question: Why are our eyes colored?
comment: we're not really sure why humans have different eye colors. Most humans have dark / brown eyes. There are also animals with different eye colors. The coolest of them are arctic reindeers. Their eye color changes with the season - they are golden in summer and blue in winter. So one could think that blue eyes see better when there's less light, or that the brown / dark hue is an adaptation to protect the eye from too much sunlight - just like dark skin. Green eyes might be just blue eyes with some brown in it. relevant reindeer study [here]( URL_0 ) & #x200B;
passages: Section::::Pigmentation.
  
  The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related—although problems of convergence do exist.
  
  Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides.
  
  The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.
   Section::::In mammals.
  
  The only pigments that mammals produce are melanins. For a mammal to be unable to chemically manufacture melanin renders it completely pigmentless. This condition is more commonly called albinism. Amelanistic mammals have white hair, pink skin, and eyes that have a pink, red, or violet appearance. Reddish eyes are due to the lack of pigment in the iris pigment epithelium. When the stroma is unpigmented but the iris pigment epithelium is not, mammalian eyes appear blue. Melanin in the pigment epithelium is critical for visual acuity and contrast.
   Section::::Mate selection and traits that have been linked to iris color.
  
  Section::::Mate selection and traits that have been linked to iris color.:Selection for rare iris colors.
   Section::::Eye color.:Genetic and physical factors determining iris color.
  
  Iris color is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, fibrous tissue and blood vessels within the iris stroma, which together make up an individual's epigenetic constitution in this context. A person's "eye color" is actually the color of one's iris, the cornea being transparent and the white sclera entirely outside the area of interest.
   The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids. Humans and other animals have many phenotypic variations in eye color. The genetics of eye color are complicated, and color is determined by multiple genes. So far, as many as 15 genes have been associated with eye color inheritance. Some of the eye-color genes include "OCA2" and "HERC2". The earlier belief that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. However, OCA2 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation.
   Most new-born babies who have European ancestry have light-colored eyes. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continually produce pigment, in theory eye color can be changed. Adult eye color is usually established between 3 and 6 months of age, though this can be later. Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages. An iris that appears golden contains some melanin even at this early age and is likely to turn from blue to green or brown as the infant ages.
   The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, or blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculo-cutaneous albinism, or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal human iris color is the dark pigment melanin. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of a person. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair. Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue and green-eyed people.
   Section::::Human adaptation.:Evolutionary origins.
   Section::::Genetic determination.
   Melanin is a compound found in plants, animals, and protists, and is derived from the amino acid tyrosine. Melanin is a photoprotectant, absorbing the DNA-damaging ultraviolet radiation of the sun. Vertebrates have melanin in their skin and hair, feathers, or scales. They also have two layers of pigmented tissue in the eye: the stroma, at the front of the iris, and the iris pigment epithelium, a thin but critical layer of pigmented cells at the back of the iris. Melanin is also present in the inner ear, and is important for the early development of the auditory system. Melanin is also found in parts of the brain and adrenal gland.
   Eye color is determined primarily by the concentration and distribution of melanin within the iris tissues. Although the processes determining eye color are not fully understood, it is known that inherited eye color is determined by multiple genes. Environmental or acquired factors can alter these inherited traits.
  
  The human iris can be seen in a number of various colors. There are three true colors in human eyes that determine the outward appearance: brown, yellow, and grey. The amount of each color an individual has determines the appearance of the eye color.
   BULLET::::- The vascular tunic (or uvea) is made up of the choroid, the ciliary body, and the iris. The choroid has a great deal of pigment, and is almost entirely made of blood vessels. It forms the tapetum lucidum when it crosses over the fundus of the eye, causing the yellowish-green eye shine when light is directed into the animal's eyes at night. The tapetum lucidum reflects light back onto the retina, allowing for greater absorption in dark conditions. The iris lies between the cornea and the lens, and not only gives the eye its color, ("see "eye color," below") but also allows varying amounts of light to pass through its center hole, the pupil.
   Melanin in the eyes, in the iris and choroid, helps protect them from ultraviolet and high-frequency visible light; people with gray, blue, and green eyes are more at risk for sun-related eye problems. Further, the ocular lens yellows with age, providing added protection. However, the lens also becomes more rigid with age, losing most of its accommodation — the ability to change shape to focus from far to near — a detriment due probably to protein crosslinking caused by UV exposure.
   Section::::Eye color chart (Martin scale).:Brown.
  
  In humans, brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.
  
  Dark brown eyes are dominant in humans and in many parts of the world, it is nearly the only iris color present. Dark pigment of brown eyes is common in Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, Africa and the Americas. The majority of people in the world overall have brown eyes to dark brown eyes.
   Iris (anatomy)
  
  In humans and most mammals and birds, the iris (plural: irides or irises) is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm.
  
  Section::::Structure.
  
  The iris consists of two layers: the front pigmented layer known as a stroma and, beneath the stroma, pigmented epithelial cells.
   Eye color in non-human animals is regulated differently. For example, instead of blue as in humans, autosomal recessive eye color in the skink species "Corucia zebrata" is black, and the autosomal dominant color is yellow-green.
  
  As the perception of color depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color.
  
  Section::::Changes in eye color.
   Melanin is yellowish-brown to dark brown in the stromal pigment cells, and black in the iris pigment epithelium, which lies in a thin but very opaque layer across the back of the iris. Most human irises also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an overt influence on the overall color. The degree of dispersion of the melanin, which is in subcellular bundles called melanosomes, has some influence on the observed color, but melanosomes in the iris of humans and other vertebrates are not mobile, and the degree of pigment dispersion cannot be reversed. Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see heterochromia, below). Colors other than brown or black are due to selective reflection and absorption from the other stromal components. Sometimes lipofuscin, a yellow "wear and tear" pigment, also enters into the visible eye color, especially in aged or diseased green eyes.
   Many songbirds (like evening grosbeak, yellow warbler, common yellowthroat and Javan green magpie) deposit lutein obtained from the diet into growing tissues to color their feathers.
  
  Section::::Role in human eyes.
  
  Although lutein is concentrated in the macula – a small area of the retina responsible for three-color vision – the precise functional role of retinal lutein has not been determined.
  
  Section::::Role in human eyes.:Macular degeneration.
   The sclera, also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the human eye containing mainly collagen and some elastic fiber. In humans, the whole sclera is white, contrasting with the coloured iris, but in other mammals the visible part of the sclera matches the colour of the iris, so the white part does not normally show. In the development of the embryo, the sclera is derived from the neural crest. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, fatty deposits on the sclera can make it appear slightly yellow. Many people with dark skin have naturally darkened sclerae, the result of melanin pigmentation.
   Section::::Mate selection and traits that have been linked to iris color.:Selection though imprinting of parental eye color.
  
  In contrast to the phenomenon of selection for rarity, scholarship has implied the existence another form of eye color involvement in mate selection. A study found a significant incidence of the partners of heterosexual people possessing similar eye and hair color to that of their opposite-sex parent. This is suggestive of a form of parental imprinting on eventual mate selection.
  
  Section::::See also.
  
  BULLET::::- Hair color
  
  BULLET::::- Limbal ring
  
  BULLET::::- Iridology
  
  BULLET::::- Human skin color
  
  BULLET::::- Xanthophore
  
  BULLET::::- List of Mendelian traits in humans
   The gene "OCA2" (), when in a variant form, causes the pink eye color and hypopigmentation common in human albinism. (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within "OCA2" are strongly associated with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an "OCA2" regulatory sequence, where they may influence the expression of the gene product, which in turn affects pigmentation. A specific mutation within the "HERC2" gene, a gene that regulates "OCA2" expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are SLC24A4 and TYR. A 2010 study on eye color variation into hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, and now about 50% of eye colour variation can be explained.
   Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color; lighter eye color is also associated with an increased risk of ARMD progression. A gray iris may indicate the presence of a uveitis, and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes. However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight.
  
  Section::::Medical implications.:Wilson's disease.
   It is fairly common for mammalian eyes to change color in the first years of life. This happens, with human infants and kittens being some well-known examples, because the eyes of the baby, just like the rest of its body, are still developing. This change can be as simple as blue to brown, or can involve multiple color changes in which neither the child's parents nor his/her doctors know when the changes will stop and what the final eye color will be.
   As early as 2004, evidence was compiled showing that the bulk of phenotypic variation in human iris color could be attributed to polymorphisms in the OCA2 gene. This paper, and the work it cited, laid the foundation for the inference of human iris color from DNA, first carried out on basic level by DNAPrint Genomics Beginning in 2009, academic groups developed and reported on more accurate predictive models for eye color and, more recently, hair color in the European population. More recently, companies such as Parabon NanoLabs and Identitas have begun offering forensic DNA phenotyping services for U.S. and international law enforcement.
   Section::::Blood supply.:In bony fish.
  
  Teleosts bear a body of capillary adjacent to the optic nerve called the choroidal gland. Though its function is not known, it is believed to be a supplemental oxygen carrier.
  
  Section::::Mechanism.
  
  Melanin, a dark colored pigment, helps the choroid limit uncontrolled reflection within the eye that would potentially result in the perception of confusing images.
  