Wonder what strong ethnic identities lead to?
An example in the animal world is Argentine ants. They are all very closely related. They get along with other Argentine ants, even if not from the same colony. But when they enounter other ants, they fight to the death.
The result is that Argentine ants displace other species of ants. If an area has a diverse population of ants, with different species in different colonies, the Argentine ants will eventually take over and wipe out the other ants.
If you were a non-Argentine ant in charge of an ant colony, would you tolerate Argentine ants? No, that would be suicidal. The sensible strategy for non-Argentine ants is to kill Argentine ants at every opportunity.
Evolution is described as "survival of the fittest", but "fittest" means having a winning survival strategy. That's what the Argentine ants have, as long as they are tolerated by others.