Sam Tazzyman answered on 20 Jun 2011:
The first thing to say is that different races in humans are not like different subspecies in other types of animal. If you look at the world before humans started moving about so much, the variation between people was gradual from place to place, rather than being an abrupt change. And on top of this, if you look at the genetics of people, there really is very little difference between people of different races – the differences are superficial, and only for a few genes. The average difference in genes between two people of the same race is not much less than the average distance between two people of different races.
However, we can often look at someone and say what race they are, and certainly people often define themselves as being of a certain race. Quite a lot of this is down to skin colour, which is a result of evolution. Human skin colour is down to how much of a chemical called melanin that people have in their skin. Of course this can change from day to day – this is what happens when you get a tan, it’s your skin producing more melanin, to protect you from the sun. And this is a clue to why some people have darker skin than others. The other clue is the fact that people need to produce vitamin D, and this happens when the skin absorbs sunlight.
The human species originated in Africa, where the sun is very hot, and having darker skin gave an evolutionary advantage because it protected people from the sun. But then when people moved out of Africa, they moved north (if you look on a world map you can see that people must have moved out into the Middle East, and from there into Asia and Europe, because they couldn’t cross the sea as they hadn’t yet invented boats). As they went north, the sun became less hot where they lived, and so people’s skin colour started to vary a bit more, and they became lighter. When they got to northern Europe, the sun was very weak, and a mutation occurred that made people’s skin very light. This was beneficial because it enabled people to make vitamin D even in the far north. Ancestors of people who went to Asia from Africa never developed this mutation and so stayed a darker colour. When they went down into Southern Asia and Australia it was again very hot so again they developed a darker skin. The Siberian people went across to the Americas by walking from the eastern end of Russia to Alaska when the sea there was frozen. They then spread into the Americas, and as you would expect the ones who ended up nearer the equator (where it was hotter) developed darker skin.
Of course, nowadays, for historical reasons, there are lots of countries (like the UK) in which there are people of all different races living together. And nowadays the slight evolutionary benefit to having a certain skin colour in a certain place is more than counteracted by modern medical technology and good diet and so on.
As for animals, there are animals that vary from one region to another – a good example is the fox. The different species of fox in different places look very different, from arctic foxes with white fur, to Fennec foxes in the desert which have huge ears to help them cool down.