Given that we spent most of our time as a species living in small Paleolithic nomadic bands of less than sixty or so, it would seem entirely plausible that we are hardwired to regard our own kin and tribal group as inately preferable to others. I believe that we are all, likewise, inately predisposed to look upon other social and ethnic groups as somewhat inferior to our own. However, like the ability to learn not to wet the bed we acquired as children, we are all just as capable of overriding a mild biological imperative to recognize the common humanity we share with every other person.

It's when we think ourselves immune from prejudice and bigotry that we descend into racism - witness Roseanne Barr tweeting that she is not racist, instead merely responsible for telling "a bad joke".

Oh, probably. Humans are terrible people.

Maybe “racist” is too strong a term, but IMO, those with similar genetic traits tend to stick together. Perhaps there is an evolutionary reason for that. Every person alive descends from a long line of survivors. Every genetic group has precious survival traits, perhaps unique to that group. Place two groups in proximity, and they will not immediately blend together. Doing so might mean disaster for one or both groups.

On the other hand, nature likes to experiment. There will be a few adventurous individuals who go against the norm, and gradually the groups might merge, resulting in a hardier and better adapted population. It doesn’t happen suddenly because time is needed to weed out bad combinations.

I'd say we're all tribal.