simple questions. please write clearly. if you use source, put the link. How has
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simple questions. please write clearly. if you use source, put the link.
How has natural selection affected the distribution of (native) skin piqmentation across the world? In this informative essay, I expect an exploration of the natural phenomena responsible for the distribution of skin pigmentation across the globe as well as the selective mechanisms by which these natural p altered ski henomena have in pigmentation over human history. When you are trying to understand and explain how natural selection has altered a characteristic, you should always think of how a selective mechanism has (or could h impacted a group of organisms (i.e. who is more likely to reproduce because of this, who is less likely, and why?) ave)Explanation / Answer
Human skin color is varies considerably around the world, ranging from very dark brown in Africans, Australian Aborigines, and Melanesians to a near yellowish pink in Northern Europeans. However, there are no people with true black, white, red, or yellow skin. Before mass globalization, dark skin color was predominant in the southern hemisphere near the equator and light color progressively increased as people migrated away.
Individuals near the equator required protection against the sun’s rays that damages the body. Hence, they started developing darker skin to protect against the sun’s damaging rays. Melanin is the pigment in the skin the gives brown color. All people have melanin, they just differ in the amount of this pigment produced. It is a natural sunscreen for people in the tropics against ultraviolet rays. The relative intensity of UV radiation may be responsible for changes in skin color. UV rays can strip folic acid that is essential for human fetal development. UV rays penetrate the skin and are however important for vitamin D production. Vitamin D is required for calcium absorption for bone development.
Migration from equator to the colder regions with less UV rays favored selection of people with lighter skin color. Light skin color allowed UV rays to penetrate the skin more and produce more vitamin D. However, UV rays had not much effect on evolution of skin color, as it is known to cause skin cancer. Natural selection is responsible for evolution changes that favor improved reproductive success.
Folate is required by the body for DNA biosynthesis, repair, DNA methylation, amino acid metabolism, and melanin production. Folate deficiencies can result in fatal birth defects such as neural tube defects. Dark skin evolved to prevent reduction of fertility as folate can undergo photolysis in cutaneous blood vessels. Dark skin allowed for more folate production. Folate is required in rapidly dividing cells of the embryo and seminiferous tubules. Hence, folate deficiencies caused by UV radiations could affect both female and male fertility
African populations majorly lack any functionally significant variation in the coding region of the melanocortin 1 receptor (MC1R). This gene majorly regulates human eumelanin production, and can help maintain dark pigmentation under intense selective pressure. African however have increased polymorphisms in other loci. Dark skin occurs due to tanning that evolved secondarily under positive selection in people that moved from low- to high-UVR environments. These changes in pigment may have appeared the lightly and moderately pigmented “Ancestral North Indians” into darkly pigmented people of the Indian subcontinent. Similarly, pigmented changes would cause the lightly and moderately pigmented East Asians moving into the high UV radiation region of Central and mountainous South America to become darker (Nine and Chaplin, PNAS 2010) .
Vitamin D is a product of sunlight at 290–310 nm in the UVB range and is absorbed by the skin to form preVitaminD3. The vitamin D endocrine system regulates bone metabolism, the innate immune response and cell proliferation/differentiation. Vitamin D acted as a selective pressure as its deficiency can cause reduction in fertility. Vitamin D deficiency causes rickets, risk of automimmune diseases such as multiple sclerosis and diabetes, and increase in bacterial infections.
Natural selection promoted vitamin D production by loss of constitutive pigmentation under conditions where reduced UVR was strong. UVB is required for vitamin D3 synthesis. Loss of melanin pigmentation favoured more UVB absorption by light-skinned people that allowed vitamin D synthesis. Hence, there was a positive selection for depigmentation. Similarly, there was evolution of hominine population of depigmented skin. The diet of seafood is rich in vitamin D. Hence, Artic people in Alaska and Canada can remain dark skinned even in low UV area, as their diet is rich in seafood. High levels of UV rays in summer are reflected from the snow and ice surfaces. Dark skin helps to prevent the skin from reflected light.
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