Melanism (
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
:
μελανός = dark-colored) is the occurrence of an increased amount of dark pigmentation (as of skin, feathers, eyes or hair) in an organism, resulting from the presence of
melaninMelanin is a class of compounds found in plants, animals, and protists, where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine. Many melanins are insoluble salts and show affinity to water...
. It is the opposite of leucism and
albinismAlbinism is a form of hypopigmentary congenital disorder, characterized by a partial or total lack of melanin pigment in the eyes, skin and hair, or more rarely...
, which occur because of a lack of melanin or other types of pigment.
Abundism is the occurrence of excessively abundant dark markings (such as spots, stripes or other patch types) due to increased dark pigmentation on the coat, fur or skin of some animal
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
.
Melanism (
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
:
μελανός = dark-colored) is the occurrence of an increased amount of dark pigmentation (as of skin, feathers, eyes or hair) in an organism, resulting from the presence of
melaninMelanin is a class of compounds found in plants, animals, and protists, where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine. Many melanins are insoluble salts and show affinity to water...
. It is the opposite of leucism and
albinismAlbinism is a form of hypopigmentary congenital disorder, characterized by a partial or total lack of melanin pigment in the eyes, skin and hair, or more rarely...
, which occur because of a lack of melanin or other types of pigment.
Abundism is the occurrence of excessively abundant dark markings (such as spots, stripes or other patch types) due to increased dark pigmentation on the coat, fur or skin of some animal
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
. As the number and/or size of these markings increases, some overlap and merge causing the effect of
pseudomelanism where the animal appears completely black. The background color may still be discerned between the markings.
Melanism (and pseudomelanism) is often the result of genetic
mutationIn biology, a mutation is a randomly derived change to the nucleotide sequence of the genetic material of an organism.Mutations can be caused by copying errors in the genetic material during cell division, or by exposure to mutagens , or can be induced by the organism itself, by cellular processes...
, but can result from other stimuli, such as exposure to abnormal temperature changes during gestation which transiently alter gene
transcriptionTranscription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes...
or
translationTranslation is the first stage of protein biosynthesis . Translation is the production of proteins by decoding mRNA produced in transcription. Translation occurs in the cytoplasm where the ribosomes are located. Ribosomes are made of a small and large subunit which surrounds the mRNA...
.
In the context of diseases, the condition in which abnormally dark pigmentation of some tissues occurs is coined
melanosis.For a description of melanin-related disorders see
melaninMelanin is a class of compounds found in plants, animals, and protists, where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine. Many melanins are insoluble salts and show affinity to water...
,
melanosis coliMelanosis coli is a disorder of pigmentation of the wall of the colon, identified at the time of colonoscopy. It is benign, and has no significant correlation with disease...
and
ocular melanosisOcular melanosis , also known as ocular melanocytosis or melanosis oculi, is a disease of the eye which in dogs is almost found exclusively in the Cairn Terrier. Until recently it was known as pigmentary glaucoma. The disease is caused by an increase of melanocytes in the iris, sclera, and...
.
Adaptive melanism
Melanism related to the process of
adaptationAdaptation is the process whereby a population becomes better suited to its habitat. This process takes place over many generations, and is one of the basic phenomena of biology....
is called adaptive. Most commonly, dark individuals become
fitterFitness is a central concept in evolutionary theory. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual's genes in all the genes of the next generation...
to survive and reproduce in their environment as they are better camouflaged. This makes some species less conspicuous to predators while others use it as a foraging advantage (for example during night hunting). Typically adaptive melanism is
heritableIn genetics, Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Variation among individuals may be due to genetic and/or environmental factors...
: A dominant
geneA gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cells and pass genetic traits to offspring...
(which is entirely or nearly entirely expressed in the
phenotypeA phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions...
) is responsible for the excessive amount of melanin.
Adaptive melanism has been shown to occur in a variety of animals, including mammals (squirrels, many felines, many canids), reptiles (coral snakes) and insects (peppered moth).
Adaptive melanism can lead to the creation of
morphsPolymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
, the most notable example being the
peppered mothThe peppered moth is a temperate species of night-flying moth. Peppered moth evolution is often used by educators as an example of natural selection.- Ecology and life cycle :...
whose evolutionary history in the United Kingdom is offered as a classic instructional tool for teaching the principles of
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
.
Industrial melanism
Industrial melanism is adaptive melanism caused by
anthropogenicAnthropogenic effects, processes or materials are those that are derived from human activities, as opposed to those occurring in biophysical environments without human influence....
alteration of the
natural environmentThe natural environment, commonly referred to simply as the environment, is a term that encompasses all living and non-living things occurring naturally on Earth or some region thereof....
in terms of industrial
pollutionPollution is the introduction of contaminants into an environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms . Pollution can take the form of chemical substances, or energy, such as noise, heat, or light...
. As
sootSoot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
, smoke and other industrial pollutants from factories darken the landscape and because many organisms rely on camouflage to avoid
predationIn ecology, predation describes a biological interaction where a predator feeds on its prey, . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey...
, this sudden change in their environment makes them highly vulnerable to predators. This creates a strong
selectiveNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
pressure which will see any organism with a darker colour much more likely to survive and contribute to the
gene poolIn population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...
of the next generation. Rare mutations are hence selected for and over time the population will adjust to a new equilibrium.
Peppered moth evolutionThe evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens which they rested upon...
is commonly used as an example of industrial melanism.
Examples
Many examples of melanism are known among
felineFelidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the sixteen mammal families in the order Carnivora...
s. Melanism is due to changes in the
agouti geneAgouti signalling peptide, also referred to as Agouti, is a peptide consisting of 131 amino acids. Its discovery was published in 1994 in the scientific magazine Nature where its functional properties were described...
which controls banding of black and light areas on the hair shaft.
LeopardThe leopard , Panthera pardus, is a member of the Felidae family and the smallest of the four "big cats" in the genus Panthera; the other three being the tiger, lion and jaguar...
s and
jaguarThe Jaguar, Panthera onca, is a big cat, a feline in the Panthera genus. It is the only Panthera species found in the Americas. The jaguar is the third-largest feline after the tiger and the lion, and the largest and most powerful feline in the Western Hemisphere...
s with this condition are often called
black pantherA black panther is a large black cat. Black panthers are color variants of several species of larger cat. Wild black panthers in Latin America are black jaguars , in Asia and Africa they are black leopards , and in North America reported black panthers may be black jaguars or possibly black...
(although cougars are also known as panthers, there are no verified cases of melanism in that species). However, the leopard, the jaguar, the lion and the tiger are all members of the
PantheraPanthera is a genus of the family Felidae , which contains four well-known living species: the Tiger, the Lion, the Jaguar, and the Leopard. The genus comprises about half of the Pantherinae subfamily, the big cats. One meaning of the word panther is to designate cats of this subfamily...
genus. One good example of melanism expressed within a certain animal community is that of the leopard population in Malaysia, South East Asia, in which case up to 50% of the population has melanism. That is apparently due to them being more
crypticCryptic can refer to:* Crypsis, of animals that are difficult to observe* Cryptic crossword, a crossword with cryptic clues* Cryptic era, earliest period of the Earth* Cryptic, an album by Edge of Sanity* Cryptic Writings, an album by Megadeth...
in their dusky rainforest
habitatThe term habitat has a number of meanings:* Habitat , a place where a species lives and grows** Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
. Better resistance to
virusA virus is an infectious agent too small to be seen directly with a light microscope. They are not made of cells and can only replicate inside the cells of another organism . Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es may also explain the greater prevalence of black leopards in those areas.
In the Jaguar, melanism is due to a dominant gene mutation meaning that black jaguars may produce spotted offspring. In the leopard, melanism is due to a recessive gene mutation meaning that two spotted leopards carrying the gene may produce black cubs, but black leopards will breed true when mated together. It is believed that Tigers and Lions may also have the potential to be melanistic, but while there may have been black lions and tigers in the past, it is also quite possible melanistic bloodlines are extinct.
Melanism and the immune system
Melanism has been found to be linked to beneficial changes in the immune system.
The Smithsonian Answer Book: Cats notes that genes for melanism in felines may provide resistance from viral infections and that a viral epidemic may explain the prevalence of black leopards in Java and Malaysia, and the relatively high incidence of black leopards and black servals in the Aberdares region of Africa. Previously, black furred felines in the Aberdares had been considered a high altitude adaptation due to absorbing more heat.
Studies reported in
New ScientistNew Scientist is a weekly international science magazine and website covering recent developments in science and technology for a general English-speaking audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of Reed Elsevier. New Scientist has maintained a...
magazine in 2003 also suggested that recessive-gene melanism is linked to disease resistance rather than altitude. According to Eduardo Eizirik and Stephen O'Brien of the United States National Cancer Institute in Maryland, the melanism mutations involve the same gene family as those involved in human. Melanistic cats may therefore have better resistance to disease than cats with "normal" colour coats. This would explain why recessive melanism persists when melanistic individuals are disadvantaged due to being poorly camouflaged in open areas.
In the United States National Cancer Institute studies, black cats were found to have changes to a gene known as MC1R. MC1R is a member of a family of genes that includes the human gene CCR5 which codes for a protein on the cell membrane. Melanism could make black cats less susceptible to certain viral infections making melanism an evolutionary advantage.
Melanism as a socio-political movement
The term
melanism has been used on
usenetUsenet, a portmanteau of "user" and "network" , is a worldwide distributed Internet discussion system. It evolved from the general purpose UUCP architecture of the same name....
,
internet forumAn ', or message board, is an online discussion site. It originated as the modern equivalent of a traditional bulletin board, and a technological evolution of the dialup bulletin board system. From a technological standpoint, forums or boards are web applications managing user-generated content...
s and
blogA blog is a type of website, usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse-chronological order...
s to mean an African-American
social movementSocial movements are a type of group action. They are large informal groupings of individuals and/or organizations focused on specific political or social issues, in other words, on carrying out, resisting or undoing a social change....
holding that dark-skinned humans are in some measures superior to those of other skin colour. The term melanism has been used in this context as early as the mid-1990s and was promoted by some Afrocentrists, such as
Frances Cress WelsingFrances Cress Welsing is an African American psychiatrist practicing in Washington, D.C. She is noted for her "Cress Theory of Color Confrontation", which explores the practice of white supremacy...
.
Further reading
See also
- Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
- Albinism
Albinism is a form of hypopigmentary congenital disorder, characterized by a partial or total lack of melanin pigment in the eyes, skin and hair, or more rarely...
- Leucism
- Vitiligo
Vitiligo is a chronic disorder that causes depigmentation in patches of skin. It occurs when the melanocytes, the cells responsible for skin pigmentation which are derived from the neural crest, die or are unable to function. The precise pathogenesis, or cause, of vitiligo is complex and not yet...