Mark makes a point echoed by many readers. I think there’s a lot of truth to the idea that parents will always want to create/have children like them. Though it would be interesting to see what happens to the numbers of gay children under such a scenario since demographically it would be very hard for gays to maintain a TFR of 2.1 (i.e. replacement rate).
Also, if you carry this sort of thinking out into the far-flung future, you could see how the human race becomes much more diverse, as in the Dune books (though there it was natural selection which made, for example, the Fremen different from other humans).
Oh, and I thought Mark’s point was going to be that “nativist” is another word that is used for policies pro-immigration types don’t like. It’s a trickier example since pro-immigration and anti-immigration views straddle both left and right. But I think one could make the case nonetheless.