Melanism

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Melanistic black eastern grey squirrel Melsquir.jpg
Melanistic black eastern grey squirrel
Melanistic guinea pigs are rare, and are used in rituals by Andean curanderos. Cavia porcellus-Licorice.jpg
Melanistic guinea pigs are rare, and are used in rituals by Andean curanderos .

The term melanism refers to black pigment and is derived from the Greek : μελανός. [2] Melanism is the increased development of the dark-colored pigment melanin in the skin or hair.

Contents

Pseudomelanism, also called abundism, is another variant of pigmentation, identifiable by dark spots or enlarged stripes, which cover a large part of the body of the animal, making it appear melanistic. [3]

The morbid deposition of black matter, often of a malignant character causing pigmented tumors, is called melanosis. [4]

Adaptation

A melanistic European adder (Vipera berus) compared to a normal-colored adder Hugorme.jpg
A melanistic European adder (Vipera berus) compared to a normal-colored adder

Melanism related to the process of adaptation is called adaptive. Most commonly, dark individuals become fitter to survive and reproduce in their environment as they are better camouflaged. This makes some species less conspicuous to predators, while others, such as black panthers, use it as a foraging advantage during night hunting. [5] Typically, adaptive melanism is heritable: A dominant allele, which is entirely or nearly entirely expressed in the phenotype, is responsible for the excessive amount of melanin.

Adaptive melanism has been shown to occur in a variety of animals, including mammals such as squirrels, many cats and canids, and coral snakes. Adaptive melanism can lead to the creation of morphs, the most notable example being the peppered moth, whose evolutionary history in the United Kingdom is offered as a classic instructional tool for teaching the principles of natural selection. [6]

Industrial melanism

Industrial melanism is an evolutionary effect in insects such as the peppered moth, Biston betularia in areas subject to industrial pollution. Darker pigmented individuals are favored by natural selection, apparently because they are better camouflaged against polluted backgrounds. When pollution was later reduced, lighter forms regained the advantage and melanism became less frequent. [7] [8] [9] [10] [11] [12] Other explanations have been proposed, such as that the melanin pigment enhances function of immune defences, [13] or a thermal advantage from the darker coloration. [14] [15] [16]

In cats

Melanistic and normally coloured jaguars Melanism in Panthera Onca.jpg
Melanistic and normally coloured jaguars

Melanistic coat coloration occurs as a common polymorphism in 11 of 37 felid species and reaches high population frequency in some cases but never achieves complete fixation. The black panther, a melanistic leopard, is common in the equatorial rainforest of Malaya and the tropical rainforest on the slopes of some African mountains, such as Mount Kenya. The serval also has melanistic forms in certain areas of East Africa. In the jaguarundi, coloration varies from dark brown and gray to light reddish. Melanic forms of jaguar are common in certain parts of South America. [17] In 1938 and 1940, two melanistic bobcats were trapped alive in sub-tropical Florida. [18]

Pseudomelanism on a cheetah Acinonyx jubatus King Cheetah.jpg
Pseudomelanism on a cheetah

In 2003, the dominant mode of inheritance of melanism in jaguars was confirmed by performing phenotype-transmission analysis in a 116-individual captive pedigree. Melanistic animals were found to carry at least one copy of a mutant MC1R sequence allele, bearing a 15-base pair inframe deletion. Ten unrelated melanistic jaguars were either homozygous or heterozygous for this allele. A 24-base pair deletion causes the incompletely dominant allele for melanism in the jaguarundi. Sequencing of the agouti signalling peptide in the agouti gene coding region revealed a 2-base pair deletion in black domestic cats. These variants were absent in melanistic individuals of Geoffroy's cat, oncilla, pampas cat and Asian golden cat, suggesting that melanism arose independently at least four times in the cat family. [19]

Melanism in leopards is inherited as a Mendelian, monogenic recessive trait relative to the spotted form. Pairings of black animals have a significantly smaller litter size than other possible pairings. [20] Between January 1996 and March 2009, Indochinese leopards were photographed at 16 sites in the Malay Peninsula in a sampling effort of more than 1000 trap nights. Of 445 photographs of melanistic leopards, 410 were taken south of the Kra Isthmus, where the non-melanistic morph was never photographed. These data suggest the near fixation of the dark allele in the region. The expected time to fixation of this recessive allele due to genetic drift alone ranged from about 1,100 years to about 100,000 years. [21] Melanism in leopards has been hypothesized to be causally associated with a selective advantage for ambush. [22] Other theories are that genes for melanism in felines may provide resistance to viral infections, or a high-altitude adaptation, since black fur absorbs more heat. [23]

In birds

White Silkie rooster Poule Soie gris perle.jpg
White Silkie rooster
Black Silkie rooster Coq poule soie noir.png
Black Silkie rooster

The Silkie chicken commonly exhibits this trait. In April 2015, an extremely rare black flamingo was spotted on the Mediterranean island of Cyprus. [24]

Ayam Cemani is an uncommon and relatively modern breed of chicken from Indonesia. They have a dominant gene that causes hyperpigmentation (Fibromelanosis), making the chicken entirely black; including feathers, beak, and internal organs.

In humans

Melanism, meaning a mutation that results in completely dark skin, does not exist in humans[ citation needed ]. Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmful ultraviolet radiation. The same ultraviolet radiation is essential for the synthesis of vitamin D in skin, so lighter colored skin - less melanin - is an adaptation related to the prehistoric movement of humans away from equatorial regions, as there is less exposure to sunlight at higher latitudes. People from parts of Africa, South Asia, Southeast Asia, and Australia may have very dark skin, but this is not melanism.

Peutz–Jeghers syndrome

This rare genetic disorder is characterized by the development of macules with Hyperpigmentation on the lips and oral mucosa (melanosis), as well as benign polyps in the gastrointestinal tract. [25]

Socio-politics

The term melanism has been used on Usenet, internet forums and blogs to mean an African-American social movement holding that dark-skinned humans are the original people from which those of other skin color originate. The term melanism has been used in this context as early as the mid-1990s [26] and was promoted by some Afrocentrists, such as Frances Cress Welsing.

See also

Related Research Articles

Peppered moth Species of insect

The peppered moth is a temperate species of night-flying moth. Peppered moth evolution is an example of population genetics and natural selection.

A black panther is the melanistic colour variant found in members of the genus Panthera, particularly of the leopard in Asia and Africa, and the jaguar in the Americas. Black panthers of both species have excess black pigments, but their typical spotted markings are also present. Melanism in the leopard is caused by a recessive allele, and in the jaguar by a dominant allele.

E. B. Ford

Edmund Brisco "Henry" Ford was a British ecological geneticist. He was a leader among those British biologists who investigated the role of natural selection in nature. As a schoolboy Ford became interested in lepidoptera, the group of insects which includes butterflies and moths. He went on to study the genetics of natural populations, and invented the field of ecological genetics. Ford was awarded the Royal Society's Darwin Medal in 1954.

Bernard Kettlewell

Henry Bernard Davis Kettlewell was a British geneticist, lepidopterist and medical doctor, who performed research on the influence of industrial melanism on peppered moth coloration, showing why moths are darker in polluted areas. This experiment is cited as a classic demonstration of natural selection in action. After live video record of the experiment with Niko Tinbergen, Sewall Wright called the study as "the clearest case in which a conspicuous evolutionary process has actually been observed."

Michael Eugene Nicolas Majerus was a British geneticist and professor of evolution at the University of Cambridge. He was also a teaching fellow at Clare College, Cambridge. He was an enthusiast in Darwin's theory of evolution by natural selection and became a world authority in his field of insect evolutionary biology. He was widely noted for his work on moths and ladybirds and as an advocate of the science of evolution. He was also an enthusiastic educator and the author of several books on insects, evolution and sexual reproduction. He is best remembered as an ardent supporter and champion of experiments on peppered moth evolution.

Peppered moth evolution significance of the peppered moth in evolutionary biology

The evolution of the peppered moth is an evolutionary instance of directional colour change in the moth population as a consequence of air pollution during the Industrial Revolution. The frequency of dark-coloured moths increased at that time, an example of industrial melanism. Later, when pollution was reduced, the light-coloured form again predominated. Industrial melanism in the peppered moth was an early test of Charles Darwin's natural selection in action, and remains as a classic example in the teaching of evolution. In 1978 Sewall Wright described it as "the clearest case in which a conspicuous evolutionary process has actually been observed."

Bruce S. Grant is emeritus professor of biology at the College of William and Mary. He has a particular research interest in the peppered moth, He is a defender of the teaching of evolution and has criticized creationist Jonathan Wells, who has cited his work, as "dishonest."

<i>Melanism: Evolution in Action</i>

Melanism: Evolution in Action (ISBN 0-19-854982-2) is a book by Dr. Mike Majerus, published in 1998. It is an update of Bernard Kettlewell's book The Evolution of Melanism.

Plumage

Plumage is a layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, there can be different colour morphs. The placement of feathers on a bird is not haphazard, but rather emerge in organized, overlapping rows and groups, and these feather tracts are known by standardized names.

Equine coat color genetics

Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. Extension and agouti are particularly well-known genes with dramatic effects. Differences at the agouti gene determine whether a horse is bay or black, and a change to the extension gene can make a horse chestnut instead. Most domestic horses have a variant of the dun gene which saturates the coat with color so that they are bay, black, or chestnut instead of dun, grullo, or red dun. A mutation called cream is responsible for palomino, buckskin, and cremello horses. Pearl, champagne and silver dapple also lighten the coat, and sometimes the skin and eyes as well. Genes that affect the distribution of melanocytes create patterns of white such as in roan, pinto, leopard, white, and even white markings. Finally, the gray gene causes premature graying, slowly adding white hairs over the course of several years until the horse looks white. Some of these patterns have complex interactions.

<i>Adalia bipunctata</i> Species of beetle

Adalia bipunctata, the two-spot ladybird, two-spotted ladybug or two-spotted lady beetle, is a carnivorous beetle of the family Coccinellidae that is found throughout the holarctic region. It is very common in western and central Europe. It is also native to North America but it has heavily declined in many states and provinces. It is commonly introduced and imported as a biological control agent.

Black squirrel

Black squirrels are a melanistic subgroup of squirrels with black coloration on their fur. The phenomenon occurs with several species of squirrels, although it is most frequent with the eastern gray squirrel and the fox squirrel. Black morphs of the eastern gray and fox squirrels are the result of a abnormal pigment gene. Several theories have surfaced as to why the black morph occurs, with some suggesting that the black morph is a selective advantage for squirrels inhabiting the northern ranges of the species, with the black-fur providing a thermal advantage over its non-melanistic counterpart.

Industrial melanism

Industrial melanism is an evolutionary effect prominent in several arthropods, where dark pigmentation (melanism) has evolved in an environment affected by industrial pollution, including sulphur dioxide gas and dark soot deposits. Sulphur dioxide kills lichens, leaving tree bark bare where in clean areas it is boldly patterned, while soot darkens bark and other surfaces. Darker pigmented individuals have a higher fitness in those areas as their camouflage matches the polluted background better; they are thus favoured by natural selection. This change, extensively studied by Bernard Kettlewell, is a popular teaching example in Darwinian evolution, providing evidence for natural selection. Kettlewell's results have been challenged by zoologists, creationists and the journalist Judith Hooper, but later researchers have upheld Kettlewell's findings.

Kettlewells experiment

Kettlewell's experiment was a biological experiment in the mid-1950s to study the evolutionary mechanism of industrial melanism in the peppered moth. It was executed by Bernard Kettlewell, working as a research fellow in the Department of Zoology, University of Oxford. He was investigating the cause of the appearance of dark-coloured moth since Industrial Revolution in England in the 19th century. He conducted his first experiment in 1953 in the polluted woodland of Birmingham, and his second experiment in 1955 in Birmingham as well as in the clean woods of Dorset.

Judith Hooper is an American journalist.

Light skin Human skin color

Light skin is a human skin color, which has little eumelanin pigmentation and which has been adapted to environments of low UV radiation. Light skin is most commonly found amongst the native populations of Europe and Asia as measured through skin reflectance. People with light skin pigmentation are often referred to as "white" or "fair", although these usages can be ambiguous in some countries where they are used to refer specifically to certain ethnic groups or populations.

<i>Biston strataria</i> Species of moth

Biston strataria, the oak beauty, is a moth of the family Geometridae. It is native to Europe, but is primarily found in the United Kingdom. The species was first described by Johann Siegfried Hufnagel in 1767. B. strataria is found in a variety of habitats, but is mostly found in woodlands where it rests on the bark of trees, camouflaged by its mottled black and grey wings. The male has feather-like antennae while those of the female are more thread-like. The moth has a wingspan of 40 to 56 mm.

Amelanism

Amelanism is a pigmentation abnormality characterized by the lack of pigments called melanins, commonly associated with a genetic loss of tyrosinase function. Amelanism can affect fish, amphibians, reptiles, birds, and mammals including humans. The appearance of an amelanistic animal depends on the remaining non-melanin pigments. The opposite of amelanism is melanism, a higher percentage of melanin.

Dark skin Human skin color

Dark skin is a human skin color that is rich in melanin pigments, especially eumelanin. People with very dark skin are often referred to as "black people", although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.

The agouti gene (ASIP) is responsible for variations in color in many species. Agouti works with extension to regulate the color of melanin which is produced in hairs. The agouti protein causes red to yellow pheomelanin to be produced, while the competing molecule α-MSH signals production of brown to black eumelanin. In wildtype mice, alternating cycles of agouti and α-MSH production cause agouti coloration. Each hair has bands of yellow which grew during agouti production, and black which grew during α-MSH production. Wildtype mice also have light-colored bellies. The hairs there are a creamy color the whole length because the agouti protein was produced the whole time the hairs were growing.

References

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Bibliography