3 ms·

Well, if you're an evolutionary extremist, it actually becomes tautological. Imagine half of mankind flies off to colonize space and the two halves of mankind n

by xamuel 6y ago

Well, if you're an evolutionary extremist, it actually becomes tautological. Imagine half of mankind flies off to colonize space and the two halves of mankind never inter-breed ever again (the prototypical example of a future with no "reverse IA point"). Well, the evolutionary extremist would say that under those circumstances, inevitably the two branches would evolve separately, so that eventually, they would no longer be the same species. So at most one of them would be "human", and so the split would not prevent a "reverse IA point" like it initially seems (because of the word "human" in the definition of the IA point). This is formalized in the internodal species concept of Willi Hennig. According to Hennig's species notion, speciation occurs exactly when a permanent split occurs. If you could somehow contrive that mankind split in half, whether by space colonization, a gigantic wall along the equator, or by some segregationist law (but good luck enforcing that law in perpetuity!), then it would no longer be "mankind" at all, but two new species.

PaulDavisThe1st 6y ago
But if you're not an evolutionary extremist ... merely someone who notes that our previous IA was around when the human population was a tiny fraction of the current level, and that there is no reason to think that it had been anywhere close to our current level before the IA. Essentially, for there to be a future IAP, there would need to be a massive near-extinction level event for humans. Obviously, there's no reason to insist that this cannot happen, but if it doesn't happen, I don't see there will ever be an IAP in the future.
- xamuel 6y ago
  Why would there need to be a massive near-extinction event? Maybe we're talking past each-other. What I mean by "reverse IA point" is a point Y in the future such that every human in 2020 is either an ancestor of every human in Y or of no human in Y. As an over-simplified example, imagine starting this generation, everyone produces exactly 2 kids, so the total population remains static forever. In 1 generation, you'll have 2 kids, in 2 generations, you'll have 4 grandkids, ..., in 33 generations, you'll have over 7 billion grand^32kids, i.e., everyone on earth will be your descendant. Of course that's inaccurate because the growth will slow when your descendants start marrying your other descendants. But the point is, when you think of it this way, it would actually be a miracle for a reverse IA point not to exist (if we don't go extinct or colonize space). In fact, the main innovation in Rohde et al wasn't to come up with a simplistic model like the above example, but rather to factor in real things like the isolation of different continents/tiny islands for long stretches of time. With modern air travel, populations should tend to mix more, not less, and that should encourage a reverse IA point.
  - PaulDavisThe1st 6y ago
    > every human in 2020 is either an ancestor of every human in Y or of no human in Y. as the population increases in size, the likelihood that at time T' there is a living descendant of any given individual at time T increases. It never becomes 100%, but it does increase (for reasons you note). by contrast, for the likelihood to decrease to zero means that as (T' - T) increases, an ever increasing number of descendants have to die before having children. for example, if i have two children and T' is T+1 year, then both children must not have children or must die in the next year. Now increase T' to T+30 years, and we now require that both my children, plus any children they have in the intervening 30 years, die before having any more children (at which point, I will have no descendants at any time beyond T'). Increase T' to T+300000 years, and we're talking about a huge number of people who must all die or have no more children before T'+1. That's more or less what you've said above. But the problem is that it applies to every living human today also. For the situation to change such that I (or any other living human) become the IA, the descendant trees that do not include myself (or that other currently living human) must all vanish. This would be a near extinction event, since the number of humans in every descendant tree except the one connected to the IA will vastly outnumber those in that tree.
    - xamuel 6y ago
      You seem to be reasoning as if I've said there's a unique person today such that everyone in future year Y will be that person's descendant. In the above oversimplified example that I gave, everyone alive today would have the property that they are ancestors of everyone alive in year Y.
      - PaulDavisThe1st 6y ago
        Isn't that what an IA is?
        unkulunkulu 6y ago
        There couldn’t be one because one of his partners would have a descendant alive at 2020

by xamuel 6y ago

PaulDavisThe1st 6y ago

But if you're not an evolutionary extremist ... merely someone who notes that our previous IA was around when the human population was a tiny fraction of the current level, and that there is no reason to think that it had been anywhere close to our current level before the IA. Essentially, for there to be a future IAP, there would need to be a massive near-extinction level event for humans. Obviously, there's no reason to insist that this cannot happen, but if it doesn't happen, I don't see there will ever be an IAP in the future.

xamuel 6y ago
Why would there need to be a massive near-extinction event? Maybe we're talking past each-other. What I mean by "reverse IA point" is a point Y in the future such that every human in 2020 is either an ancestor of every human in Y or of no human in Y. As an over-simplified example, imagine starting this generation, everyone produces exactly 2 kids, so the total population remains static forever. In 1 generation, you'll have 2 kids, in 2 generations, you'll have 4 grandkids, ..., in 33 generations, you'll have over 7 billion grand^32kids, i.e., everyone on earth will be your descendant. Of course that's inaccurate because the growth will slow when your descendants start marrying your other descendants. But the point is, when you think of it this way, it would actually be a miracle for a reverse IA point not to exist (if we don't go extinct or colonize space). In fact, the main innovation in Rohde et al wasn't to come up with a simplistic model like the above example, but rather to factor in real things like the isolation of different continents/tiny islands for long stretches of time. With modern air travel, populations should tend to mix more, not less, and that should encourage a reverse IA point.
- PaulDavisThe1st 6y ago
  > every human in 2020 is either an ancestor of every human in Y or of no human in Y. as the population increases in size, the likelihood that at time T' there is a living descendant of any given individual at time T increases. It never becomes 100%, but it does increase (for reasons you note). by contrast, for the likelihood to decrease to zero means that as (T' - T) increases, an ever increasing number of descendants have to die before having children. for example, if i have two children and T' is T+1 year, then both children must not have children or must die in the next year. Now increase T' to T+30 years, and we now require that both my children, plus any children they have in the intervening 30 years, die before having any more children (at which point, I will have no descendants at any time beyond T'). Increase T' to T+300000 years, and we're talking about a huge number of people who must all die or have no more children before T'+1. That's more or less what you've said above. But the problem is that it applies to every living human today also. For the situation to change such that I (or any other living human) become the IA, the descendant trees that do not include myself (or that other currently living human) must all vanish. This would be a near extinction event, since the number of humans in every descendant tree except the one connected to the IA will vastly outnumber those in that tree.
  - xamuel 6y ago
    You seem to be reasoning as if I've said there's a unique person today such that everyone in future year Y will be that person's descendant. In the above oversimplified example that I gave, everyone alive today would have the property that they are ancestors of everyone alive in year Y.
    - PaulDavisThe1st 6y ago
      Isn't that what an IA is?
      - unkulunkulu 6y ago
        There couldn’t be one because one of his partners would have a descendant alive at 2020