Thanks, Setanta - really helpful. But one further question arising from your information on sexual isolation:
a. What is it?(!)
b. If it is the idea that the species Homo sapiens, already fully evolved and each alike to the other, spread all over the world and that racial difference occurred then and that, therefore, being very near descendents of the single race type human, can still interbreed, then of course I concur.
However, can Tibetan partridges and Grey partridges not interbreed? All that differentiates them are a few extra black and white feathers on the former, yet they are given separate species names.
Also, your analogy with horses, ass and mules is not quite what I'm getting at. Horses and zebras are different species. Within the species of horses themselves (Equus ferus as opposed to Equus quaggus, ie zebras) you can find Equus ferus przewalskii (a wild horse) and Equus ferus caballus (the domestic horse). These are distinct subspecies which are related and can breed, but are dealt with differently in taxonomy: They are not considered "the same".
(PS Thanks again for an interesting first post. Shame you let yourself down with the subsequent ramblings. It seems pretty immature and simplistic to revert to claims of racism. As far as I understand it, racism means suggesting one race is better than another. As far as I can see, I have not done this in any posts. I am not suggesting zebras are better than horses either, just in case the above post of mine has confused you too.)