What is the reason for regional variances in the appearance of humans in a clustered population?

Regional variances in appearances in human beings can be accounted for by the simple fact that, as a race, humans have adapted for life in different areas. For example, if you were to look at North America as a clustered population, you would notice that the closer you get to the equator in Mexico, the darker the skin color.

This is an evolutionary response to sunlight. Lighter skin lets in more sunlight so that we can make vitamin D. Darker skin is a response to living in a place with a lot of sunlight, and therefore needs a better defense against the UV rays that the sun emits. This is the best example of a regional appearance variance that can easily be explained by where people live.