BBCnews is carrying this report detailing new findings about the genetic makeup of modern Britons:
Scientists believe a small population of migrants from Germany, Holland and Denmark established a segregated society when they arrived in England.
The researchers think the incomers changed the local gene pool by using their economic advantage to out-breed the native population.
While the article doesn't mention it, I was immediately struck by the similarities of this argument to that of what may have caused the Neandertals' extinction. If you've got a long enough period of time, even a slight advantage accumulates into a significant difference.