It depends on what you mean by race.
Some scholars have seen three, or four races. Others, such as Garn, have seen several dozen micro-races.
And, the best explanation so far to the question of physical differences is adaptation to environments through time. But these adaptations are largely independent from descent. I know its confusing, but that's the way it appears at the moment.
Remember, prior to about A.D. 1500, travel was slow and somewhat limited, and groups did not intermingle a great deal except where two groups intersected.