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Corporea

Critical population decline

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So... I'm working on some post-apocalyptic material and doing research into genetics, population decline, etc.  I basically have a human population isolated on a single planet with sustainable resources.  Does anyone have any opinions on the number of individuals needed for the population to survive?

 

I killed off most of my population with mass disaster and limited the birthrate with some funky genetics causing a slow population decline.  But, I want to make certain, when the birthrate does pick up again with my sneaky genetic plan, I have enough genetic diversity to sustain a species and prevent too much genetic drift leading to an "unhealthy" and ultimately non-viable population.  I have a method to induce a few dozen new individuals with completely different genetics, but otherwise- how many do I need?

 

I glanced at the equations, but to be honest, I'm not sure how to calculate something like this.  Any thoughts? 

 

Oh- and I need to kill off at least a few thousand in wars, too, so I at least need a decent fighting force!

 

::D:  thanks for any help!!!

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Are you looking at a sustainable community, or more of a nation? Honestly, if you have 50 breeding pairs with as few genetic intersections as possible, I would buy that such a population could branch and develop into a continent-wide race.

 

Note that I have no clue of the science behind it, but as a reader or player, if the given circumstance at the outset of your scenario is that the current population grew from a common ancestry of as few as 50 people, I'd buy it. Heck, Vonnegut did it with an initial population of...7, I think? Maybe 8 (of you haven't read his /Galapagos/, it's pretty hilarious.) Not that his humans remain entirely viable, of course.

 

Your last sentence makes me think your initial population is in the thousands, though, which I would thnk is more than enough. Humans certainly didn't start off in communities of thousands, and we've made it this far.

 

My thoughts on speculative fiction, for gaming in particular, are to A)make your initial given situation as plausible as you can, B) without bogging down in science that only muddles your plot.

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oh.  I didn't mean it that way!  I writing a sci-fi book, and have been for 2 years.  Although I suppose that means my web searches look particularly odd, since I am researching some, er... yes.  hmmn.  Anyway, right now I have my population at about 20,000 and I think that is at least reasonable, but I was worried I might need more.

 

:down:

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I would totally buy 20k as viable. Again, I have no authority on the actual science of it, but I'd accept that population without any trouble. Heinlein's lunar colonies, Asimov's Pebbles in the Sky, even McCafferey's PERN started with smaller colonies than that (though still in the thousands, in the first two examples...which are likely far more scientifically reasonable).

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Check this link out, seems to discuss precisely what you are thinking of:
https://en.wikipedia.org/wiki/Population_bottleneck

 

It suggests that the subsaharan population at one point dropped to 2k people.

Also there's the whole thing with the "7 daughters of Eve", suggesting that you don't need very many people at all.

But really, with a population of 20k, I'd totally buy the idea that you are skirting the line between recovery and extinction. It only takes one major catastrophe to off colony at that point.

Also, might I suggest not being super specific when writing about technical stuff. It's probably the easiest way to avoid the problem. Instead of having 20k people, you might just have few enough people as to be worried. 

Edited by vejlin
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"Hello? NSA? Yeah I'm on these forums, see, and...."

 

Well, it's not as bad as the SpaceBattles forums, where they actually have to have a rule against advocating genocide/xenocide.

 

2k sounds like bare minimum, and I think other things would be limiting factors first.

 

For example, what standard of living do you want them to have? 2,000 means everyone's down to either farming or hunting/gathering, at least barring technological gifts from the pre-apocalypse past.

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Corporea;

I would suggest that you look at the genetic studies of Iceland.

The have done the whole gene pool, and the population of Iceland (as of 2013 is 323,002 people).

They have mapped all the families, their illnesses.

That should provide you with some viability studies.

I would suggest that 323,002 is an ideal, they are surviving, and multiplying.

Hope that helps; but do look it up.

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 As long as they still have the pre-Apoc tech and the knowledge to use it (assuming at least a concurrent tech level with our real world), 20.000 peeps should be enough - particularly if they've decided to enforce an active solution such as controlled and/or mandatory breeding, and especially if they still have the capability to do genetic modification...

 

   If this was originally a purposely-settled colony world (rather than someplace where the human population was based around a particular industry or evolved naturally), chances are good that the colony was founded and originally settled by people with enough genetic diversity to sustain themselves even when taking into account a potential mass-extinction event such as an epidemic or natural disaster. In a society advanced enough to be colonizing worlds, chances are good they may have thought to bring extra genetic material with them beyond what was represented by their physical population...

If the population evolved naturally on the planet, genetic diversity is going to rely heavily on the size of the original Pre-Apoc population, how long the humans have had advanced cultures and how much intermingling there was between different cities/states/cultures before the Apoc event - in a society like the modern US, you wouldn't need nearly as many people to restart a race as you would if, say, you were trying to do it in rural Afghanistan, India or some other place where the entire population is a single genetic group and much more likely to be at least loosely genetically related within the past ten or fifteen generations.

On the other hand, if the planet was originally settled by people involved in some industry or the exploitation of some resource (spice farming or mining planet, for example), then it really becomes a crap-shoot as to whether or not the population was even genetically sustainable before the Apoc event without massive infusions of new genetic material on a regular basis from incoming workers/support personnel...

Edited by Mad Jack
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I was a cruel, evil writer and destroyed a bunch of colonies with my antagonists, leaving only one small planet hidden away.  They have a high level of technology, good genetic diversity with a few specific exceptions that are plot points, and are well-established.  But, I'm socking them with a double whammy and cutting down their birth rate with another plot point, so over about an 800 year period I want to knock their population down to the 20K.  They get sucked into an unavoidable war after that and lose almost half of their fighting force.  But, I have new allies that can interbreed, hence the new gene flow.

 

Thanks everyone!!  I just want to try to be realistic, even if the number is only mentioned offhand.  For science!

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Honestly, you can get away with some very small numbers. There are a lot of good number suggestions in this thread, but even the 50/500 number kitchen_wolf mentions is entirely possible in your scenario for a simple reason. Your setting is post apocalyptic. I don't know how far into the future you are planning on making it, but who knows how much better humans will be at genetic manipulation in even a single generation? Changing genetic material could suddenly become trivial, and a single small family tree could become genetically sustainable with enough gene manipulation.

 

Of course you mention genetics a lot in your first post, it sounds like maybe people tried this and failed, but it could conceivably take just one scientist to break through the problems and fully figure out the code. If you are taking the route of people messing around and that's what caused the problem in the first place, it's possible genetic manipulation has even been banned, but maybe one scientist working in secret with whatever futuristic tech is available makes the breakthrough needed.

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It is known that around 10,000-12,000 years ago the world population of cheetahs plummeted, possibly to a single family. They have almost no genetic diversity; other species reduced to a couple dozen individuals a century ago today have more genetic diversity than cheetahs, suggesting that the number of survivors was brutally small.

 

http://cheetah.org/about-the-cheetah/genetic-diversity/

 

Drastic reduction is possible and survivable, apparently, even if it leaves the descendants with a host of common genetic defects, as cheetahs are prone to.

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