The opposite of albinism: melanism

Everyone has heard of albinism. Animals with albinism have white fur or white feathers and red eyes. People with albinism also exist. They have very fair skin and a translucent iris. These people often also have other limitations such as reduced vision. Their skin is very sensitive to the sun and therefore burns quickly. This is all caused by the lack of melanin. The lack of melanin is caused by a genetic mutation. However, the opposite of this also exists and is much less known. This is called melanism and it occurs in humans and animals, just like albinism.

General features of melanism

Normally, melanin causes people to tan in the sun and this pigment causes birthmarks. The amount of melanin also determines the color of your iris and hair. The more melanin, the darker the color. Unlike albinism, melanism is a gene mutation that causes an abundance of the pigment melanin. The main characteristic of this condition is that the skin, fur or feathers are black. Organisms with melanin can tolerate a large amount of sunlight, unlike organisms with albinism. Animals where melanism often occurs include squirrels, lizards and zebras. In general, the gene variant (allele) for melanism is recessive, except in the jaguar (leopard). Melanism is dominant in this species .

Evolutionary melanism

Evolutionary melanism can arise when melanism is adaptive. This means that the condition (particularly the color) ensures a better chance of survival. When animals with melanism reproduce, a new subbreed may even arise. An example of this is the black panther. If it hunts in the dark, it is less noticeable, meaning its prey is easily caught and there is therefore a greater chance of survival. In fact, the black panther is a subbreed of the leopard. Cold-blooded animals (reptiles and amphibians) with melanism gain an advantage over their normal counterparts due to the condition. The black color makes them warm up faster, making them active more quickly. However, this condition also has a disadvantage: the black color causes heat to radiate more quickly.
A well-known description of evolutionary melanism comes from England around the time of industrialization. A type of moth, the salt-and-pepper moth, was found throughout Britain at this time. This butterfly was originally white-colored and in some cities appeared completely black-colored. These black-colored butterflies appeared to only occur in cities where coal was processed. The black soot emitted by these factories turned almost all trees darker. These butterflies normally sheltered on these trees. The white butterflies were noticed much earlier through the dark trees, which reduced their chance of survival, while the chance of survival of the black butterflies increased. As a result, the white specimen disappeared and the entire salt and pepper butterfly population became melanistic. This also explains the fact that melanism prevalences often vary by region.

Melanism in humans

A humanid form of melanism also exists, but it differs slightly from the animal form of melanism. In humans, the condition is called melanosis . The condition manifests itself in humans as abnormal dark pigmentation spots on the skin or other tissues. So these are not people with a black background. Another characteristic is that people with melanosis often have black spots in the eye around the iris. This condition is not adaptive in humans and can sometimes even cause damage to the body, such as malignant melanocytes. The dark spots are usually only malignant when they are not flat (like moles), but lie on the skin or the eye. This is not melanosis (benign), but a melanoma. These must be cut away.

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