Polymorphism (biology)
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2008-06-24T19:39:28Z
Kungfuadam
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Correcting spelling: occurence->occurrence
[[Image:Jaguar head shot.jpg|thumb|right|Light-morph [[Jaguar]] (typical)]]
[[Image:Black jaguar.jpg|thumb|right|Dark-morph or melanistic Jaguar (about 6% of the South American population)]]
'''Polymorphism''' in biology occurs when two or more clearly different [[phenotypes]] exist in the same population of a species where the frequency of the rarer form/s is greater than can be maintained by recurrent mutation alone — in other words, the occurrence of more than one ''form'' or ''morph'' where the rarer one has an occurrence rate of at least 1% in the population. The caste system in [[ant]]s is an example.
''Polymorphism'' ([[Classical Greek|Greek]]: poly = many, and morph = form) is defined as discontinuous<ref>{{cite journal | last = Clark | first = W. C. | year = 1976 | title = The environment and the genotype in polymorphism | journal = Zool. J. Linn. Soc. | volume = 58 | pages = 255–262 | doi = 10.1111/j.1096-3642.1976.tb00831.x}}</ref> variation in a single population<ref name=EcoGen>{{cite book | last = Ford | first = E. B. | year = 1964 | title = Ecological Genetics | publisher = Chapman & Hall | location = London}}</ref> — meaning, the population is in the same location and is interbreeding. The term was first used to describe ''visible forms'', but by extension we now use the term to include ''cryptic morphs'', for instance [[blood types]], which can be made manifest by a test.
The shorter term ''morphism'' may be more accurate than ''polymorphism'', but is not often used. It was the preferred term of the evolutionist [[Julian Huxley]].<ref>Huxley J.S. 1955. Morphism and evolution. ''Heredity'' '''9''', 1-52.</ref>
Polymorphism is extremely common; it is a kind of variation related to [[biodiversity]], [[genetic variation]] and [[adaptation]]. Polymorphism usually functions to retain variety of form in a population living 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]]), certain [[cryptic]] forms of moths, the banding pattern on snail shells, human blood groups and many other cases.
Polymorphism results from an evolutionary process, as does any aspect of a species. Polymorphism is [[heritable]], and is modified by [[selection]] (either [[artificial selection|artificial]] or [[natural selection|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 somewhat differently by molecular biologists to describe certain point mutations in the [[genotype]], such as [[single nucleotide polymorphism|SNP]]s (see also [[RFLP]]s). This usage is not discussed in this article.
===What polymorphism is not===
Although in general use polymorphism is quite a broad term, in biology it has been given a specific meaning. This section indicates its proper use.
*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.<ref>Sheppard P.M. 1975. ''Natural selection and heredity''. 4th ed, Hutchinson, London.</ref> The use of the words ''morph'' or ''polymorphism'' for what is a visibly different ''geographical race or variant'' is common, but incorrect. The significance of geographical variation ([[allopatry]]) is that interbreeding between different locations is reduced or eliminated, a possible prelude to ''species splitting''. True polymorphism takes place in [[panmictic]] populations, and has to do with the adaptation of a species to its environment.
*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 [[mutation]]s<ref name=Ford1940>{{cite book | last = Ford | first = E.B. | year = 1940 | authorlink = E.B. Ford | title = The New Systematics | chapter = Polymorphism and taxonomy | editor = [[Julian Huxley|J. Huxley]], ed. | publisher = Clarendon Press | location = Oxford | pages = 493–513}}</ref><ref name=EcoGen/> 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).<ref>Sheppard P.M. 1975. ''Natural selection and heredity''. 4th ed Hutchinson, London, Chapter 5.</ref>
=== Nomenclature ===
*'''Terminology'''. <br>This topic crosses several discipline boundaries: ecology and genetics, at least; evolution theory, taxonomy, cytology and biochemistry also occur. Different disciplines may give the same concept different names, and different concepts may be given the same name. We use here the terms established in ecological genetics by Ford,<ref>Ford E.B. 1975. ''Ecological genetics'', 4th ed. Chapman & Hall, London.</ref> and for classical genetics Maynard Smith.<ref>Maynard Smith J. 1998. ''Evolutionary genetics'', 2nd ed. Oxford.</ref></br>
*'''Synonyms''' for the different forms. <br>They can be called morphs or morphotypes; [[trait (biology)]] and [[characters]] are also possible descriptions, though that might imply just a limited aspect of the body. ''Phase'' is sometimes used, but ''morph'' or ''form'' is best.</br>
*'''Taxonomic nomenclature'''. <br>In '''[[zoology]]''' the word "morpha" plus a Latin name for the morph can be added to a [[binomial nomenclature|binomial]] or [[trinomial nomenclature|trinomial]] name. However, this invites confusion with [[ring species|geographical variation]], especially with polytypic species or [[sub-species]]. Morphs have no formal standing in the [[ICZN]].<br>In '''[[botany]]''', morphs may be named with the terms "[[Variety (botany)|variety]]", "[[subvariety]]", and "[[Form (botany)|forms]]" which are formally regulated by the [[ICBN]]. There might be confusion with the term ''variety''.</br>
==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.<ref name=Ford1940/> Diver (1929) found banding morphs in ''Cepaea nemoralis'' could be seen in [[sub-fossil|pre-fossil shells]] going back to the [[Mesolithic]] [[Holocene]].<ref>Diver C. 1929. Fossil records of Mendelian mutants. ''Nature'' '''124''', 183.</ref><ref>Cain A.J. 1971. Colour and banding morphs in subfossil samples of the snail ''Cepaea''. In Creed R. ''Ecological genetics and evolution: essays in honour of E.B. Ford''. Blackwell, Oxford.</ref> Apes have similar blood groups to humans: human and chimpanzee blood, with compatible blood groups, can be exchanged through transfusion ([[Great Ape Project]]). This suggests rather strongly that this kind of polymorphism is quite ancient, at least as far back as the last common ancestor of the apes and man, and possibly even further.
[[Image:albino monarch butterfly.jpg|right|thumb|The white morph of the [[Monarch (butterfly)|monarch]] in [[Hawaii]] is partly a result of [[apostatic selection]]<ref>{{cite journal |last=Stimson |first=John |coauthors=Mark Berman |year=1990 |title= Predator induced colour polymorphism in Danaus plexippus L. (Lepidoptera: Nymphalidae) in Hawaii |journal=Heredity |volume=65 |issue=3 |pages=401–406 |accessdate=2008-04-06 |laysummary=http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=2472199&q=predator+induced+colour+polymorphism&uid=792450984&setcookie=yes |doi= 10.1038/hdy.1990.110 }}</ref>]]
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]]; nor does it prevent 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.
==== Polymorphism and niche diversity ====
[[G. Evelyn Hutchinson]], a founder of niche research, commented "It is very likely from an ecological point of view that all species, or at least all common species, consist of populations adapted to more than one niche."<ref>Hutchinson, G. Evelyn 1965. ''The evolutionary theater and the evolutionary play''. Yale. The niche: an abstractly inhabited hypervolume: polymorphism and niche diversity, p66-70.</ref> He gave as examples sexual size dimorphism and mimicry. In many cases where the male is short-lived and smaller than the female, he does not compete with her during her late pre-adult and adult life. Size difference may permit both sexes to exploit different niches. In elaborate cases of [[mimicry]], such as the African butterfly ''[[Papilio]] dardanus'',<ref>Ford E.B. 1975. ''Ecological genetics''. 4th ed, Chapter 13 ''Papilio dardanus'' and the evolution of mimicry. Chapman & Hall, London.</ref> female morphs mimic a range of distasteful models, often in the same region. The fitness of each type of mimic decreases as it becomes more common, so the polymorphism is maintained by frequency-dependent selection. Thus the efficiency of the mimicry is maintained in a much increased total population.
==== The switch ====
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.
====Investigative methods====
Investigation of polymorphism requires a coming together of field and laboratory technique. In the field:
*detailed survey of occurrence, habits and predation
*selection of an ecological area or areas, with well-defined boundaries
*capture, mark, release, recapture data (see [[Mark and recapture]])
*relative numbers and distribution of morphs
*estimation of population sizes
And in the laboratory:
*genetic data from crosses
*population cages
*[[chromosome]] [[cytology]] if possible
*use of [[chromatography]] or similar techniques if morphs are cryptic (for example, biochemical)
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.
== Genetics ==
==== 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.<ref name=Ford1940>{{cite book | last = Ford | first = E. B. | year = 1940 | authorlink = E. B. Ford | title = The New Systematics | chapter = Polymorphism and taxonomy | editor = [[Julian Huxley|J. Huxley]], ed. | publisher = Clarendon Press | location = Oxford | pages = 493–513}}</ref> 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.
==== Pleiotropism ====
Most genes have more than one effect on the [[phenotype]] of an organism ([[pleiotropism]]). Some of these effects may be visible, and others cryptic, so it is often important to look beyond the most obvious effects of a gene to identify other effects. Cases occur where a gene affects an unimportant visible character, yet a change in fitness is recorded. In such cases the gene's other (cryptic or 'physiological') effects may be responsible for the change in fitness. <br>"If a neutral trait is pleiotropically linked to an advantageous one, it may emerge because of a proces of natural selection. It was selected but this doesn't mean it is an adaptation. The reason is that, although it was selected, there was no selection for that trait."<ref>Sober E. 1984. ''The nature of selection: evolutionary theory in philosophical focus''. Chicago. p197</ref>
==== Epistasis ====
[[Epistasis]] occurs when the expression of one gene is modified by another gene. For example, gene A only shows its effect when allele B1 (at another [[locus]]) is present, but not if B2 is absent. This is one of the ways in which two or more genes may combine to produce a co-ordinated change in more than one characters (in, for instance, mimicry). Unlike the supergene, epistatic genes do not need to be closely [[Genetic linkage|linked]] or even on the same [[chromosome]].
Both pleiotropism and epistasis show that a gene need not relate to a character in the simple manner that was once supposed.
==== The origin of supergenes ====
Although a polymorphism can be controlled by [[alleles]] at a single [[locus]] (eg human [[ABO]] blood groups), the more complex forms are controlled by [[supergene]]s consisting of several tightly [[linked genes]] on a single [[chromosome]]. [[Batesian mimicry]] in butterflies and [[heterostyly]] in angiosperms are good examples. There is a long-standing debate as to how this situation can have arisen, and the question is not yet resolved.
Whereas a [[gene family]] (several tighly linked genes performing similar or identical functions) arises by duplication of a single original gene, this is usually not the case with supergenes. In a supergene some of the constituent genes have quite distinct functions, so they must have come together under selection. This process might involve suppression of crossing-over, translocation of chromosome fragments and possibly occasional cistron duplication. That crossing-over can be suppressed by selection has been known for many years.<ref>Detlefsen J.A. and Roberts E. 1921. Studies on crossing-over I. The effects of selection on crossover values. ''J Exp Zoology''. 32, 333-54.</ref><ref>Darlington C.D. 1956. ''Chromosome botany'', p36. Allen & Unwin, London.</ref>
Debate has tended to centre round the question: Could the component genes in a super-gene have started off on separate chromosomes, with subsequent reorganization, or is it necessary for them to start on the same chromosome? Originally, it was held that chromosome rearrangement would play an important role.<ref>Darlington C.D. & Mather K. 1949. ''The elements of genetics'', p335-6. Allen & Unwin, London.</ref> This explanation was accepted by [[E.B. Ford]] and incorporated into his accounts of ecological genetics.<ref>Ford E.B. 1965. ''Genetic polymorphism'', p17-25. MIT Press 1965.</ref><ref>Ford E.B. 1975. ''Ecological genetics''. 4th ed, Chapter 6 The theory of genetic polymorphism. Chapman & Hall, London.</ref>
However, today many believe it more likely that the genes start on the same chromosome.<ref>Charlesworth D. and Charlesworth B. 1975. Theoretical genetics of Batesian mimicry I. Single-locus models. ''J. Theoret. Biol.'' '''55''', 283-303; II. Evolution of supergenes. ''J. Theoret. Biol.'' '''55''', 305-324; III. Evolution of dominance. ''J. Theoret. Biol.'' '''55''', 325-337.</ref> They argue that supergenes arose ''in situ''. This is known as Turner's sieve hypothesis.<ref>Turner JRG 1984. Mimicry: the palatability spectrum and its consequences. Chap. 14. In Vane-Wright R.I. & Ackery P.R. (eds) ''The Biology of Butterflies''. Symposia of the Royal Entomological Society of
London #11. Academic Press, London.
</ref> Maynard Smith agreed with this view in his authoritative textbook,<ref>Maynard Smith J. 1998. ''Evolutionary genetics'', 2nd ed. Oxford.</ref> but the question is still not definitively settled.
== Relevance for evolutionary theory ==
Polymorphism was crucial to research in [[ecological genetics]] by [[E.B. Ford]] and his co-workers from the mid-1920s to the 1970s (similar work continues today, especially on [[mimicry]]). The results had a considerable effect on the mid-century [[evolutionary synthesis]], and on present [[evolutionary theory]]. The work started at a time when [[natural selection]] was largely discounted as the leading mechanism for evolution,<ref>Bowler P.J. 1983. ''The eclipse of Darwinism: anti-Darwinian evolutionary theories in the decades around 1900''. Johns Hopkins, Baltimore.</ref><ref>Bowler P.J. 2003. ''Evolution: the history of an idea''. 3rd ed revised and expanded, University of California Press.</ref> continued through the middle period when [[Sewall Wright]]'s ideas on [[genetic drift|drift]] were prominent, to the last quarter of the 20th century when ideas such as [[Motoo Kimura|Kimura]]'s [[neutral theory of molecular evolution]] was given much attention. The significance of the work on ecological genetics is that it has shown how important selection is in the evolution of natural populations, and that selection is a much stronger force than was envisaged even by those population geneticists who believed in its importance, such as [[J.B.S. Haldane|Haldane]] and [[Ronald Fisher|Fisher]].<ref>Cain A.J. and Provine W.B. 1991. Genes and ecology in history. In Berry R.J. et al (eds) ''Genes in ecology'': the 33rd Symposium of the British Ecological Society. Blackwell, Oxford.</ref>
In just a couple of decades the work of [[Ronald Fisher|Fisher]], [[E.B. Ford|Ford]], [[Arthur Cain|Cain]], [[Philip Sheppard|Sheppard]] and [[Cyril Clarke|Clarke]] promoted natural selection as the primary explanation of variation in natural populations, instead of genetic drift. Evidence can be seen in Mayr's famous book ''Animal species and evolution'',<ref>Mayr E. 1963. ''Animal species and evolution''. Harvard.</ref> and Ford's ''Ecological genetics''.<ref>Ford E.B. 1964, 4th edn 1975. ''Ecological genetics''. Chapman and Hall, London</ref> Similar shifts in emphasis can be seen in most of the other participants in the evolutionary synthesis, such as [[G. Ledyard Stebbins|Stebbins]] and [[Theodosius Dobzhansky|Dobzhansky]], though the latter was slow to change.<ref>Stebbins G.L. 1950 ''Variation and evolution in plants''. Columbia.</ref><ref>Stebbins G.L. 1966. ''Processes of organic evolution''.</ref><ref>Dobzhansky T. 1937. ''Genetics and the origin of species''. Columbia, New York. (3rd ed 1951: note contrast between these two editions)</ref><ref>Dobzhansky, Th. 1970. ''Genetics of the evolutionary process''. Columbia, New York.</ref>
[[Motoo Kimura|Kimura]] drew a distinction between molecular evolution, which he saw as dominated by selectively neutral mutations, and phenotypic characters, probably dominated by natural selection rather than drift.<ref>Kimura M. 1983. ''The neutral theory of molecular evolution''. Cambridge.</ref> This does not conflict with the account of polymorphism given here, though most of the ecological geneticists believed that evidence would gradually accumulate against his theory.
== Examples==
====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]], usually 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 the [[evolution of sex]] occur from an original situation of asexual reproduction, which has the advantage that every member of a species could reproduce? Why the visible differences between the sexes? These questions have engaged the attentions of biologists such as [[Charles Darwin]], [[August Weismann]], [[Ronald Fisher]], [[George C. Williams]], [[John Maynard Smith]] and [[W.D. Hamilton]], with varied success.
Although this huge topic cannot be treated here in detail, it is fair to say there is widespread agreement on the following: the advantage of [[sexual reproduction]] over [[asexual reproduction]] lies in the way [[Genetic recombination|recombination]] increases the [[genetic diversity]] of the ensuing population. This enables the population to better meet the challenges of [[infection]], [[parasitism]], [[predation]] and other hazards of the varied environment.<ref>Fisher R. 1930. ''The genetical theory of natural selection''</ref><ref>Hamilton W.D. 2002. ''Narrow roads of gene land vol. 2: Evolution of sex''. Oxford</ref><ref>Maynard Smith J. 1978. ''The evolution of sex''. Cambridge</ref> See also [[evolution of sex]].
====Other human polymorphisms====
Apart from sexual dimorphism, there are many other examples of human genetic polymorphisms. Infectious disease has been a major factor in human mortality, and so has affected the evolution of human populations. Evidence is now strong that many polymorphisms are maintained in human populations by balancing selection.<ref>Cooke GS and Hill AVS 2001. Genetics of susceptibility to human infectious disease. ''Nat Rev Genet'' '''2''', 967–977</ref><ref>Sykes B. 1999. ''The human inheritance: genes, language and evolution''. Oxford University Press.</ref>
*'''Human blood groups.''' <br>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; the reason came from a study of disease statistics.<br>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.<ref>Clarke C.A. 1964. ''Genetics for the clinician''. Blackwell, Oxford</ref><ref>Crow J. 1993. Felix Bernstein and the first human marker locus. ''Genetics'' '''133''', 1, 4-7</ref><ref>Meade S.M. and Earickson R.J. 2005. ''Medical geography''. Guilford.</ref><br>Geographical distribution of blood groups (the differences in gene frequency between populations) is broadly consistent with the classification of races developed by anthropologists on the basis of visible features.<ref>Dobzhansky T. 1970. ''Genetics of the evolutionary process''. Columbia, N.Y. p283-291</ref>
*'''Sickle-cell anaemia.''' <br>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). <br>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]] (see<ref>Allison A.C. 1956. The sickle-cell and Haemoglobin C genes in some African populations. ''Ann. Human Genet.'' '''21''', 67-89.</ref><ref>Ford E.B. 1942; 7th ed 1973. ''Genetics for medical students''. Chapman & Hall, London. </ref> and refs in [[Sickle-cell disease]]).</br>
*'''G6PD''' <br>G6PD (Glucose-6-phosphate dehydrogenase) human polymorphism is also implicated in malarial resistance. G6PD alleles with reduced activity are maintained at a high level in endemic malarial regions, despite reduced general viability. Variant A (with 85% activity) reaches 40% in sub-Saharan Africa, but is generally <1% outside Africa and the Middle East.<ref>Beutler E. 1994. G6PD deficiency. ''Blood'' 84, 3613–36.</ref><ref>Verrill B.C. et al 2002. Evidence for balancing selection from nucleotide sequence analyses of human G6PD. ''Am J Hum Genet''. '''71''', 1112–28.</ref>
*'''Human taste morphisms'''. <br>A famous puzzle in human genetics is the genetic ability to taste [[phenylthiourea]] (phenylthiocarbamide or PTC), a morphism which was discovered in 1931. This substance, which to some of us is bitter, and to others tasteless, is of no great significance in itself, yet it is a genetic dimorphism. Because of its high frequency (which varies in different ethnic groups) it must be connected to some function of selective value. <br>The ability to taste PTC itself is correlated with the ability to taste other bitter substances, many of which are toxic. Indeed, PTC itself is toxic, though not at the level of tasting it on litmus paper. Variation in PTC perception may reflect variation in dietary preferences throughout human evolution, and might correlate with susceptibility to diet-related diseases in modern populations. There is a statistical correlation between PTC tasting and liability to thyroid disease.<br>[[Ronald Fisher|Fisher]], [[E.B. Ford|Ford]] and [[Julian Huxley|Huxley]] tested [[orang-utan]]s and [[chimpanzees]] for PTC perception with positive results, thus demonstrating the long-standing existence of this dimorphism.<ref>Fisher R.A, Ford E.B. & Huxley J.S. 1939. Taste-testing the anthropoid apes. ''Nature'' '''144''', 750.</ref> The recently identified PTC gene, which accounts for 85% of the tasting variance, has now been analysed for sequence variation with results which suggest selection is maintaining the morphism.<ref>Wooding S, Kim Un-Kyung, Bamshad M.J, Larsen J, Jorde L.B. & Drayna D. 2004. Natural selection and molecular evolution in PTC, a bitter-taste receptor gene. ''Am J Human Genetics''. '''74''', 637-46.</ref>
====The Cuckoo====
Over fifty species in this family of birds practise [[brood parasitism]]; the details are best seen 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.
[[Image:Reed warbler cuckoo.jpg|right|230px|thumb|Reed warbler feeding a cuckoo chick (''Cuculus canorus'')]]
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.
*[[European 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]], singular "gens"), each parasitises the host to which it is adapted. The male Cuckoo has its own territory, and mates with females from any gens; thus the population (all gentes) is interbreeding.
The standard explanation of how the inheritance of gens works is as follows. The egg colour is inherited by sex chromosome. In birds [[sex determination]] is ZZ/ZW, and unlike mammals, the heterogametic sex is the female.<ref>Ellegren, Hans 2001. Hens, cocks and avian sex chromosomes: a quest for genes on Z or W? ''EMBO reports'' '''2''', 3, 192-196.</ref> The determining gene (or super-gene) for the inheritance of egg colour is believed to be carried on the W chromosome. The W chromosome, of course, is directly transmitted in the female line. The female behaviour in choosing the host species is set by [[Imprinting (psychology)|imprinting]] after birth, a common mechanism in bird behaviour.<ref>Ford E.B. 1975. ''Ecological genetics'', 4th ed. Chapman & Hall, London.</ref><ref>Ford E.B. 1981. ''Taking genetics into the countryside''. Weidenfeld & Nicolson, London.</ref>
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<ref>Chance E. 1922. ''The Cuckoo's secret''. London.</ref> and continues to the present day; in particular, the inheritance of gens is still a live issue.
==== Grove snail ====
The Grove Snail, ''[[Cepaea nemoralis]]'', is famous for the rich polymorphism of its shell. The system is controlled by a series of [[multiple alleles]]. The shell colour series is brown (genetically the top [[dominant]] trait), dark pink, light pink, very pale pink, dark yellow and light yellow (the bottom or universal [[recessive]] trait). Bands may be present or absent; and if present from one to five in number. Unbanded is the top dominant trait, and the forms of banding are controlled by modifier genes (see [[epistasis]]).
[[Image:Schneckesnail1.jpg|thumb|200px|left|Grove snail, dark yellow shell with single band.]]
In England the snail is regularly predated by the [[Song Thrush]] ''Turdus philomelos'', which breaks them open on ''thrush anvils'' (large stones). Here fragments accumulate, permitting researchers to analyse the snails taken. The thrushes hunt by sight, and capture selectively those forms which match the habitat ''least well''. Snail colonies are found in woodland, hedgerows and grassland, and the predation determines the proportion of phenotypes (morphs) found in each colony.
[[Image:Cepaea nemoralis active pair on tree trunk.jpg|thumb|200px|right|Two active Grove snails]]
A second kind of selection also operates on the snail, whereby certain heterozygotes have a physiological advantage over the homozygotes. In addition, apostatic selection is likely, with the birds preferentially taking the most common morph. This is the 'search pattern' effect, where a predominantly visual predator persists in targeting the morph which gave a good result, even though other morphs are available.
Despite the predation, the polymorphism survives in almost all habitats, though the proportions of morphs varies considerably. The alleles controlling the polymorphism form a [[super-gene]] with linkage so close as to be nearly absolute. This control saves the population from a high proportion of undesirable recombinants, and it is hypothesised that selection has brought the loci concerned together.
To sum up, in this species predation by birds appears to be the main (but not the only) selective force driving the polymorphism. The snails live on heterogenous backgrounds, and thrush are adept at detecting poor matches. The inheritance of physiological and cryptic diversity is preserved also by heterozygous advantage in the super-gene.<ref>Cain A.J. and Currey J.D. Area effects in ''Cepaea''. ''[[Phil. Trans. R. Soc. B]]'' '''246''': 1-81.</ref><ref>Cain A.J. and Currey J.D. 1968. Climate and selection of banding morphs in ''Cepaea'' from the climate optimum to the present day. ''[[Phil. Trans. R. Soc. B]]'' '''253''': 483-98.</ref><ref>Cain A.J. and Sheppard P.M. 1950. Selection in the polymorphic land snail ''Cepaea nemoralis'' (L). ''Heredity'' '''4''':275-94.</ref><ref>Cain A.J. and Sheppard P.M. 1954. Natural selection in ''Cepaea''. ''Genetics'' 39: 89-116.</ref><ref>Ford E.B. 1975. ''Ecological genetics'', 4th ed. Chapman & Hall, London</ref> Recent work has included the effect of shell colour on thermoregulation,<ref>Jones J.S., Leith B.N. & Rawlings P. 1977. Polymorphism in ''Cepaea'': a problem with too many solutions. ''Annual Reviews in Ecology and Systematics'' '''8''', 109-143.</ref> and a wider selection of possible genetic influences is considered by Cook.<ref>Cook L.M. 1998. A two-stage model for ''Cepaea'' polymorphism. ''[[Phil. Trans. R. Soc. B]]'' '''353''', 1577-1593.</ref>
A similar system of genetic polymorphism occurs in the White-lipped Snail ''[[Cepaea hortensis]]'', a close relative of the Grove Snail. In Iceland, where there are no song thrushes, a correlation has been established between temperature and colour forms. Banded and brown morphs reach higher temperatures than unbanded and yellow snails.<ref>Owen D. 1980. ''Camouflage and mimicry''. Oxford.</ref> This may be the basis of the physiological selection found in both species of snail.
==== Scarlet Tiger Moth ====
The [[Scarlet tiger moth|Scarlet Tiger Moth]] ''Callimorpha'' (''Panaxia'') ''dominula'' (family [[Arctiidae]]) occurs in continental Europe, western Asia and southern England. It is a day-flying moth, noxious-tasting, with brilliant warning colour in flight, but cryptic at rest. The moth is colonial in habit, and prefers marshy ground or hedgerows. The preferred food of the larvae is the herb [[Comfrey]] (''Symphytum officinale''). In England it has one generation per year.
[[Image:Scarlet tiger moth.jpg|thumb|250px|''Callimorpha dominula'' morpha ''typica'' with spread wings. The red with black rear wings, revealed in flight, warn of its noxious taste. The front wings are cryptic, covering the rear wings at rest. Here the moth is resting but alert, and has jinked the front wings forward to reveal the warning flash.]]
The moth is known to be polymorphic in its colony at [[Cothill]], about five miles from [[Oxford]], with three forms: the typical [[homozygote]]; the rare homozygote (''bimacula'') and the [[heterozygote]] (''medionigra''). It was studied there by [[E.B. Ford]], and later by [[Philip Sheppard|P.M. Sheppard]] and their co-workers over many years. Data is available from 1939 to the present day, got by the usual field method of capture-mark-release-recapture and by genetic analysis from breeding in captivity. The records cover gene frequency and population-size for much of the twentieth century.<ref>Ford E.B. 1971. ''Ecological genetics''. 3rd ed London 1971, chapter7.</ref>
In this instance the genetics appears to be simple: two [[alleles]] at a single [[locus (genetics)|locus]], producing the three [[phenotypes]]. Total captures over 26 years 1939-64 came to 15,784 homozygous ''dominula'' (ie ''typica''), 1,221 heterozygous ''medionigra'' and 28 homozygous ''bimacula''. Now, assuming equal viability of the genotypes 1,209 heterozygotes would be expected, so the field results do ''not'' suggest any heterozygous advantage. It was Sheppard who found that the polymorphism is maintained by selective mating: each genotype preferentially mates with other morphs.<ref>Sheppard P.M. 1952. A note on non-random mating in the moth Panaxia dominula (L.). ''Heredity'' '''6''': 239-41.</ref> This is sufficient to maintain the system despite the fact that in this case the heterozygote has slightly lower viability.<ref>Sheppard P.M. and Cook L.M. 1962. The manifold effects of the medionigra gene in the moth ''Panaxia dominula'' and the maintenance of polymorphism. ''Heredity'' '''17''':415-26.</ref>
====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. E.B. Ford described it as "one of the most striking, though not the most profound, evolutionary changes ever actually witnessed in nature". <ref>Ford E.B. 1976. ''Genetics and adaptation''. Arnold, London. p14</ref>
Although the moths are cryptically [[camouflage]]d 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. This was a rapid change for a species that has only one generation a year.
[[Image:Biston.betularia.7200.jpg|thumb|260px|''Biston betularia'' morpha ''typica'', the standard light-coloured Peppered Moth.]]
[[Image:Biston.betularia.f.carbonaria.7209.jpg|thumb|260px|''Biston betularia'' morpha ''carbonaria'', the melanic Peppered Moth.]]
In Europe, there are three morphs: the typical white morph (''betularia'' or ''typica''), 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 ''insularia'', controlled by other alleles (see Majerus 1998). <ref>Majerus, Michael 1998. ''Melanism: evolution in action''. Blackwell, Oxford.</ref>
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.
Another interesting feature is that the carbonaria had noticeably darkened after about a century. This was seen quite clearly when specimens collected about 1880 were compared with specimens collected more recently: clearly the dark morph has been adjusted by the strong selection acting on the gene complex. This might happen if a more extreme allele was available at the same locus; or genes at other loci might act as modifiers. We do not, of course, know anything about the genetics of the original melanics from the nineteenth century.
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 (over 100 species of moth in Britain with melanic forms were known by 1980).<ref>Ford E.B. 1981. ''Taking genetics into the countryside''. Weidenfeld & Nicolson, London.</ref> No species which hide during the day, for instance, among dead leaves, is affected, nor has the melanic change been observed among butterflies.<ref>Ford E.B. ''Genetic polymorphism'' Faber & Faber, London.1965</ref><ref>Kettlewell H.B.D. 1973. ''The evolution of melanism''. Oxford.</ref><ref>Majerus, Michael 1998. ''Melanism: evolution in action''. Blackwell, Oxford.</ref>
This is, as advertised in many textbooks, 'evolution in action'. Much of the work was done by [[Bernard Kettlewell]], whose methods came under scrutiny later on. The entomologist [[Michael Majerus]] discussed criticisms made of Kettlewell's experimental methods in his 1998 book ''[[Melanism: Evolution in Action]]''.<ref name="dd">Majerus, M.E.N. (2004)[http://www.gen.cam.ac.uk/Research/Majerus/Darwiniandisciple.doc The Peppered moth: decline of a Darwinian disciple.] (.doc download)</ref> This book was misrepresented in some reviews, and the story picked up by [[creationism|creationist]] campaigners. In her controversial book ''[[Of Moths and Men]]'', Judith Hooper (2002) implied that Kettlewell's work was fraudulent or incompetent. Careful studies of Kettlewell's surviving papers by Rudge (2005) and Young (2004) found that Hooper's allegation of fraud was unjustified, and that "Hooper does not provide one shred of evidence to support this serious allegation”.<ref name="rudge2005">Rudge D.W. (2005). "Did Kettlewell commit fraud? Re-examining the evidence.", ''Public Understanding of Science'' 14 (3) (pp. 249–268).</ref><ref name="young">Young, M. (2003). ''[http://www.talkreason.org/articles/moonshine.cfm Moonshine: Why the Peppered Moth Remains an Icon of Evolution].''</ref> Majerus himself described ''[[Of Moths and Men]]'' as "littered with errors, misrepresentations, misinterpretations and falsehoods".<ref name="dd"/>. A suitably restrained summary of latest opinion mostly favours predation as the main selective force. <ref>Ruxton G.D., Sherratt T.N. & Speed M.P. 2004. ''Avoiding attack: the evolutionary ecology of crypsis, warning signals and mimicry''. Oxford. p9-10</ref>
Conclusion: The Peppered Moth is a valid example of [[natural selection]] and [[adaptation]]. It illustrates a polymorphic species maintaining adaptation to a varied and sometimes changing environment.
==== Two-spotted lady beetle ====
<gallery>
Image:Adalia_bipunctata01.jpg|red morph
Image:Adalia.bipunctata.7222.jpg|black morph
Image:Adalia.bipunctata.melanic.form.jpg|black morph
</gallery>
''[[Adalia bipunctata]]'', the two-spotted ladybird, is highly polymorphic. Its basic form is red with two black spots, but it has many other forms, the most important being melanic, with black elytra and red spots. The curious fact about this morphism is that, although the melanic forms are more common in industrial areas, its maintenance has nothing to do with cryptic camouflage and predation. The [[Coccinellidae]] as a whole are highly noxious, and experiments with birds and other predators have have found this species quite exceptionally distasteful.<ref>Fraser J.F.D. and Rothschild, M. 1960. Defence mechanisms in warningly-coloured moths and other insects. ''Proc 11th Int Cong Entomology'' p248-256.</ref> Therefore, their colour is warning ([[aposematic]]) colouration, and all the morphs are quite conspicuous against green vegetation. The field studies identify differing proportions of morphs at different times of year and in different places, which indicates a high level of selection. However, the basis of that selection is still not known for sure, though many theories have been proposed.<ref>Timofeev-Ressovsky-Ressovsky N.W. 1940. Zur analyse des polymorphismus bei ''Adelia bipunctata'' L. ''Biol Zbl''. '''60''', 130-7.</ref><ref>Creed E.R. Melanism in the Two-spot Ladybird, ''Adelia bipunctata'', in Great Britain. In Creed E.R. (ed) ''Ecological genetics and evolution''. Blackwell 1971.</ref><ref>Brakefield P.M. 1985. Polymorphic Müllerian mimicry and interactions with thermal melanism in ladybirds and a soldier beetle—a hypothesis. ''Biological J. Linnaean Soc''. '''26''', 243-267.</ref> Since all the morphs are aposematically coloured, it seems unlikely that the difference between the colour of morphs is directly under selection. Perhaps [[pleiotropic]] effects of the genes acting on colour also affect the beetle's physiology, and hence its relative fitness.
==== Hoverfly polymorphism ====
<gallery>
Image:Hoverfly September 2007-8.jpg|''Xanthogramma pedissiquum'' <br> wasp mimic
Image:Volucella September 2007-2.jpg|''Volucella zonaria'', a large bumblebee mimic
Image:Syrphid_fly_Mallota_1.jpg|''Mallota'' sp <br> Bumblebee mimic
</gallery>
Hoverfly mimics can be seen in almost any garden in the [[temperate zone]].
The [[Syrphidae]] are a large (5600+ species) family of flies; their [[imagos]] feed on nectar and pollen, and are well-known for their mimicry of social [[hymenoptera]]. The mimicry is [[mimicry|Batesian]] in nature: hoverflies are palatable but hymenoptera are generally unpalatable and may also be protected by stings and/or armour.
Many social wasp species ([[Vespidae]]) exhibit [[mimicry|Mullerian]] mimicry, where a group of unpalatable species benefit from sharing the same kind of warning ([[aposematic]]) colouration. Wasps are decidedly noxious: nasty-tasting and with a painful sting. They form a Mullerian 'ring' of similarly coloured models; the wasps are often accompanied by clusters of hover-fly mimics, who tend to arrive at the flowers at a similar time of day, and whose flight pattern is passably similar to wasp flight.
Observers in a garden can see for themselves that hoverfly mimics are quite common, usually many times more common than the models, and are (to our sight) relatively poor mimics, often easy to distinguish from real wasps. However, it has been established in other cases that imperfect mimicry can confer significant advantage to the mimic, especially if the model is really noxious.<ref>Brower L.P. 1988. ''Mimicry and the evolutionary process''. Chicago.</ref> Also, not only is polymorphism absent from these mimics, it is absent in the wasps also: these facts are presumably connected.<ref>Edmunds M. 2000. Why are there good and poor mimics? ''Biological J. Linnaean Soc''. '''70''', 459-56.</ref>
The situation with [[bumblebees]] (''Bombus'') is rather different. They too are unpalatable, in the sense of being difficult to eat: their body is covered with [[setae]] ([[pile]]) and is armoured; they are sometimes described as being 'non-food'. Mostler in 1935 carried out tests of their palatability: with the exception of specialist bee-eaters, adults of 19 species of birds ate only 2% of 646 bumblebees presented to them. After various trials, Mostler attributed their avoidance mainly to mechanical difficulties in handling: one young bird took 18 minutes to subdue, kill and eat a bumblebee.<ref>Mostler G. 1935. Beobachtungen zur Frage der Wespenmimikrey. ''Zeitschrift für Morphologie und Ökologie der Thiere'' '''29''', 381-454.</ref>
Bumblebees form Mullerian rings of species, and they do often exhibit polymorphism. The hoverfly species mimicking bumblebees are generally accurate mimics, and many of their species are polymorphic. Many of the polymorphisms are different between the sexes, for example by the mimicry being limited to one sex only.
The question is, how can the differences between social wasp mimics and bumblebee mimics be explained? Evidently if model species are common, and have overlapping distributions, they are less likely to be polymorphic. Their mimics are widespread and develop a kind of rough and ready jack-of-all-trades mimicry. But if model species are less common and have patchy distribution they develop polymorphism; and their mimics match them more exactly and are polymorphic also. The issues are currently being investigated.<ref>Gilbert, Francis. 2004. The evolution of imperfect mimicry in hoverflies. In Fellows M., Holloway G. and Rolff J (eds) ''Insect evolutionary biology''.</ref><ref>Sherratt T.N. The evolution of imperfect mimicry. ''Behavioral Ecology'' '''13''', 6, 821-26. CABI</ref><ref>Mallet J. and Joron M. 1999. The evolution of diversity in warning color and mimicry: polymorphisms, shifting balance, and speciation. ''Annual Review of Ecology and Systematics'' '''30''': 201-33.</ref>
==== Chromosome polymorphism in ''Drosophila'' ====
In the 1930s [[Dobzhansky]] and his co-workers collected ''[[Drosophila pseudoobscura]]'' and ''[[Drosophila persimilis|D. persimilis]]'' from wild populations in [[California]] and neighbouring states. Using Painter's technique<ref>Painter T.S. 1933. A new method for the study of chromosome rearrangements and the plotting of chromosome maps. ''Science'' '''78''': 585-586.</ref> they studied the [[polytene|polytene chromosomes]] and discovered that the wild populations were polymorphic for [[chromosomal inversions]]. All the flies look alike whatever inversions they carry: this is an example of a cryptic polymorphism. Accordingly, Dobzhansky favoured the idea that the morphs became fixed in the population by means of [[Sewall Wright]]'s [[Genetic drift|drift]].<ref>Dobzhansky T. 1937. ''Genetics and the origin of species''. Columbia University Press, New York. (2nd ed 1941; 3rd ed 1951)</ref> However, evidence rapidly accumulated to show that natural selection was responsible:
[[Image:Drosophila polytene chromosomes 2.jpg|right|thumb|250 px|''Drosophila'' polytene chromosome]]
1. Values for heterozygote inversions of the third chromosome were often much higher than they should be under the null assumption: if no advantage for any form the number of heterozygotes should conform to N<sub>s</sub> (number in sample) = p<sup>2</sup>+2pq+q<sup>2</sup> where 2pq is the number of heterozygotes (see [[Hardy-Weinberg equilibrium]]).
2. Using a method invented by L'Heretier and Teissier, Dobzhansky bred populations in ''population cages'', which enabled feeding, breeding and sampling whilst preventing escape. This had the benefit of eliminating [[insect migration|migration]] as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions. It was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which they become stabilised. With ''D. persimilis'' he found that the caged population followed the values expected on the Hardy-Weinberg equilibrium when conditions were optimal (which disproved any idea of non-random mating), but with a restricted food supply heterozygotes had a distinct advantage.
3. Different proportions of chromosome morphs were found in different areas. There is, for example, a polymorph-ratio [[cline]] in ''[[D. robusta]]'' along an 18-mile transect near [[Gatlingsburg]] [[TN]] passing from 1,000 feet to 4,000 feet.<ref>Stalker H.D and Carson H.L. 1948. An altitudinal transect of ''Drosophila robusta''. ''Evolution'' '''1''', 237-48.</ref> Also, the same areas sampled at different times of year yielded significant differences in the proportions of forms. This indicates a regular cycle of changes which adjust the population to the seasonal conditions. For these results selection is by far the most likely explanation.
4. Lastly, morphs cannot be maintained at the high levels found simply by mutation, nor is drift a possible explanation when population numbers are high.
By the time Dobzhansky published the third edition of his book in 1951 he was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms. Later he made yet another interesting discovery. One of the inversions, known as PP, was quite rare up to 1946, but by 1958 its proportion had risen to 8%. Not only that, but the proportion was similar over an area of some 200,000 square miles in California. This cannot have happened by migration of PP morphs from, say, Mexico (where the inversion is common) because the rate of dispersal (at less than 2km/year) is of the wrong order. The change therefore reflected a change in prevailing selection whose basis was not yet known.<ref>Dobzhansky T. 1970. ''Genetics of the evolutionary process''. Columbia University Press N.Y.</ref><ref>[Dobzhansky T.] 1981. ''Dobzhansky's genetics of natural populations''. eds Lewontin RC, Moore JA, Provine WB and Wallace B. Columbia University Press N.Y.</ref><ref>Ford E.B. 1975. ''Ecological genetics''. 4th ed. Chapman & Hall, London.</ref>
==== Chromosomal polymorphism in general ====
In 1973, [[M.J.D. White]], then at the end of a long career investigating [[karyotypes]], gave an interesting summary of the distribution of chromosome polymorphism.
:"It is extremely difficult to get an adequate idea as to what fraction of the species of [[eukaryote]] organisms actually are polymorphic for structural rearrangements of the chromosomes. In [[Diptera|Dipterous]] flies with [[Polytene|polytene chromosomes]]... the figure is somewhere between 60 and 80 percent... In [[grasshopper]]s [[Chromosomal inversion|pericentric inversion]] polymorphism is shown by only a small number of species. But in this group polymorphism for [[B chromosomes|super-numerary chromosomes]] and chromosome regions is very strongly developed in many species."
:"It is clear that the nature of natural populations is a very complicated subject, and it now appears probable that [[annidation]] (adaptation of the various [[genotypes]] to different [[ecological niches]]) and [[frequency-dependent selection]] are at least as important, and probably more important in many cases, than simple [[heterosis]] (in the sense of increased viability or fecundity of the [[heterozygote]])."<ref>White M.J.D. 1973. The chromosomes. Chapman & Hall, London. 6th ed, p166-7.</ref>
This suggests, once again, that polymorphism is a common and important aspect of adaptive evolution in natural populations.
==== Heterostyly ====
[[Image:Distyly primula.jpg|thumb|left|180px|Dissection of thrum and pin flowers of ''[[Primula vulgaris]]'']]
An example of a botanical genetic polymorphism is [[heterostyly]], in which flowers occur in different forms with different arrangements of the [[Flower#Flower anatomy|pistil]] and the [[Flower#Flower anatomy|stamen]]s. The system is called [[self-incompatibility in plants#heteromorphic self-incompatibility|heteromorphic self-incompatibility]], and the general 'strategy' is known as [[herkogamy]].
[[Pin and thrum]] heterostyly occurs in dimorphic species of [[Primula]], such as ''[[Primula 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'' 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.<ref>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.</ref><ref>Darwin, Charles 1877. ''The different forms of flowers on plants of the same species''. Murray, London.</ref>
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 out-crossing, as described by Darwin. <br>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]].<ref>Ford E.B. ''Ecological genetics''. 3rd ed, Chapman & Hall, London 1971 chapter 10.</ref><ref>Sheppard P.M. 1975. ''Natural selection and heredity''. 4th ed Hutchinson, London.</ref><ref>Maynard Smith J. ''Evolutionary genetics''. 2nd ed, Oxford 1998 p86.</ref> All sections of the genus ''Primula'' have heterostyle species, altogether 354 species out of 419.<ref>Bruun H.G. 1938. Studies on heterostyle plants 2. ''Svensk. Bot. Tidskr.'' '''32''', 249-260.</ref> Since heterostyly is characteristic of nearly all races or species, the system is therefore at least as old as the genus.<ref>Darlington C. 1958. ''Evolution of genetic systems'', 2nd ed, p120 et seq: The genetic promotion of crossing. Oliver & Boyd, London.</ref></br>
Between 1861 and 1863 Darwin found the same kind of structure in other groups: [[flax]] (and other species of ''[[Linum]]''); and in [[purple loosestrife]] and other species of ''[[Lythrum]]''. Some of the Lythrum species are trimorphic, with one style and two stamens in each form.<ref>Barrett PH (ed) 1977. ''The collected papers of Charles Darwin''. Chicago University Press.</ref>
Heterostyly is known in at least 51 genera of 18 families of Angiosperms.<ref>Darlington C. 1971. The evolution of polymorphic systems. In Creed R. (ed) ''Ecological genetics and evolution''. Blackwell, Oxford.</ref><ref>Charlesworth B & D. 1979. The evolutionary genetics of sexual systems in flowering plants. ''Proc Royal Soc B'' '''205''', 513-30.</ref>
====Darwin's finches====
Whereas [[Charles Darwin|Darwin]] spent just five weeks in the [[Galápagos]], and [[David Lack]] spent three months, [[Peter and Rosemary Grant]] and their colleagues have made research trips to the Galápagos for about thirty years, particularly studying [[Darwin's finch]]es. Here we look briefly at the case of the large cactus finch ''[[Geospiza conirostris]]'' on Isla [[Genovesa]] (formerly Tower Island) which is formed from a shield volcano, and is home to a variety of birds. These birds, like all well-studied groups,<ref>Huxley J. 1955. Morphism in birds. ''11th Int Ornith Cong'' (Basel 1954, p309-328) touches on this theme.</ref> show various kinds of morphism.
Males are dimorphic in song type: songs A and B are quite distinct. Also, males with song A have shorter bills than B males. This is also a clear difference. With these beaks males are able to feed differently on their favourite cactus, the prickly pear ''[[Opuntia]]''. Those with long beaks are able to punch holes in the cactus fruit and eat the fleshy [[aril]] pulp which surrounds the seeds, whereas those with shorter beaks tear apart the cactus base and eat the pulp and any insect larvae and pupae (both groups eat flowers and buds). This dimorphism clearly maximises their feeding opportunities during the non-breeding season when food is scarce.
Territories of type A and type B males are random if not mated but alternate if mated: no two breeding males of the same song type shared a common boundary. This initially suggested the possibility of [[assortative mating]] by [[female choice]].<ref>Grant B.R. & P.R. 1979. Darwin's finches: population variation and sympatric speciation. ''Proc Natl Acad Sci USA''. '''76''', 2359–2363.</ref><ref>Grant P.R & B.R. 1989. Sympatric speciation and Darwin's finches. In Otte D & Endler J (eds) ''Speciation and its consequences''. Sinauer.</ref> However, further work showed that "the choice of a male by a female is independent of any conditioning influence of her father's song type {and] there is no evidence of assortative mating by bill type... Hence there is no direct evidence of reproductive subdivision in the population."<ref>B. Rosemary Grant and Peter R. Grant 1989. ''Evolutionary dynamics of a natural population: the large cactus finch of the Galápagos''. Chicago, p241 first para.</ref> In 1999 Peter Grant agreed that "sympatric speciation [in this example] is unlikely to occur".<ref>Grant, Peter R. 1999. ''Ecology and evolution of Darwin's finches''. Princeton NJ, p428 in Afterword.</ref>
If the population is [[Panmixia|panmixic]], then ''Geospiza conirostris'' exhibits a balanced genetic polymorphism and not, as originally supposed, a case of nascent [[sympatric speciation]]. The selection maintaining the polymorphism maximises the species' niche by expanded its feeding opportunity. The genetics of this situation cannot be clarified in the absence of a detailed breeding program, but two loci with [[linkage disequilibrium]]<ref>Maynard Smith J. 1998. ''Evolutionary genetics''. 2nd ed, Chapter 5, Oxford.</ref> is a possibility.
Another interesting dimorphism is for the bills of young finches, which are either 'pink' or 'yellow'. All species of Darwin's finches exhibit this morphism, which lasts for two months. No interpretation of this phenomenon is known.<ref>Grant, Peter R. 1999. ''Ecology and evolution of Darwin's finches''. Princeton NJ. (see plate 7)</ref>
== Relative frequency ==
Endler's survey of natural selection gave an indication of the relative importance of polymorphisms among studies showing natural selection.<ref>Endler J.A. 1986. ''Natural selection in the wild''. Princeton: Table 5.2 and Sections 5.2 & 5.3, p154-162. [Endler's Table 5.1 gives details of the individual investigations used to compile Table 5.2]</ref> The following is a precis of key points:
*Number of species demonstrating natural selection: 141<br>Number showing quantitative traits: 56<br>Number showing polymorphic traits: 62<br>Number showing both Q and P traits: 23</br>
This summary shows that polymorphisms are found to be at least as common as continuous variation in studies of natural selection, and hence just as likely to be part of the evolutionary process.
==See also==
*[[CEPH]]
*[[Single nucleotide polymorphism]]
==References==
{{reflist}}
==External links==
*[http://www-biol.paisley.ac.uk/bioref/Genetics/Primula_heterostyly.html Heterostyly in the Cowslip (''Primula veris'' L.)]
* {{cite web | last = McNamara | first = Don | title = Notes on Rearing Scarlet tiger moth ''Callimorpha dominula'' (L.)|publisher=[[Amateur Entomologists' Society]] | date = 1998 | url = http://www.amentsoc.org/mcnamara.htm | accessdate = 2006-08-15}}
*[http://www.meine-molekuele.de/polymorphismus Molecular individuality (German online-book)]
[[Category:Evolution]]
[[Category:Biological evolution]]
[[Category:Subfields and areas of study related to evolutionary biology]]
[[Category:Evolutionarily significant biological phenomena]]
[[Category:Genetics]]
[[Category:Population ecology]]
[[Category:Zoological nomenclature]]
[[de:Polymorphismus]]
[[es:Polimorfismo genético]]
[[fr:Polymorphisme (génétique)]]
[[lt:Polimorfizmas]]
[[nl:Polymorfisme (genetica)]]
[[ja:多型]]
[[pl:Polimorfizm (genetyka)]]
[[ru:Полиморфизм (биология)]]
[[uk:Поліморфізм (біологія)]]