Most Dreaded Terror 3 Sibly
https://www.youtube.com/watch?v=hVATVF-816U
It is 21st of February 2018, and you cannot possibly imagine how excited I am until some day you get to the end and understand it. Then you may wonder why I am not running down the street screaming.
- We are in pursuit of whatever makes inbreeding kill babies, because it does not stop there. We have seen that the mechanism is inherited but not genetic and it stabilizes a population at some moderate size, neither too small to survive nor too large. It is time for some data.
- A team led by a man named Richard Sibly wrote “On the Regulation of Populations of Mammals, Birds, Fish and Insects,” Richard M. Sibly, Daniel Barker, Michael C. Denham, Jim Hope and Mark Pagel SCIENCE vol. 309 July 22, 2005 page 609. They collected every paper published before 2005 that was about serial field counts of wild animals. If you went into a field periodically and counted rabbits and published your results your paper was analyzed.
- In all they found 1,700 studies they thought suitable and discussed what as found. If a study ran for 5 or 10 years. That would be something like ten thousand years of data collected by professionals. That is a lot of work. The result is not going to go away.
- Sibly did not publish an overall result, but he looked at the collection and gave a typical example. I have made bold to do a sketch of it here:
Feast your mind for a moment on the idea that holds it together. The horizontal axis is easy. It’s just the animals counted. The vertical axis is the population growth rate. You might be able to contrive an explanation based on predators or disease, but over the next few talks that will not hold up. The cause is a difference in fertility.
- The line is incomplete. There is no evidence for inbreeding depression. The line should be falling toward the left very steeply at the red arrow here.
We agree that inbreeding depression happens. Sibly says he did not observe it in the wild, but he excluded from analysis populations that went extinct. So I think it more than fair to say that there is an undemonstrated falloff in fertility at very low population sizes.
- The important point is indicated with a purple arrow here:
Here there is neither positive nor negative growth. For humans that is ideal. Opinions may vary on just how many people we want on the planet, but there can be no disagreement on the right growth rate. Negative growth eventually leads to extinction. Positive growth eventually leads to the kind of environmental and resource exhaustion that we have been warned of so much.
- Crucially, at smaller population sizes than the zero growth point the fertility rises rapidly. At smaller sizes the fertility falls slowly. This is what we needed to see. The curve stabilizes the population size at some moderate value. So long as the displacement from the zero point is not too great the population will return to that point.
- If it were possible to divide human kind into a number of mutually exclusive societies, each the size of the population at this point, and if they were to reach equilibrium then we would have a stable world population. Just what that population size should be is not known. It might be possible to work it out from the Icelandic genealogies such as we shall be looking at next. How it could be set up in a way that left everybody happy or even whether there will ever be the will to try are questions that are as baffling as they are important.
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