Richard,
Very interesting question! One ive often thought about myself. On many levels the wide vareity of human "races" are quite different. Obviously visually we are all quite different. This vareity between us is heightened by the fact that we are tuned into the minute differences between us, making individual recognition possible. Im quite sure this happens in many organisms. Many social structures, even rudimentary ones, would have trouble functioning without such an ability to identify individuals.
Even so its hard to ignore the racial differences that exist in humans. Beyond the visual, differences exist on the molecular, immunological and genetic levels to some degree.
However, it should be noted that such differences take up a tiny amount of the total genetic variation thats out there.
As the only technologically intelligent species on this planet we are slightly different from every other species on the planet. We have been able to adapt to environments using technology (such as clothing and fire) with a bare minimum of physical adaptation.
Its entirely possible that, without further technological advancement, separate species of humans may have developed on the various remote regions of the planet.
But the same ability to traverse the planet has improved with time, ensuring that the flow of genetic material is constantly returning back into the central pool, no matter how far apart we get.
We are the only species with a cultural paramater (language, tradition, religion etc). And to many people that can be seen as a difference, but its important to remember that having a culture of any kind is encompassed in the realm of the human condition.
From a social point of view and from a genetic security view it makes no sense to separate humans into separate races or species. We need that genetic variety. Otherwise some pandemic or other would wipe us out.
Socially, we dont need to see eachother as different, otherwise we would wipe eachother out!!
But on the simplest level. The simplest genetically. The simplest socially. The simplest psychologically. And the simplest evolutionary....it all comes down to one little thing....sex.
The male of our species is not fussy. The cross breeding paramater, often so crucial in determining levels of speciation these days will never be satisfied in our species.
Owen