Angela
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If neolithic farming in Ireland was always relatively marginal then farmers from the Atlantic Megalith culture might not have fully covered the territory leaving surviving WHG populations on the fringes.
If subsequent to their settlement changing conditions led to a partial or complete farming collapse then you'd have the conditions for a small, partially LP population of incoming cattle herders to expand into the remaining territory (alongside strong selection for LP).
A dramatic founder effect and expansion from a small population seems to fit better with things like the very high frequency of the hemochromatosis C282Y allele (and did they check for their red haired frequency?)
If the pre-herder population of Ireland included a surviving whg population (or if the some parts of the megalith culture farmer population included a higher than usual whg percentage) then that could explain the WHG resurgence.
If it is a case of massive founder effect and not a massive tribal invasion then the seemingly male dominated nature of the expansion can easily be explained by the invaders coming from a male dominated occupation like
- mercenaries
- traders
- prospectors
- miners
Interestingly, I didn't see any derived MCIR for red hair in the tables. As I said above, the two groups seemed to both be derived homozygotes for the two major skin depigmentation genes. The Bronze Age men were heterozygotes for HERC2, so brown eyed, but this is only three men, so there might have been people who were actually blue eyed among them. As the authors state, one of them probably had lighter hair with brown eyes.
The severity of the farming collapse in Britain is well-documented, so we're talking about a very small population remaining, unlike in other areas of Europe. The authors don't see, as I pointed out before, any input from the Irish Neolithic people into the Bronze Age people, but as Maciamo pointed out these are three samples from one area. We might find samples from other areas that do show some input from the prior people.
It's difficult to know how many actual migrants made their way into Ireland. If this clade did start expanding five hundred years earlier, that might have been in central Europe or northwest France where it can also be found.
The mtDna of these Bronze Age men is also interesting. It's not the HV of the Irish Megalithic woman, but neither is it very steppe like. The U5b is western and central European Mesolithic and also found in central European farmers. The J2b likewise is common in central European farmers. It's only the U5a that we could say is steppe related.
If I have time I'll take a look at the modern breakdown of mtDna in Ireland.