You seem to be arguing that this is a protective notion, when really it argues the opposite. If you bring two large groups of people together, their respective cultures are not going to change quickly. And if their cultures clash with each other, you have a recipe for enduring intra-national strife.

Is there an actual historical example of this? Most civil wars throughout history aren't due to immigrants, it's usually due to geographical and cultural divides that have long existed in a country.

Most countries in history didn't have immigrants from the other side of the world either.

The United States imported Africans as slaves for 100+ years. While some flair ups occured, black people never fully revolted into a situation that could be called a civil war. The US Civil War was fought over slavery, but it was fought between the North and the South, not black vs. white.

Ditto on Chinese immigrants in the later 1800s, Germans, etc. The point is that immigrants are always a minority in their host country, so they naturally have less of a base and thus power.

I didn't argue it as a protective notion.

I would argue that most intra-national strife comes from histories of wars and blood feuds. The Balkans being a case of this.

> And if their cultures clash with each other, you have a recipe for enduring intra-national strife.

Feels logical but also feels unproven.

This is something which always gets me.

So it is widely accepted that there is no/negligible genetic difference, but there are cultural differences at least when it comes to prosperity.

However it is also the case that cultures are extremely slow to change - multiple generations - almost to the actual speed of evolution. Essentially conceding the point that even if it isn't genetic there is some kind of heritable negative/missing positive traits which can last multiple generations.

No, humans have not evolved in hundreds of thousands of years, well beyond the beginning of recorded history.

That's preposterous and obviously false. We're seeing fast evolution on the scale of thousands or even hundreds of years. See Baujau, for example.