1.)The Romans conquer Britannia and Mediterranean genes are introduced.
2.) Centuries pass. The Roman Empire collapses and Britannia is invaded by multiple waves of Germanic and Scandinavian invaders.
3.) The surviving Romano-Britons are pushed out of the indefensible lowlands but are able to mount a successful defense in the mountainous regions of Wales just as the Asturians mounted a successful defense against the invading Moors in their mountainous region of northwest Spain.
4.) Mediterranean genes survive in Wales and are concentrated in Wales.
Roman civilisation started its climb maybe 500 BC or so at its oldest. This study says these genes were introduced circa 2000 BC.
Seems logical to me.
...and you didn’t even have a half-million dollar research grant.
If this genetic marker is 4,000 years old, then it would have been pre Roman. Was this an area of Wales that could have supplied tin to be alloyed with copper to create the Bronze Age. Perhaps ancient eastern Mediterranean peoples were traveling here by boat to either mine tin, or to trade for tin. An while there, enjoying the local female companionship. They could have had a settlement with extensive mining and commercial activity centered around the importance of the tin trade.