If you look at a relief map of Britain, it's interesting to note that the Pennines act as a natural barrier between the east & west of northern England, or potentially, Anglo-Saxon (east) & Brythonic Celt (west). If you draw a line following the higher ground from Derby down to Poole (in Dorset), that seems to be where 6th C maps show the split tbetween the Anglo-Saxons & Brythonic Celts in the south.
The Welsh mountainous regions obviously made attack by various invaders difficult. Historically, it was easier to eradicate an enemy on flat, open terrain. Mountains are a great bolt-hole to defend when under pressure. I think the mountains of Cumbria were of similar benefit to Brythonic Celts there, when Angles , Danes & others moved into the region.
My view of the upcoming POBI results? I'd guess there'll be some sweeping statements & maps showing clearly defined tribal groupings. I'm concerned that the map they released showed England, south of Cumbria & Northumberland, being almost completely red, with the exception of Devon & Cornwall.
The Welsh map appears bizarre to me. I can understand the Pembrokeshire presence of Flemish & Normans (if that's what the map is indicating). However, surely there should be very similar DNA found in the western counties of England & Wales generally? To have only tested Anglesey, Pembrokeshire & some hot spots on the Anglo-Welsh northern & southern borders seems very remiss! Surely the Welsh in the interior would have DNA more representative of ancient Welsh DNA , so of greater relevance to the country than areas near ports & borders that traditionally attract a more mobile population? Also, shouldn't we be seeing some similarity in DNA results when comparing Cornwall & Wales?
Finally, what the hell are we supposed to make of the Scottish map?!
I hope POBI have only released appetizers so far, & that they have far more to offer.
You raise some interesting points.
Geography is often important in explaining the history of a region and this upland versus lowland division in Britain has often been commented on.
If you look at the history of Britain , invaders/settlers usually take the best agricultural land for themselves, confining the natives to the poorer soils of the hills that are only good for grazing sheep. The Romans dominated lowland England but largely ignored the uplands of Wales and the Pennines. The Anglo-Saxons appear to have done the same, settling on land better suited to crops.
Celtic speakers clearly survived most in the uplands of Britain, in Cumbria it probably survived longer (Cumbrian sheep counting contains a Celtic remnant) than elsewhere due to its remoteness. Of course, Viking burials and placenames indicate a later Scandinavian influence in Cumbria.
As for Wales, I think the POBI tested Anglesey because they were at an agricultural show on the island and lots of volunteers came forward. I agree that the lack of samples in central Wales is frustrating because it is reasonable to assume that the most ancient Welsh dna would be found in the the remote villages and farmsteads of the Welsh uplands.
The coast of Wales has always attracted settlers, from Viking and Irish raiders in medieval times through to more modern immigrants.
Having said all that, Wales did still manage to remain separate from England until the late 1200's, and not until industrial times was there significant further immigration into Wales. This relative isolation might account for the different genetic clusters with western English counties.