Thanks for the great question; it’s one that we should all think about.

To begin, it is a fact that people categorize others on the basis of their physical appearance, ethnicity, ancestry, social relations, and the interaction of all of these which we call race. However, race is not a defined term in biology and cannot solely be explained by different groups having different genes. Indeed there is much more genetic variation within any given population of humans than all the variation between human populations. Therefore, genetically, it is the case that humans share much more in common than they diverge. This is why we are all a part of the same species: Homo sapiens. Our perception of people as belonging to different races may then be a product of human psychology rather than biological reality.

But your question still needs to be answered, how do we explain the observed, or phenotypic, differences between different groups of humans? Evolution by natural selection is one means. When modern humans left Africa some 100,000 to 50,000 years ago, they migrated quickly all over the world to climates very much different than where they evolved. In these new conditions, different traits were better suited for survival and reproduction in different areas. For instance near the equator where we evolved, it was beneficial to have darker skin to resist the intense sunlight. However as people migrated to extreme northern latitudes, those with lighter skin (due to random genetic variation) were better able to survive as they could absorb more sunlight needed to synthesize important vitamins.

Lighter skin phenotypes therefore spread in these regions. Another example: people in Eurasia domesticated herd animals like cows and sheep, and eventually evolved the ability to digest milk beyond infancy, whereas people in other regions are lactose intolerant for life.

So over time natural selection has changed the traits of different groups of humans based on their local environment. It is important to note however that not many traits have been selected for, and humans in fact demonstrate shockingly low genetic diversity compared to other primates. All humans share more genes in common than any two groups of chimpanzees for instance.

In brief, I would answer your question by saying that populations of humans have evolved traits that make them suited to their environment, but we have not evolved into different races or even different sub-species. Rather each human is a different expression of the same set of genes, with some expressions more common based on geographical location.

Excellent question.

Good question. In some cases, it's natural selection caused by the differing amounts of sunlight on different parts of the Earth.

In tropical areas like Africa, Central America, southern India, and Indonesia, there is a lot more sunlight than in far-northern places like Canada and Scandinavia or far-southern places like Patagonia and New Zealand. Melanin, the pigment that gives human skin its color, provides some help in resisting sunburn, but in small amounts also allows skin to make vitamin D from sunlight. This means that people in these sun-baked tropical climates tended to evolve darker skin while people in the sun-starved areas tended to evolve lighter skin: it's why Kenyan skin is very dark while Norwegian skin is very pale.

Skin color isn't the only difference between the different races of humans however: for example, people from eastern Asia like Japan and China tend to have slanted eyes while people from elsewhere tend to have rounder eyes. I'm not sure if an explanation for this has ever been identified. It is possible that these differences are the result of an evolutionary process known as genetic drift: random evolutionary changes that have nothing to do with making an organism more or less able to survive. Genetic drift is known to happen to traits that are evolutionarily neutral, i.e. they neither benefit their owners nor harm them. It is possible that eye shape in humans does not affect the ability to survive and so is not subject to natural selection, with the result that it evolves due to genetic drift instead.

To drive the point on genetic drift home: it is possible today for dark-skinned people to buy foods in the supermarket that have vitamin D, and for light-skinned people to wear sunscreen when they go out into the sun. This would remove the effects of natural selection on human skin color, since it is no longer valuable to survive. I predict that, in the future, human skin color may begin to evolve in response to genetic drift instead of due to natural selection as it has in the past.