3 ms·

OK, so this is the key assumption that does not hold up in the real world? > With fully random matings the number of your ancestors doubles every generation.

lolinder 3y ago
No, because fully random would still cause duplication, because the population of Earth is finite not infinite. With matings specifically selected to create no common ancestors back to X generations, then you theoretically have 2^X ancestors.
- kaycebasques 3y ago
  I'm kinda grokking it but still have no clue how far off the calculation is. To go back to the blog title, are you saying that the number of your ancestors is more like hundreds rather than thousands? Or is it just impossible to even put a number behind it due to how complicated the real-world relationships are? Edit: Just saw your other comment here. The answer seems to be more along the lines of "impossible to put a number behind": https://news.ycombinator.com/item?id=37387655 https://news.ycombinator.com/item?id=37387655
  - lolinder 3y ago
    The latter. It's impossible to make any generalizable statement about everyone's ancestry, you really have to look at the family tree and count them up one by one. People whose families have been in one place for generations are much more likely to have a smaller set of ancestors than people whose families migrated many times over the last few centuries. And any given person's family tree is likely to have some subtrees that were sedentary and others that migrated, further complicating any attempt at an abstract calculation.
- mlyle 3y ago
  Fully random only appreciably diverges from 2^n when 2^n becomes a sizable share of the world's population. When the population is roughly 2^2n, you expect to lose one ancestor in that generation.
dietr1ch 3y ago
I guess it doesn't, or you could go back to the time where the early human lived expecting an astonishingly high number too, or even before the big bang if you wanted.
Tuna-Fish 3y ago
It absolutely does not. Going backwards in time like this, most people will have many generations where the number of their ancestors in that generation go down, not up. For the majority of the past four thousand years, almost everyone lived in farming villages that had ~50-100 families living in them, and mostly married inside those communities. Even just going to a neighboring village for a bride was something that only happened occasionally, but even when that happened, it was almost always the same few neighboring villages. This means that when you trace ancestry backwards, you always end up related to everyone living in a few villages, and then every few generations this extends to a new village. Then if you are related to a successful trader or soldier or something, it will suddenly double the amount of ancestors in a generation, but I really need to stress that this is rare. The introduction of the bicycle did a lot for reducing inbreeding in farming communities, because it was something that a normal people could own, and allowed day visits to neighboring communities. Before that, it was entirely ordinary to live and die without ever traveling more than 10 miles from the place where you were born.

lolinder 3y ago

No, because fully random would still cause duplication, because the population of Earth is finite not infinite. With matings specifically selected to create no common ancestors back to X generations, then you theoretically have 2^X ancestors.

kaycebasques 3y ago
I'm kinda grokking it but still have no clue how far off the calculation is. To go back to the blog title, are you saying that the number of your ancestors is more like hundreds rather than thousands? Or is it just impossible to even put a number behind it due to how complicated the real-world relationships are? Edit: Just saw your other comment here. The answer seems to be more along the lines of "impossible to put a number behind": https://news.ycombinator.com/item?id=37387655 https://news.ycombinator.com/item?id=37387655
- lolinder 3y ago
  The latter. It's impossible to make any generalizable statement about everyone's ancestry, you really have to look at the family tree and count them up one by one. People whose families have been in one place for generations are much more likely to have a smaller set of ancestors than people whose families migrated many times over the last few centuries. And any given person's family tree is likely to have some subtrees that were sedentary and others that migrated, further complicating any attempt at an abstract calculation.
mlyle 3y ago
Fully random only appreciably diverges from 2^n when 2^n becomes a sizable share of the world's population. When the population is roughly 2^2n, you expect to lose one ancestor in that generation.

dietr1ch 3y ago

I guess it doesn't, or you could go back to the time where the early human lived expecting an astonishingly high number too, or even before the big bang if you wanted.

Tuna-Fish 3y ago

It absolutely does not. Going backwards in time like this, most people will have many generations where the number of their ancestors in that generation go down, not up. For the majority of the past four thousand years, almost everyone lived in farming villages that had ~50-100 families living in them, and mostly married inside those communities. Even just going to a neighboring village for a bride was something that only happened occasionally, but even when that happened, it was almost always the same few neighboring villages. This means that when you trace ancestry backwards, you always end up related to everyone living in a few villages, and then every few generations this extends to a new village. Then if you are related to a successful trader or soldier or something, it will suddenly double the amount of ancestors in a generation, but I really need to stress that this is rare. The introduction of the bicycle did a lot for reducing inbreeding in farming communities, because it was something that a normal people could own, and allowed day visits to neighboring communities. Before that, it was entirely ordinary to live and die without ever traveling more than 10 miles from the place where you were born.