Skin Color
Skin color is the amount of melanin we have in our skin created by cells called melanocytes. It is a spectrum.
Skin color evolved so that we fit in with our environment. There is lower UV radiation from the Sun in high latitude areas (meaning away from the equator) There is greater UV radiation in lower latitude areas (near the equator). Of course UV radiation also changes seasonally, with solar weather and with weather patterns on Earth.
Lighter skin color advantage is that there is better Vitamin D synthesis in higher latitude areas.
Vitamin D is necessary for many physiological processes such as calcium development/bone integrity.
Lighter skin disadvantage is that when exposed to high levels of UV radiation such as near the equator or during the summer, the individuals burn and increase their risk for skin/eye cancer. There is also evidence that people with lighter eyes are more prone to diseases such as macular degeneration because melanin tends to be protective in some cases.
Dark skin advantage is basically the opposite of lighter skin as it is exponentially less common to get skin/eye cancer and macular degeneration the more darker pigmented you are.
Dark skin disadvantage is that you cannot absorb sufficient quantities of UV light to synthesize Vitamin D in high latitude areas. Vitamin D must be absorbed from the diet as a substitute.
Evolution basically shows us that the extremely low chances of skin/eye cancer do not outweigh the needs for Vitamin D absorption in high latitude areas necessary for bone structure integrity and other processes. Macular degeneration tends to affect individuals only later in life so again evolution mostly focuses on humans during their reproductive age. This is the opposite in equatorial areas.
There is a genetic component to skin color as well as an epigenetic component (environmental effect).
The genetic component is mainly regulated by the gene SLC24A5.
Melanocortin 1 receptor is the receptor behind epigenetic regulation of our skin color. Commonly referred to as tanning.
There is a medical scale of skin color (skin pigmentation). Its called the Fitzpatrick scale:
Fitzpatrick Skin Types
SKIN COLOR CHARACTERISTICS
I White Always burns, never tans
II White Usually burns, tans with difficulty
III White Sometimes burns, easily tans
IV Brown Rarely burns, easily tans
V Dark Brown Very rarely burns, easily tans
VI Black Does not burn, easily tans
East Asians also evolving lighter skin pigmentation is an example of convergent evolution.
Sources:
1) Lamason RL, Mohideen MA, Mest JR, et al. (December 2005). "SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans". Science. 310 (5755): 1782–6. Bibcode:2005Sci...310.1782L. doi:10.1126/science.1116238. PMID 16357253.
2) Fitzpatrick Skin Types:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwinwN3Gj8bXAhUN0IMKHZpzBIoQFggvMAE&url=http%3A%2F%2Fimg.medscape.com%2Fpi%2Femed%2Fckb%2Fotolaryngology%2F834279-837993-842489-1470476.doc&usg=AOvVaw3AMtfzUAbNILkc4xhf2-QG