Both, but mostly nurture. Yes, we have a biological mechanism to discriminate against outsiders since acceptance of perceived outsiders may lead to our group breaking apart (think monkeys, they rarely let others join and they especially wouldn't let different people join). Not to mention, when we are part of a social group, we want people in the group to be as similar as possible.
However, it's mostly nurture. If you are raised to say "that race is a horrible group of people", you're going to believe it. There is also in-group out-group behaviour, as in some people act like racist idiots as a form of aggressive "competition" if the in-group is mostly, if not all, white. There is also the media moral panics over immigration, which makes the general population act racist and perceive the offending racist in an offensive and stereotypical way. Which the moral panics only work because we, as a human species, love stability and the idea of everything around us changes because of a group frightens us, especially if we're told it'll make our lives worse (even if they present no proof). We just perceive the news as being correct and just run with it. After all, to the every-day man it's their jobs to tell us what the truth is. It's even worse when they use evidences which, only to a more cynical and/or trained eye, can see the results have been skewed and false assumptions have been made. As off topic as it is, I once watched an environmental awareness clip where the person made the illogical assumption that correlation means causation. I was genuinely annoyed at the time that they were acting like professionals of their field, but yet was skewing data and doing their evaluations generally incorrectly. However, on a daily basis, news programs make basic mistakes, yet they're regarded as more worth listening to by the general masses than professionals in the field of sociology.
