this dosent' sound very PC but maybe its true, you don't know!!!! there are other differences in ethnic groups, skin colour, culture, religious beliefs, physical make up, thought processes so why not intellegence???
The reasons are simple when you look carefully at the list you proposed. Let's see:
skin color - adaptation to climate, less sunny climates require white skin so that sufficient amount of vitamin D is produced (white skin allows for better penetration of sun rays).
culture - this doesn't appear to have anything to do with IQ. Similar myths, similar legends. So what that Africans believe that having sex with a virgin cures HIV. Many whites believe gays are the reason of HIV.
religious beliefs - same difference. Religious beliefs are regarded by some as dumb, whether the believer is white or black. After all, to an atheist there is no difference between the belief in Santa Claus, Jesus, or a thousand year old witch living in an old baobab tree
physical make up - again, adaptation to the external conditions. Cold climate - bulky body to keep the heat in, hot climate - slim body to help get rid of the heat. A fools trancend climates. So do smart people.
Intelligence is also a skill, to some extend. Regardless where you go, various ethnicities survive in the conditions in which they have to live - from the Arctic to the hottest places in Africa and elsewhere. Some manage to survive even in their burbs with the oil prices hitting all time highs, and withe driving restrictions imposed by their parents. They'd probably be leading fools in the streets of Soweto, of the wilderness if Nigeria. If they survived more than 24 hours, that is.