Minimum Viable Population vs. Effective Population Size

Life has not always been so easy for humans. Human populations had to squeeze through several bottlenecks before emerging as the dominant species on Earth. The ultimate survival of humanity was uncertain through most of human history. And it could be again, should a meteor or comet impact the planet, or a similar scale disaster strike.

At the Society for Creative Apocalyptology (SCA), we spend a good deal of time discussing population sizes and compositions for various types of disasters and locales. Healthy populations contain enough genetic variety to avoid high rates of congenital diseases that prey on inbred populations.

Longtime Al Fin readers may recall this 2007 posting: http://alfin2100.blogspot.com/2007/12/planning-for-apocalypse-minimum-viable.html , where we discussed the need for a large enough population of survivors after a catastrophe. We suggested that about 10,000 breeding individuals would suffice as a minimum viable population after large scale disasters.

Up until now, our main concern has been to achieve effective population sizes to avoid “genetic drift,” or the loss of genetic variability necessary to avoid inbreeding.

Another concept of “effective population size” was recently discussed by Greg Cochran. But he was discussing the number of breeders necessary to retain advantageous mutations that make populations more fit. Cochran suggests that 20,000 breeders would be enough to constitute this type of effective population size.

The distinction is between holding on to the genetic variety you have, versus being able to hold on to nifty new genetic variations.

The blue, solid line shows the sequence of numbers X1, X2, X3, etc., up to X40 starting with the initial condition X1 = 0.4. Now suppose we had made a small error in the initial condition and started with X1 = 0.4001. The red, dotted line shows what happens. For the first 10 or so steps all seems well – a small initial error doesn’t affect the result. But then the two sequences of numbers become quite different. This is chaos – small initial differences can lead to completely different predictions. _Population Chaos

In reality, it is not that simple, as breeding populations are prone to a wide range of factors both internal and external. Real populations are prone to chaotic population shifts, which can quickly reduce small breeding populations below the effective population size, before allowing the trend to reverse.

But by the time populations recover, they are likely to have lost a portion of earlier genetic variability. This can leave them more vulnerable to future events and trends.

This suggests that disaster planners at the SCA will need to allow an extra margin of error — not only to provide for retention of favorable mutations, but also to prevent inadvertent bottleneck effects caused by chaotic population shifts.

We could approach the problem from one of two directions:

We could plan in a paternalistic fashion in terms of “providing for a larger number of breeding individuals.”

Or we could take the approach of developing larger numbers of breeding individuals who are capable of providing for themselves, through any number of changing circumstances.

Modern governments will inevitably take the paternalistic approach, and end up making a disaster of the whole thing.

At the Al Fin Institute for the Dangerous Child, we believe in adding maximum value to each child, according to his aptitude, interests, and capacity. This is a more complex approach toward mid- to long- term disaster preparedness, but it is the only approach that is likely to work, as long as it is self-sustaining over generations — which is how it is designed.

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4 Responses to Minimum Viable Population vs. Effective Population Size

  1. Abelard Lindsey says:

    So, the colonizer starship needs 20,000 people to make for a viable interstellar colony.

    • Stephen says:

      If we could create artificial wombs and carry them on board such a vessel along with stored semen and eggs, perhaps less than 20,000 would be needed for the journey.

  2. swampie says:

    There are experts whose specialty involves breeding as healthy and genetically diverse population as possible from very few individuals. The ones I know work in veterinary research, but remember that the Jews have greatly reduced the incidence of Tay-Sachs disease through genetic testing.

  3. Matt Musson says:

    “Mr. President, we must not allow a Mine Shaft Gap!”

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