''Genetic polymorphism'' Faber & Faber, London.1965
17. Kettlewell H.B.D. 1973. ''The evolution of melanism''. Oxford.
18. Majerus, Michael 1998. ''Melanism: evolution in action''. Blackwell, Oxford.
19. Darwin, Charles 1877. ''The different forms of flowers on plants of the same species''. Murray, London.
20. Ford E.B. ''Ecological genetics''. 3rd ed, Chapman & Hall, London 1971 chapter 10.
21. Sheppard P.M. 1975. ''Natural selection and heredity''. 4th ed Hutchinson, London.
22. Insect Ecology, , Peter Wilfrid, Price, John Wiley & Sons, 1997, ISBN 0-471-16184-5
23. Ecological Entomology, , J. R., Nechols, John Wiley & Sons, 1999, ISBN 0-471-24483-X

External links

★ Heterostyly in the Cowslip (''Primula veris'' L.)

★ Information on Genetic Variation and Polymorphism