It's hard to argue for nature when the immigrants come from among the most diverse ethnic groups. There is literally no homogeneity among them apart from the fact that their parents weren't American citizens.
While I definitely buy the nurture side of that argument more than the nature side, it's still possible to be selecting for a set of genetic traits across a diverse group as long as those traits are distributed pretty evenly.
Much like a randomized controlled trial, the high-pass filters tend to render the nature-nurture argument a moot point. In fact, raising it mostly makes me worry about the more racist aspects of anthropology leaking into the discussion.
