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, 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 Dipterous flies with polytene chromosomes... the figure is somewhere between 60 and 80 percent... In grasshoppers pericentric inversion polymorphism is shown by only a small number of species. But in this group polymorphism for 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 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)".
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 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. Geographical distribution of blood groups (the differences in gene frequency between populations) is broadly consistent with the classification of "races" developed by early anthropologists on the basis of visible features.
Sickle-cell anaemia Sickle-cell anaemia is found mostly in tropical populations in Africa and India. An individual homozygous for the recessive sickle hemoglobin, HgbS, has a short expectancy of life, whereas the life expectancy of the standard hemoglobin (HgbA) homozygote as well as heterozygote is normal (though heterozygote individuals will suffer periodic problems). The sickle-cell variant survives 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.
Duffy system The Duffy antigen is a protein located on the surface of red blood cells, encoded by the FY (DARC) gene. 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.
In humans, a mutant variant at a single site in the FY cis-regulatory region abolishes all expression of the gene in erythrocyte precursors. As a result, homozygous mutants are strongly protected from infection by P. vivax, and a lower level of protection is conferred on heterozygotes. The variant has apparently arisen twice in geographically distinct human populations, in Africa and Papua New Guinea. It has been driven to high frequencies on at least two haplotypic backgrounds within Africa. Recent work indicates a similar, but not identical, pattern exists in baboons (Papio cynocephalus), which suffer a mosquito-carried malaria-like pathogen, Hepatocystis kochi. Researchers interpret this as a case of convergent evolution.
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.
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 orangutans and chimpanzees for PTC perception with positive results, thus demonstrating the long-standing existence of this dimorphism. The 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.
MHC molecules The genes of the major histocompatibility complex (MHC) are highly polymorphic, 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. DNA repair gene polymorphisms also appear to influence the risk of lung cancer.
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 Rana limnocharis, Rana ridibunda, Rana sylvatica and Rana arvalis; this means that individuals that are homozygous dominate or heterozygous display the stripe, whereas only homozygous recessive individuals are non-striped. The proportions of striped specimens in populations of some frogs show clinal variations. For example, the proportion of striped Rana sylvatica in North America generally increases towards the west and north. The variations in the proportion of different color may relate to either genetic-stochastic processes. or their adaptive importance. For different colour morphs of Acris crepitans, the hypothesis about the direct adaptive value of different colour morphs (for escaping predation) competes with the hypothesis that these morphs correlate with thermotolerance. Striped specimens Rana sylvatica, striped specimens better perform in open areas. Differences in the proportion of striped frogs in Rana arvalis are explained with physiological differences between the morphs. Striped recently metamorphosed frogs have a relatively large liver, in comparison with unstriped ones, and their weight increases more rapidly. Tadpoles of striped Rana arvalis need more time for completing metamorphosis but, after metamorphosis, their growth is faster than that of unstriped froglets. In a frog widespread in Turkey and the Caucasus, Rana macrocnemis, the proportion of frogs with the stripe increases with the altitude in mountains of the Lesser Caucasus, but not in the Greater Caucasus. Given the same altitude, non-striped frogs from the Greater Caucasus grow slower and maturate later than the striped frogs from the Lesser Caucasus, which provides them selective advantage in high mountains, but their tadpoles are likely to be less resistant to overheating than those of the non-striped frogs.Tarkhnishvili, D. N., 1996. Genetic relationships in local populations of brown frogs — analysis of distribution of a character under selection. In: Population Genetic Group, 30th annual meeting, University of Edinburgh, 17-20 Dec. 1996, Paper Abstr., p.42
Birds Cuckoo
Over fifty species in this family of birds practice brood parasitism; the details are best known in the common cuckoo (Cuculus canorus). In a season the female lays one egg in 15–20 other bird nests. She removes some or all of the host's clutch of eggs, and lays an egg which closely matches the host eggs. 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 parasitising the host to which it is adapted. The male cuckoo mates with females from any gens; thus the population (all gentes) is interbreeding.
Egg colour is inherited by sex chromosome. In birds sex determination is ZZ/ZW, and unlike mammals, the heterogametic sex is the female. The determining gene (or super-gene) for the inheritance of egg colour is believed to be carried on the W chromosome, which is directly transmitted in the female line. The female behaviour in choosing the host species is set by imprinting after birth, a common mechanism in bird behaviour.
Ecologically, the system of multiple hosts maximises the egg-laying capacity of the population of cuckoos and extends the range of habitats where the cuckoo eggs may be raised successfully. Detailed work on the cuckoo started with E. Chance in 1922, and continues to the present day.
Darwin's finches Whereas 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 finches. The Española cactus finch (Geospiza conirostris) lives 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, show various kinds of morphism.