I accept Ken Nordtvedt's conclusions about the timing of the founding of I2a2a and b.
It is the location that seems a problem to me.
The founding, migration and near extinction of I2a2b.
If I am reading his Warped Founders Tree correctly, I2a2a and I2a2b branched
off a proto I2a2 (that no longer exists?) about 13,000 years ago. That
requires that the founders were in the same geography at that time. If
northern Germany, how did all those !2a2a get all the way back down into the
Balkans? Difficult, if not impossible. Especially with the Carpathians in the way.
Much simpler to see the location as being the area of deltas of the Danube
and Dneister. Then, perhaps in the warm spell before the Younger Dryas, the
two groups migrated upriver - I2a2a going up the Danube and I2a2b going up
the Dneister - leading to a quick clean split. The impassibility of the
Carpathians reinforced and maintained the split over time.
The I2a2b moved on, seems likely, to Doggerland and the I2a2a were contained in the Balkans.
Doggerland Genetic Bottleneck: N >>> N*
Caused by the combined effects of the Younger Dryas, the 8.200 kiloyear climate event, Lake Agassiz drainage sharply raising sea level by 3 meters, and the three Storegga tsunamis.
As Doggerland submerged and the land divided, the I2a2b were split - a few
on the west side and a very few, perhaps as few as a single individual, on the east but the majority were trapped and slowly dying out on shrinking Doggerland islands where they were running out of firewood (as happened at Easter Island). They had neither the marine technology to escape the slowly rising water nor the wood to make boats. This was happening during the Younger Dryas glacial period so the population would have had a hard time maintaining themselves during this long time of land subsidence and bitter cold. A population collapse would seem very likely as a result.
Then, after the end of the Younger Dryas, there were four catastrophic events over the span of less than a couple hundred years. Lake Agassiz drained, raising the sea level 1 to 3 meters in a matter of only a few days time and causing the "8.2 kiloyear event", a cold period, perhaps 5˚ below normal, lasting about 3 centuries. This would have caused major habitat and resource destruction in the low flat islands and shorelines of Doggerland and severe disruption of the food supply resulting great loss of life.
Then the remnants of Doggerland were destroyed and the remaining I2a2b were nearly exterminated by the three devastating Storegga Tsunamis about 6,200 BC creating a major genetic "bottleneck" (e.g. N*=small). This "bottleneck" might go a long ways toward explaining the very long time between the founding of I2a2b about 13,000 ya and and the TMRCA only about 5,000 ya. It might also explain the relatively low numbers of I2a2b overall.
It is thought that the sea rise from Lake Agassiz and the Storegga tsunamis resulted in the opening of the English Channel, isolating those peoples who were on Britain. At first the channel opening may have been narrow enough and shallow enough to walk across at low tide, but the sea level continued rising at the rate of a meter a century, so the walking period did not last very long.
Thriving on Britain
Over on the west bank - in a large area now under water off East Anglia - Isles C was founded and thrived. The continued rise of sea level drove them to the west where they dispersed throughout Britain.
Isles A split off from a remnant of Isles B about 3000 BC. Then Isles C2 and D
split off from C about 2000 BC in Ireland.
Some 6,000 years after the tsunamis, and 2,500 years after the split of C and D, the Anglo-Saxon and other "late" invasions started conceivably containing some B from those very few folks left on the continent 6,000 years before. Isles is a minuscule part of the continental gene pool and therefore any contribution to the gene pool of Isles B in Britain and Ireland would be minuscule and add none at all to groups A, C, and D.
This is speculation, logical deduction, and conjecture, but it seems to fit the
currently known (to me) facts.
Thoughts? Poke holes in it so I can improve the hypothesis.
Excellent paper on the Storegga Tsunami:
The catastrophic final flooding of Doggerland by the Storegga Slide tsunami