List of polymorphisms
In biology, polymorphism is the occurrence of two or more clearly different forms or phenotypes in a population of a species. Different types of polymorphism have been identified and are listed separately.
General
Chromosomal polymorphism
In 1973,
- "It is extremely difficult to get an adequate idea as to what fraction of the species of super-numerary chromosomesand 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 adaptation of the various heterozygote)".[1]
This suggests, once again, that polymorphism is a common and important aspect of adaptive evolution in natural populations.
Sexual dimorphism
Humans
Human blood groups
All the common blood types, such as the ABO blood group system, are genetic polymorphisms. Here we see a system where there are more than two morphs: the phenotypes 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.
Statistical research has shown that an individual of a given phenotype will generally be, compared to an individual of a differing phenotype, more resistant to certain diseases while less resistant to others. For example, an individual's susceptibility to
Sickle-cell anaemia
Duffy system
The Duffy antigen is a protein located on the surface of red blood cells, encoded by the FY (DARC) gene.[8] The protein encoded by this gene is a non-specific receptor for several chemokines, and is the known entry-point for the human malarial parasites Plasmodium vivax and Plasmodium knowlesi. Polymorphisms in this gene are the basis of the Duffy blood group system.[9]
In humans, a mutant variant at a single site in the FY cis-regulatory region abolishes all expression of the gene in
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 less than 1% outside Africa and the Middle East.[11][12]
Human taste morphisms
A famous puzzle in human genetics is the genetic ability to taste phenylthiocarbamide (phenylthiourea or PTC), a morphism which was discovered in 1931. This substance, which is bitter to some people and tasteless to others, 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. 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. 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.
Fisher, Ford and Huxley tested
MHC molecules
The genes of the major histocompatibility complex (MHC) are highly polymorphic,[15] and this diversity plays a very important role in resistance to pathogens. This is true for other species as well.
Cancer
DNA repair gene polymorphisms are associated with increased risk of colorectal cancer development.[16][17] DNA repair gene polymorphisms also appear to influence the risk of lung cancer.[18]
Amphibians
Mid-dorsal stripe in frogs
Some frog species display polymorphism by presence/absence of a light stripe going along the central part of their back. A light mid-dorsal stripe has been shown to be
determined by a simple dominant gene in
Birds
Cuckoo
Over fifty species in this family of birds practice
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 scirpaceus): light dull green eggs blotched with olive.
- Common 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
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. It also extends the range of habitats where the cuckoo eggs may be raised successfully. Detailed work on the cuckoo started with E. Chance in 1922,[34] and continues to the present day; in particular, the inheritance of gens is still a live issue.
Darwin's finches
Whereas Darwin spent just five weeks in the
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
If the population is 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 expanding 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[40]: ch. 5 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.[39]: plate 10
White-throated sparrows
The
Their heads are either white-striped or tan-striped. These differences in plumage result from a balanced chromosomal inversion polymorphism; in white-striped (WS) birds, one copy of chromosome 2 is partly inverted, while in tan-striped (TS) birds, both copies are uninverted.[41]
The plumage differences are paralleled by differences in behavior and breeding strategy. WS males sing more, are more aggressive and more frequently engage in extra-pair copulation than their TS counterparts.[42] TS birds of both sexes provide more parental care than WS birds.
The polymorphism is maintained by negative assortative mating—each morph mates with its opposite.[43] Dimorphic pairs may have an advantageous balance between parental care and aggressive territorial defense. In addition, as in many other polymorphisms, heterozygote advantage seems to help maintain this one; the proportion of WS birds homozygotic for the inversion is even lower than would be expected from the low frequency (4%) of pairings of the same morph.[44]
In the underlying chromosomal polymorphism, the standard (ZAL2) and alternative (ZAL2m) arrangements differ by a pair of included pericentric inversions at least. ZAL2m suppresses recombination in the heterokaryotype and is evolving as a rare nonrecombining autosomal segment of the genome.[45]
Fish
Arctic char
Insects
Ants
Chromosome polymorphism in Drosophila
In the 1930s Dobzhansky and his co-workers collected
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 Ns (number in sample) = p2+2pq+q2 where 2pq is the number of heterozygotes (see
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 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 (29 km) transect near Gatlinburg, TN passing from 1,000 feet (300 m) to 4,000 feet.[55] 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 (520,000 km2) 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 2 km/year) is of the wrong order. The change therefore reflected a change in prevailing selection whose basis was not yet known.[5][32][56]
Hoverfly polymorphism
Hoverfly mimics can be seen in almost any garden in the
Many social wasp (
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.[57] Also, not only is polymorphism absent from these mimics, it is absent in the wasps also: these facts are presumably connected.[58]
The situation with
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.[60][61][62]
Peppered moth
The peppered moth,
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. 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 or typica), and carbonaria, the melanic black morph. They are controlled by
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).[33] No species which hide during the day, for instance, among dead leaves, is affected, nor has the melanic change been observed among butterflies.[66][64][67] This is, as shown in many textbooks, "evolution in action".
Much of the early 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.[68] This book was misrepresented in some reviews, and the story picked up by creationist campaigners.
Judith Hooper, in her controversial book Of Moths and Men (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 accusation of fraud was unjustified, and that "Hooper does not provide one shred of evidence to support this serious allegation".[69][70] Majerus himself described Of Moths and Men as "littered with errors, misrepresentations, misinterpretations and falsehoods".[68] A suitably restrained 2004 summary of opinion mostly favoured predation as the main selective force.[71]
Starting in 2000, Majerus conducted a detailed seven-year study of moths, experimenting to assess the various criticisms. He concluded that differential bird predation was a major factor responsible for the decline in carbonaria frequency compared to typica in Cambridge during the study period,[72] and described his results as a complete vindication of the peppered moth story. He said, "If the rise and fall of the peppered moth is one of the most visually impacting and easily understood examples of Darwinian evolution in action, it should be taught. It provides after all the proof of evolution."[73]
Current interpretation of the available evidence is that the peppered moth is in fact a valid example of natural selection and adaptation. It illustrates a polymorphic species maintaining adaptation to a varied and sometimes changing environment.
Two-spotted ladybird beetle
Scarlet tiger moth
The
The moth is known to be polymorphic in its colony at
In this instance the genetics appears to be simple: two
Mammals
Reindeer and caribou
Genetic polymorphism of serum
Molluscs
Cepaea snails
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 stand out against the background. So predation may be one factor influencing the different proportion of phenotypes (morphs) found in woodland than in open habitats.[82]
In addition, there is evidence of apostatic selection, with the birds preferentially taking the most common morph.[83] This is the 'search image' effect, where a visual predator persists in targeting the morph which it had most often found. Apostatic selection could explain the polymorphism within sites, but song thrushes are not everwhere present.
Dark coloured shells are at greater risk of overheating in the sun.[84] This "climatic selection" provides an alternative explanation for the observed pattern of paler shells in open habitats,[85] and also explains the broad trend of paler shells at more southern latitudes (in C. nemoralis, but not C. hortensis).[81][86]
Different colour morphs occupy different microhabitats, which provides another explanation for why different morphs coexist within a site.
Plants
Heterostyly
An example of a botanical genetic polymorphism is
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 halfway down; and the thrum flower has a short style, so the stigma is halfway 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.[90][91]
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. 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
Between 1861 and 1863, Darwin found the same kind of structure in other groups:
Heterostyly is known in at least 51 genera of 18 families of Angiosperms.[96][97]
Heterocarpy
Another example is heterocarpy, in which two or more different fruit morphs occur within an individual or population.
Heterospermy
Heterospermy is the occurrence of two or more different types of seed within an individual (e.g. as in Calendula officinalis) or population.
Heterophylly
Heterophylly is the presence of two or more different forms of leaves within an individual.
Reptiles
Common side-blotched lizards
Male common side-blotched lizards (Uta stansburiana) exhibit polymorphism in their throat pigmentation, and these different phenotypes are correlated with different mating strategies. Orange-throated males are the largest and most aggressive, defending large territories and keeping harems of females. Blue-throated males are of intermediate size, and guard smaller territories containing only a single female. Yellow-throated males are the smallest, and instead of holding territories they mimic females in order to sneak matings away from the other two morphs. The balance between these three morphs is maintained by frequency-dependent selection.[98][99]
Common wall lizards
The
Ctenophorus decresii
The Ctenophorus decresii lizard displays polymorphism with varying colors of their throats. The throat colors range from white and gray to bright colors of red, orange, or blue. The diversity in throat color is due to a combination of sexual selection and natural selection.
Ctenophorus pictus
Male Ctenophorus pictus lizards display different colors. The most common are red and yellow, but colors can range from brown to orange to red/orange. These morphs are maintained in nature through a combination of selective factors: natural selection and sexual selection.
Viviparous lizards
Viviparous lizards display color polymorphism in three ventral colors: yellow, orange, and a mixture of the two. These color morphs respond to variation in density frequency-dependence within their environment.
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