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What is the selective pressure for darker skin color at the equator?
People living near the equator generally have darker skin color than those living further away. This is due to natural selection acting on human populations based on UV radiation exposure. Areas with intense sunshine have higher levels of UV radiation which can damage the skin and cause health problems. Having more melanin, the pigment that makes skin darker, helps protect against UV damage. Thus, over time human populations near the equator evolved to have darker skin color through natural selection.
UV Radiation Exposure Increases Closer to the Equator
The Earth receives the most direct sunlight at the equator. UV radiation intensity increases exponentially as latitude decreases and the equator is approached. Areas closer to the equator receive more UV exposure for a few key reasons:
– The sunlight hits the Earth most directly at the equator without being scattered or absorbed as the rays pass at an angle through the atmosphere. Areas further away from the equator receive sunlight at lower angles.
– The ozone layer, which absorbs UVB radiation, is naturally thinner near the equator.
– There is less cloud cover and rainfall near the equator, so the sun’s rays reach the surface more often.
– The hours of sunlight are more constant throughout the year near the equator. Locations further away have fewer daylight hours in winter months.
The difference in UV exposure can be dramatic. For example, on the summer solstice, at noon:
– Quito, Ecuador (1° south of the equator) receives 2953 μW/cm2 of UV radiation
– Oslo, Norway (60° north of the equator) receives 1099 μW/cm2 of UV radiation
That’s nearly 3 times as much UV radiation in Quito compared to Oslo. The equator clearly experiences far higher levels of UV exposure year-round.
High UV Radiation Damages Skin and Health
The primary concern with intense UV radiation exposure is that it damages skin and can lead to various health problems:
– UVB radiation is absorbed by DNA and causes genetic mutations and skin cancer. Near the equator, rates of skin cancer and other UV related cancers are much higher than further away.
– UVA radiation destroys folate in the bloodstream and causes dangerous folate deficiency. Lack of folate is especially damaging during pregnancy.
– UV exposure also suppresses immune system functions making infectious diseases more likely.
– The sun’s rays harm the eyes by causing cataracts, macular degeneration, and other vision problems. These issues are more prevalent near the equator.
In addition to cancer and nutrient deficiency risks, severe sunburns from excessive UV radiation can be quite painful and dangerous on their own. Unprotected light-skinned individuals at the equator can experience blistering sunburns very quickly.
Melanin Pigmentation Protects Against UV Damage
Melanin is a pigment that gives skin, hair, and eyes their color. It absorbs and scatters UV radiation, protecting deeper layers of skin. There are two types of melanin:
– Eumelanin comes in shades of brown and black. It provides stronger UV protection.
– Pheomelanin is red and yellow. It has a weaker shielding effect.
People with dark brown or black eumelanin-dominated skin and hair have the best protection against UV radiation. Albinos who completely lack melanin are the most vulnerable.
Melanin concentration is determined by melanocyte cells which produce and distribute melanin granules. More active melanocytes result in increased melanin levels and darker coloration.
When the skin is exposed to UV radiation, melanocytes ramp up melanin production to protect the DNA in skin cells from damage. This melanin boosting process results in tanning. Indirectly, vitamin D synthesis which is powered by UVB rays also upregulates melanin production.
Natural Selection for Dark Skin Near the Equator
Populations living for long periods near the equator received intense, year-round UV exposure. This created evolutionary selective pressure that favored individuals with high levels of protective eumelanin.
Conversely, at higher latitudes with less UV exposure, survival was easier for both darkly pigmented and light pigmented people. Thus no single skin color was strongly selected for.
Over thousands of generations, natural selection produced darkly pigmented equatorial populations and lighter skin in cloudier northern regions:
| Location | UV Exposure | Average Skin Color |
|---|---|---|
| Central Africa | Very high | Deep brown |
| Northern Europe | Low | Pale pink |
This general gradation of indigenous skin color by latitude has been confirmed by global surveys, though other factors also influence local skin tones.
Some key mechanisms of natural selection for dark skin near the equator include:
– **Survival advantage** – Darker skin protected individuals from folate destruction, skin cancer, and other threats in high UV environments. Lighter skinned people were less likely survive and reproduce.
– **Sexual selection** – Darker skin was considered attractive as a sign of health. Lighter skin susceptible to burns or sores was seen as less desirable.
– **Mate selection** – People tended to choose darker skinned partners which led to darker skinned offspring.
Through this natural selection, dark skin pigmentation spread near the equator while light pigmentation dominated in the north. However, as humans migrated more recently, these tendencies have weakened allowing skin colors to blend across regions.
Other Evolutionary Factors Influencing Skin Color
While UV exposure and melanin adaptation are the key drivers of skin color differences by latitude, other selective forces have influenced local skin tones including:
– **Diet** – Natural diets high in certain nutrients and fats result in darker skin, even with low UV levels. For example, high fish intake.
– **Sexual trends** – Colorism within cultures leads people to favor lighter or darker skin aesthetically.
– **Cultural practices** – Traditions like body painting temporarily darken skin. Food processing methods affect nutrients impacting pigmentation.
– **Clothing** – More skin coverage blocks UV radiation, reducing natural tanning and selection for melanin production.
– **Altitude** – Higher elevations receive more intense UV exposure which also selects for darker skin.
So while UV radiation and melanin adaptation explain the major global skin color gradient from equator to poles, local nutrition, culture, clothing, and ecology lead to unique variations and exceptions.
Conclusion
In summary, populations near the equator evolved darker skin pigmentation over time primarily due to natural selection by intense UV radiation exposure. Higher levels of melanin protected the health and survival of early humans in these regions, leading to biological adaptation. This also demonstrates the powerful impact environmental pressures can have in shaping the physiology of organisms through natural selection. Understanding the evolutionary roots of human skin color variation helps build appreciation for how we’ve adapted to our habitats globally.
