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Barry Bishop writes:
> with a high degree of what Catherine calls 'comfortable incest', it is
> inconceivable that in only a very short time you would not be a direct
> descendant of everyone in Ireland whose genes have been passed down. Unless
> you claim that the population of Ireland has been completely genetically
> isolated, with not even one case of genetic input from elsewhere, one of

A couple of items from my own family genealogy:

My parents can almost be shown to be distantly related
(one's British roots included some Irish Quakers, one of whom
has a surname and origin similar to one of the other parent's
Irish Catholic families).  At this point I suspect we will
find some other suspected cross links, perhaps some we can
really document if we are lucky.

In looking at Quaker families in the US, they maintained a curious
kind of inbreeding for almost 200 years.  Forbidden to marry
near relations, they would marry others, but the trees cross again
and again at 4-5 generations (2nd/3rd cousins).  Since they wrote
everything down the lineages, at least the official lines of descent,
are pretty well documented.  (I think the same thing has been shown
for early Virginia settlers.)

So on the one hand, common ancestors can be there despite the mobility
problems, religious problems, social class &c.  And on the other
a community can manage its separateness (incl gene pool) for a long period of time
if it is determined to do so even when plenty of diversity is readily
available.