Generational collapse

What is generational collapse? How often does it occur?

Are you talking about pedigree collapse?

It always happens. It just means that the number of our ancestral parents N generations back is smaller than 2^N, as you might naively expect. The reason is that there are many duplications in the list of 2^N names, because of in-breeding. If your pedigree didn’t collapse, you’d have trillions of ancestors 1,000 years ago and there were far less than a trillion people alive at that time.

If you were talking about pedigree collapse, I see that the master himself has addressed this question:

http://www.straightdope.com/columns/read/412/2-4-8-16-how-can-you-always-have-more-ancestors-as-you-go-back-in-time