Read Richard Dawkins "River out of Eden". Races are akin to streams of genes that mix and match, but the set of alleles i.e. versions of particular gene, are finite and different between "streams" or races. e.g. Hair color genes, Asians only have brown hair genes, Swedes stream has a variety of hair colors, as do most Nordics. Evolution works by physical separation, i.e. mutations within a given stream. When streams mix, i.e. through migration, you get essentially a new race. Africans and non-Africans have been physically separated for nearly 200,000 years, all dogs were wolves 30,000 years ago if you think that isn't enough time to evolve into dramatically different creatures.