Is the quasi-urban group modern, e.g. with technology and everything else? Even ignoring the fact that 1000 is not a long time evolutionarily - you'd get what, 15-20 generations? - a hunter-gatherer group is going to have a lot of evolutionary pressure that a modern technological group won't. You can have a physical injury or disability in the modern group and be fine, but without support that may not be there in the hunter-gatherer group, you will die quickly.