|MadSci Network: Evolution|
This is exactly how new species evolve: when two populations are separated physically, once enough time passes, small differences will become big differences, and eventually the two will separate into two different species. The formation of cities could be a factor to separate two different populations. Remember though, that A LONG period of time needs to pass. Cities have only been present in their present form for about 100-200 years; a typical city dweller, rats, have been around for millions of years.
Try the links in the MadSci Library for more information on Evolution.