Suppose you have a well-mixed population with some set of traits that have some kind of social importance or interest. Or for that matter, suppose their traits are utterly boring: that’s unusual and would be interesting. You have to put a name to that population: race is a perfectly good word. How different from some other population do they have to be in order to be called a race? Different enough that someone gives a shit.
What kind of origins make a legitimate race? The answer is, any kind of origin. It doesn’t matter how they got here, it matters what they are. I’ve been reading a tome in which the author says that many of the groups we think of as races are fairly recent formations (a few thousand years ago). Europeans are a mix of three groups ( hunter-gatherers, Anatolian early farmers, and Indo-Europeans off the steppe) that were originally about as genetically distant from each other as Germans and Chinese today. He seems to think that somehow invalidates thinking of Europeans today as a race. Or maybe he’s just blowing smoke. Anyhow, it doesn’t matter at all. Europeans would be what they are, act as they do, be as different and as similar to each other as they are, if they’d been manufactured ( to the right spec) in a test tube 6022 years ago. The same is true for everyone else: Amerindians are mostly a mix of Sibermen and Chinesians (quite a long time ago), but although that is interesting, it hardly means that “Amerindian” isn’t a real category.