But birds of a feather flock together. Once tribes establish themselves in different places, those who agree with a tribe's ethos will naturally move to that place rather than another. In the US, the red and blue states have indeed been established for a long time.

What you said there seems to assume that immigrants are politically neutral. Of course, I can see how an environment rubs off on us, as we adapt to fit in. But there are also common sense prior decisions about choosing to live in neighborhoods that aren't alien to us.