Well, to be certain, there are clear physiological distinctions between different groups of people - coroners and archaeologists can often determine if bones have "Caucasoid" vs. "Mongoloid" features (look up Kennewick Man), and some groups are more prone to certain genetic diseases, such as Sickle-Cell in Africans and Cystic Fibrosis in Europeans.

As the idea of "Races", however, those distinctions are really pretty minor and the biological divisions between us all simply do not run as deep as they do between biologically distinct races in other animals. Today's population of humans descended from a group of about 10,000-20,000 people who survived some sort of disaster 70,000 years ago; probably the massive eruption of Mt. Toba on Sumatra. It's been speculated that the major ethnic divisions are approximately 15,000 years old, not enough time for us to diverge significantly. Humans have the lowest genetic diversity of ANY large aminal with a wide population distribution - a pretty amazing fact considering that we are so numerous and inhabit almost the entire planet! Only 25% of our genome has any variation whatsoever, and that's mostly very minor things like hair & skin color and nose shape. The other 75% of our genome is identical in all people. It's also been postulated that there's more genetic diversity on the continent of Africa than among all the other groups on Earth.

The concept of the "three main races", Caucasian, Mongoloid, and Negroid, comes from old theories of human migration that simply don't reflect the true complexity of human history. According to that idea, populations would have had to settle in the Caucasus Mountains, East Asia, and Africa, and remain isolated from the others for many thousands of years, and not come into contact with each other until very recently. Well that's just not what happened! People have been on the move since the beginning of the species, migrating, trading, fighting, resettling, etc . . ., with plenty of interaction and breeding between nearby populations, and so the current group of ethnicities shows a gradual gradation of traits as one moves geographically across any wide area. People in inner Mongolia may have not had any contact with folks in Africa, but they had contact with people in China, who had contact with people in Indochina, who had contact with people in Burma, and then India, and so on. Some groups, like the Andaman Island tribes, have been isolated from everybody else for a very long time (30,000 - 60,000 years on the Andamans, see www.andaman.org). But except for those very isolated groups, there really is no such thing as a "pure race" of any kind of people, because most ethnic groups simply have not been genetically isolated for long enough to be "pure" anything.

Dark skin color is an evolutionary adaptation to a high UV light influx; groups from tropical areas all have darker skin than those from cold or temperate climates. Light skin is probably an evolutionary response to cold weather. It's thought that a light-skinned population would eventually evolve dark skin in a few thousand years, and vice versa. Skin color is one of the most easily and rapidly changeable traits of our species, and so by itself leads to an oversimplified and incorrect classification of the groups within our species. And, as far as classifying human groups, why would we even need to do that? It IS important in science, medicine, anthropology, and a couple of other studies, but only in ways that apply to specialized aspects of those fields. For the human population as a whole, the classifications are interesting, and it helps one to connect more with one's group, but the group that we really need to connect with and understand is all of humanity. Because, aside from a couple of minor surface traits, all of us are really the same group of people!