"Orcadian" is FTDNA's British Isles reference population.
As RMS2 says, what it means in effect is that Orcadians are (a subset of) "British Isles", not the other way round. Prime example myself. 100% "Orcadian" @ 0.01%.
Although AFAIK absolutely none of my entire ancestral kin ever set foot out of England in the last 850 years, not until Elvis left the Army ..
(biologically speaking, that is; although one g-grandfather worked laying track in NZ for a few years and another spent a while down a coal pit in Tipperary, and of course just about every bloke got drafted and sent somewhere horrible (or "abroad" as we call it) for a bit, at some point in the last century).
The puzzling 'SW Europe and beyond' traces in stick-in-the-mud brits like me has had attempts to link it with the source populations of prehistory, i.e. anglo-dano-saxo-frisio-celto-belgo-dutchmen&women on the one coast (i.e. all the way up the Danube and down the Rhine, mill around at the seaside for a bit, and then cross over to the islands in the west) and franco-ibero-celto-breto-normans on the other (up the Irish sea, both sides, all the way to Shetland).
These hypotheses interpret Romania, Lithuania and the like as traces of the vast and protracted neolithic expansion and consolidation, on the Northern European Plain, of ultimately Danubian and Black Sea coast farming cultures (for instance with the "lithuanian" they're trying to say the Lithuanians, and a bit of many (?most) brits once had a common neolithic source somewhere else (as the archaeology tends to bear out)).
And the Continental Atlantic 'Facade' stuff seems to be telling a story of ultimately Cardial/Impressed Ware folks (deriving from the Mediterranean littoral and points east) nipping across from the NW peninsulas of France, and the Biscayan coast (bringing the celebrated and very cute Orkney Vole
M. arvalis with them, perhaps not inadvertently). They would have island-and-exposed-peninsula-hopped, Channel Is.>Scilly>Cornish coast>Welsh peninsulas & Anglesey, Rosslare, Howth, Lambay Island> Man> Rhinns of Galloway & Strangford, Arran, Kintyre and so on all the way up to the 60th parallel.
Main reason for that being forage and grazing for their precious animals, on the effectively evergreen machairs, and a ready supply of never-failing marine food if the catch-crop harvests went awry in the new northern climate. The boats/curraghs kept close to hand, if escape was necessary. Also the vast, tangled and frightening Wildwood covering the interior of the big islands was always soaking wet, muddy and co-o-old, the huge trees unmanageable, with hungry, potentially hostile natives flitting silently about, looking to nick one of their beasts, unlike long-settled Morbihan or Landes. Inroads would only have been made after the pioneer phase, when their numbers began to mount up, and all that
extra backbreaking, time-consuming labour and danger became un-put-offable.
You can play this game endlessly back into time, some people won't countenance anything before late neolithic/early bronze (as the relevant R1b clades, f.ex., by current estimates and data were not yet extant. Obviously this lets Adna off the hook pretty much. For now). Others fixate on legendary wandering celtic tribes, yet others d15c0unt out of hand the bulk of pre-migration-period populations, you pays yer money ..
but the fact remains, people have been (observably) coming over and settling in dribs and drabs (a few late, recorded ones being of very considerable size, much larger than the archaeological traces might lead one to expect, which bears on the scant prehistoric evidence) since the aboriginal fisher/hunters stopped filling them full of arrows, and copped on to the foreigners' talent for making yummy fruity eggy cakes and smelly brine-washed cheese and .. beer.
"Deal. You give brioche an' cidre, we not turn Ermintrude the Cow into pincushion! You give ale,
Ossau-Iraty cheese and crackers, we bring you boingy green axe-rock from Scary Mountain. By the way, is your lad there courtin' yet, 'cos if not there's *somebody* might like to meet 'im".
Or something like that).
(ahahaha, i got an easter egg! "d-i-s-c-o-u-n-t", spamguard no likee)
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