This is a fantastic visual demonstration of a simple, but surprising (if you haven't seen it before) effect! Congrats to Vi Hart & Nicky Case (I presume Nicky did the web/javascript side of things?).
This isn't just about race / skin colour either - you can see exactly the same effects in things like political viewpoints where people also tend to segregate into areas or simply social circles of people that share their own preferences. (pace the praphrased quote: "How could politician X win the election? Nobody I know would vote for them!")