It has recently been argued that there was an apartheid-like social structure operating in Early Anglo-Saxon England. This was proposed in order to explain the relatively high degree of similarity between Germanic-speaking areas of northwest Europe and England. Opinions vary as to whether there was a substantial Germanic invasion or only a relatively small number arrived in Britain during this period. Contrary to the assumption of limited intermarriage made in the apartheid simulation, there is evidence that significant mixing of the British and Germanic peoples occurred, and that the early law codes, such as that of King Ine of Wessex, could have deliberately encouraged such mixing. More importantly, the simulation did not take into account any northwest European immigration that arrived both before and after the Early Anglo-Saxon period. In view of the uncertainty of the places of origin of the various Germanic peoples, and their numbers and dates of arrival, the present study adopts an alternative approach to estimate the percentage of indigenous Britons in the current British population. It was found unnecessary to introduce any special social structure among the diverse Anglo-Saxon people in order to account for the estimates of northwest European intrusion into the British population.
This is a rejoinder to Evidence for an apartheid-like social structure in early Anglo-Saxon England. Exciting times. I don't have the time to read this right now...but I would suggest that islands are quite often rather surprisingly insulated from continuous gene flow. Granted, the English Channel is rather a narrow chasm, but sufficient to block land-based invasions since 1066. The "apartheid" argument seemed rather extreme to me at the time, but I am inclined toward accepting the reality of a significant demographic discontinuity around the 5th-6th centuries....
One indicator that mixing might have been rare is the paucity of Celtic placenames in England. One can easily see the boundary of the Danelaw on a road map with many Scandinavian (Viking) names on the north side (e.g. names ending in -toft) and almost none on the south side, but there are few Celtic placenames anywhere in England outside Cornwall (which doesn't always consider itself to be part of England).
Even in the Marches (the England-Wales border country), the Celtic names (Cefn Einion, Llanfair Waterdine) on the English side of the border are relatively few and are relatively recent re-introductions, not old names that have survived.
In "The Origins of the British," Stephen Oppenheimer argues that the post ice age Celtic migrations never reached England proper, that England itself was originally settled by Germanic/Norwegian peoples. This is based on a principal components analysis of DNA variations.
I have discussed Oppenheimer's theories in a post on gnxp classic. They are not based primarily on PCA, but on Y chromosome phylogenies.
I haven't read either the new paper or the 'apartheid' paper it is replying to. I can hardly suppose that the 'apartheid' theory assumes complete absence of mating between 'British' and 'Anglo-Saxon' genotypes, because even if such separation existed in the dark ages, it would have been obliterated in later times.