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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.
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.
Since all polymorphism has a genetic basis, genetic polymorphism has a particular meaning:
- 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).:11
The definition has three parts: a) sympatry: one interbreeding population; b) discrete forms; and c) not maintained just by mutation.
By definition, genetic polymorphism relates to a balance or equilibrium between morphs. The mechanisms that conserve it are types of balancing selection.
- Heterosis (or heterozygote advantage): "Heterosis: the heterozygote at a locus is fitter than either homozygote".:65
- Frequency dependent selection: The fitness of a particular phenotype is dependent on its frequency relative to other phenotypes in a given population. Example: prey switching, where rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs.
- Fitness varies in time and space. Fitness of a genotype may vary greatly between larval and adult stages, or between parts of a habitat range.:26
- Selection acts differently at different levels. The fitness of a genotype may depend on the fitness of other genotypes in the population: this covers many natural situations where the best thing to do (from the point of view of survival and reproduction) depends on what other members of the population are doing at the time.:17+ch7
An individual homozygous for the recessive sickle haemoglobin, HgbS, has a short expectancy of life. 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.
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. Therefore, lactase persistence is often cited as an example of recent human evolution. As lactase persistence is genetic, but animal husbandry a cultural trait, this is gene–culture coevolution.
Second, there is differentiation based mostly on feeding of larvae. This determines, for example, whether the adult is capable of reproduction.
Lastly, there is differentiation of size and 'duties' (particularly of females), which are usually controlled by feeding and/or age, but which may sometimes be genetically controlled. Thus the order exhibits both genetic polymorphism and extensive polyphenism.
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.
So when an insect in search of nectar inserts its proboscis into a long-style flower, the pollen from the stamens stick to the proboscis in exactly the part that will later touch the stigma of the short-styled flower, and vice versa.
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; these act as a single unit, a so-called super-gene.ch10p86
All sections of the genus Primula have heterostyle species, altogether 354 species out of 419. Since heterostyly is characteristic of nearly all races or species, the system is at least as old as the genus.
Studies over many years have shown that natural populations of Drosophila are polymorphic for chromosome inversions. The inversions are so common that they must be kept in the population by natural selection.
- Greek: πολύ = many, and μορφή = form, figure, silhouette
- Ford E.B. 1975. Ecological genetics. 4th ed, London: Chapman & Hall.
- Polymorphism as described here involves morphs of the phenotype. The term is also used somewhat differently by molecular biologists to describe certain point mutations in the genotype, such as SNPs (see also RFLPs). This usage is not discussed on this page.
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- Swaminathan, N. 2006. African adaptation to digesting milk is "strongest signal of selection ever". Scientific American December 11, 2006.
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- Darwin, Charles 1862. Journal of the Proceedings of the Linnaean Society (Botany) 6, 77–96.
- Darwin, Charles. 1877. The different forms of flowers on plants of the same species. London: Murray.
- 'closely linked' means close on the same chromosome.
- Sheppard, Philip M. 1975. Natural selection and heredity. 4th ed, London: Hutchinson.
- Bruun H.G. 1938. Studies on heterostyle plants 2. Svensk. Bot. Tidskr. 32, 249-260.
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- Darwin, Charles. 1977 (collection). Barrett P.H. (ed) The collected papers of Charles Darwin. Chicago: Chicago University Press.
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- When a part of a chromosome gets reversed end to end, so the genes run in the opposite direction to before.
- 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.
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