Melanism

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]

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

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 light for warmth.[23]

White Silkie rooster

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.

Melanism, meaning a mutation that results in completely dark skin, does not exist in humans. 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.

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.

• Albinism
• Albino and white squirrels
• Amelanism
• Black squirrel
• Erythrism
• Heterochromia iridum
• Leucism
• Piebaldism
• Vitiligo
• Xanthochromism
• Peutz–Jeghers syndrome
• Ocular melanosis
• Melanosis

Wikimedia Commons has media related to Melanism.

• David Attenborough (2002). The Life of Mammals (TV-Series and book). United Kingdom: BBC.
• Kettlewell, Bernard (1973). The Evolution of Melanism. Clarendon Press. ISBN 0-19-857370-7.
• Majerus, Michael (1998). Melanism: Evolution in Action. Oxford University Press. ISBN 0-19-854982-2.
• Melanism and disease resistance in insects
• Fryer, G. 2013. How should the history of industrial melanism in moths be interpreted? The Linnean. 29 (2): 15 - 22.