The effects of wealth disparity can last for centuries. In the year 1066, the Normans conquered England and placed themselves on top of the social hierarchy. More than 900 years later, people with Norman last names are still about 10% wealthier than their Anglo-Saxon counterparts.
Simple exponential math means ancestry beyond 3 centuries or so in a population becomes nationwide inbreeding with no pure non-relatives left.
....Names can be deceiving, many of the people working on estates of Norman rulers adopted the surnames of their rulers. Most never had surnames as such and usually were known as John the son as Johnson etc. when surnames were used they gave the name of the owner of the estate. Working on the William's estate you would call yourself John 'Williams'. DNA studies have shown that the Norman conquest of England did not leave any genetic evidence among the population. Celtic and Anglo Saxon and Saxon is the major makeup of the British people, the English having as much Celtic DNA as People in Scotland and Wales. see