Having "mule" offspring is sometimes harmful - worse than having no offspring at all. Queller proposes that analogous cultural mismatches can produce broadly similar harmful effects - as memes battle with incompatible companions and generally fail to work together. He gives examples and argues that mechanisms to avoid these bad outcomes could result in xenophobia - via genetic and/or cultural evolution.
David's ideas here are obviously important and worthwhile - but I'm rather skeptical about whether "isolation mismatch" is largely responsible for xenophobia. Humans cooperate in part due to reciprocity and cultural kin selection. In the absence of those effects they can behave pretty badly. If you are a caveman, you don't bash in the brains of a member of a neighboring tribe because you are concerned about cultural mismatch. You do it because they are a competitor and would likely do the same to you given half a chance. Xenophobia is pretty well explicable as a baseline state that arises when the mechanisms responsible for cooperation are absent. That's not to say that divergent selection as a result of cultural mismatches due to isolation is unimportant, but that it may be only a small part of the story of the origins of xenophobia.
Much the same argument applies to explanations for xenophobia that invoke the cost of producing genetic mules. Mules do exist and do have significant costs, but a lot of xenophobic behavior is not directly associated with the production of mules. That hypothesis would predict more female xenophobia - since females bear most of the cost of bearing mule offspring. In fact, xenophobia is more likely to be exhibited by males (see reference below). Rivalry and competition for mates seem like more appropriate explanations for that than the costs of producing mules.
Finally, I'm completely onboard with David when he writes:
Indeed understanding the roots of xenophobia might provide ways to mitigate it.This is one of the ways in which cultural kin selection is of great social and political importance. Aside from it being of scientific interest, there's also the issue of it providing scope for improving the scope of human cooperation by engineering and promoting shared memes.
- Queller, David (2017) Isolation mismatch
- McDonald, Melissa M. and Navarrete, Carlos D. (2016) Developing a Theory of Gendered Prejudice: An Evolutionary and Social Dominance Perspective