Racism is not a human only trait. In nature, animals breed with the same, uh, RACE in their genome. It's why you don't see a Magpie and Crow  cross. You don't see red and gray Sqirrels crossing. It's a natural behavior for animals really, uh, exept for us HUMAN beings, since we're not animals (ugh). There are situations where the instinct to breed within your own, uh, race becomes subdued, a situation our 'boundaryless' society we have built. Call it immersion if you like. That's only one example I can think of. Still have other issues, territoriality is a big one. Herd or tribal security. Prejudice and bigotry are learned concepts. "Racism" is actually instinct.

True, growing up we raised dairy goats, had two breads, nubians(large hanging floppy ears), and alpines(thin up ears).  The nubians were made up of two colors, brown and black, and divided themselves accordingly.  The two would battle it out for control over the herd, and both beat down the the alpines, as the alpines were less in numbers.  What was funny, was as kids, they would all get along just fine, yet would separate/segregate themselves once turned loose with the rest of the herd as yearlings.