Question

In humans, lighter skin color is adaptive at latitudes far from the equator because UV light

 stimulates synthesis of folic acid

 causes breakdown of folic acid

 stimulates synthesis of vitamin D

 causes breakdown of vitamin D

Answer 1

In humans, lighter skin color is adaptive at latitudes far from the equator because UV light:

stimulates synthesis of vitamin D.

Skin color is primarily due to the presence of a pigment called melanin. Melanin is normally located in the epidermis. It is produced at the base of the epidermis by specialized cells called melanocytes. These cells have photosensitive receptors, similar to those in the eye , that detect ultraviolet radiation from the sun and other sources. In response, they produce melanin within a few hours of exposure.

Melanin acts as a protective biological shield against UV rays since the UV rays creates sunburn diseases that could result in DNA changes and, subsequently, several kinds of maignant skin cancers. So, nature selected for people with darker skin in tropical latitudes, where ultraviolet rays from the sun is usually the most intense.

But it would be harmful if melanin acted as a complete sheild. Because a certain amount of shortwave UV B must penetrate the outer skin layer in order for the body to produce vitamin D.

That is why people who live at latitudes far from the equator, where solar radiation is relatively weak most of the year, have an advantage if their skin has little shielding pigmentation or having lighter skin color. So, Nature selects for less melanin humans when ultraviolet radiation is weak. In such an environment, very dark skin is disadvantage because it can prevent people from producing enough vitamin D, potentially resulting in rickets and osteoporesis.