snip
Well, in an exam I took at university I studied some linguistics, and I was told that genetically, we begin with the capacity to pronunciate every possible sound, and then, by listening to adults speaking our brains automatically "register" and learn to reproduce only some of them, forgetting about the others when we grow up. For example, Japanese speakers never learn how to pronunciate Ls, but only Rs, so when they try to pronunciate a word with an L in it, they don't even recognize it as an L.
So I don't think that genetics has some to do with accents. In fact, if you take for example a child born from African parents that is adopted when he's very little, less then a year for example, and grows up in England, being exposed only to English speakers, he doesn't have any different accent from that of the speakers with whom he grows up.
So I don't think that different evolutions of an accent can be ascribed to gentic differences.
That being said, I still am quite in the dark as to what they can be ascribed too...