Don't you run into the same 1st cousin / 2nd cousin genetic bottleneck issues in these small isolated groups as well?
Wouldn't the longer life spans, with some fathering children at 800 years of life, make this a non-issue?
I remember viewing a chart somewhere that showed exponentially how the human population could have increased before the Flood.
I was intrigued at how from the first couple mankind could have numbered in the millions at the time of the Flood.
There's so much we don't know about conditions back then.
I do know this: Man was created with a desire to propagate the species, and we've done just that.
Some civilizations have died out, only to be replaced with another.
If all of this was due to mere chance, then Someone could really throw the dice!
Sylvia