Let’s suppose that the world suddenly became a friendlier, friskier place to live and by some stroke of luck people would be traveling worldwide, propagating at random with great frequency.
How long do you think it would take (years or generations) before it became difficult or impossible to identify any of the common racial characteristics we observe today?
Would we be guilty of fooling with Mother Nature because people in particular climates would be in danger of losing the traits adapted over many generations by evolutionary necessity?
What kind of Tug-O-War might ensue between evolutionary necessities and random, erratic breeding?
Evolutionary necessities are mostly moot now, in the First World. While it’s true that a light-skinned person living in the tropics will sunburn or develop skin cancer more easily, and a dark-skinned person living near the poles won’t produce enough Vitamin D, we have sunscreen and vitamin supplements now.
It wouldn’t be a complete blending process, though. Things like skin color are controlled by some number of genes, and even if mating is occuring totally independently of skin color, it will still occasionally happen that a person is born with almost all light-skinned genes, or dark-skinned genes. Likewise for other traits usually associated with race. In general, for a simple trait (one controlled by a single gene), if a person with trait A and a person with trait B have a kid, the child will not be somewhere in between, but will be one or the other. Skin color is controlled by more than one gene, so there’s the possibility for some “in-betweenness”, but there’s still a limit to how thoroughly the traits can mix.
So, in other words: If a Swede marries a Zulu, their kid will probaably be lighter than a Zulu and darker than a Swede. But if that child then marries another person of similar background, their children might potentially be as dark as Zulus, or as light as Swedes.
Chronos makes some good points. Genes do tend to mix and average. I’ve also heard that some genes that seem to be “dominant” do not always remain domanant for some reason.
I’ll throw out a figure of 200 years. In a highly mobile society, I think that 10 generations would be enough not only to blend the races, but for those concerned with such things to have largely forgotten their ancestry.
Of course there will also be random variations so that even if the whole species becomes homogeneous, evolution will still tend to adapt people to their environments.