What’s the Minimum Viable Population of a Space Colony?

~ J.S. Pailly

Let’s say we’ve found a human-friendly planet orbiting another star, and we’ve decided to go colonize it. How many people should we send? In terms of maintaining a healthy human gene pool, what’s the minimum viable population for a distant, isolated space colony?

If you’re anything like me, you’ve spent many a sleepless night pondering that question.

I sincerely doubt anyone can provide us with a firm, specific number. However, there is a sort of generalized rule of thumb in the field of conservation biology called the 50/500 rule.

Originally proposed in 1980 by geneticist Ian Franklin and biologist Michael Soule, the 50/500 rule tells us:

  • Populations below 50 are under near-term threat of extinction due to inbreeding.
  • Populations below 500 are under long-term threat of extinction because the gene pool is too small to adapt to environmental changes.

Except the 50/500 rule is not a hard scientific law. It’s just a rule of thumb, and it has many, many detractors.

Even Michael Soule, one of the co-creators of the rule, seems to have gotten pretty frustrated by the way people took the rule literally. Here’s an interesting and, I think, revealing article about some endangered parrots. A team of conservationists contacted Soule, asking if they should even bother trying to save these parrots, because there were only 48 left.

There also an argument to be made that the numbers 50 and 500 are too low and that a 100/1000 rule would be more appropriate. And of course, can we really apply this rule to all species equally when some species reproduce more rapidly than others or face different kinds of environmental challenges, etc, etc….

Still, if we’re trying to imagine a colony of humans on some distant world, a colony struggling for short-terma and/or long-term survival, I think the 50/500 rule at least gives us a good place to start.

24 thoughts on “What’s the Minimum Viable Population of a Space Colony?”

  1. Huh… That’s a really interesting point. The news stories always seem to say things like “Ten astronauts will go to Mars!” Those are always for short-ish, there-and-back (or, at times, one-way) trips. The idea of long-term sustainability isn’t really talked about. This was a really interesting post.

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      1. So a quick correction to my previous comment here: I just read an article by Elon Musk called “Making Humans a Multi-Planetary Species.” In the article, he says we need to send a million people to Mars. I think he’s talking more about economic viability than just genetic viability, but still… he’s envisioning a million people, not hundreds or thousands.

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  2. Your articles always make me think and sometimes something will dredge up that’s even halfway relevant! I’m notoriously not a biologist, so I’m missing a fundamental piece of the puzzle – how frequently could a mating pair share a grandparent and it not matter. I guess ideally you’d want to push it back to great-grandparents. But if used that to provide a boundary condition you could probably do something using a Monte Carlo simulation or similar to work out how quickly the population would need to grow to stop this from happening.

    I’m almost tempted to learn coding in order to run that simulation. Almost.

    I’m with you though – probs 1000, ideally 2k, and because I’m an engineer let’s call it 5k to be on the safe side. You know, why don’t we just round that up to 10k…

    I’m also starting to think about skill-sets and knowledge base…there is a minimum number of people required to make a colony work in that basis too.

    Three takes that I would recommend to anyone interested in colony dynamics are:
    1) Kim Stanley Robinson’s Mars Trilogy
    2) Niven, Pournell and ?(doh) – Dragons of Heorot
    3) Peter Hamilton’s Night’s Dawn Trilogy

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    1. So as part of the research I’m currently doing, I ended up reading a paper about a computer simulation of bird mating behavior. The researchers were testing ideas about how a species with a low population can protect itself from extinction by changing its mating behavior. If you’re interested in that simulation, here’s the link: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2656.2006.01074.x/full

      My main take away from the 50/500 rule is that a population as low as 500 could possibly work, maybe, but you’re taking a pretty big gamble. Realistically, if you want to colonize other worlds, you probably need to send thousands of people. And if you have the ability to send 10,000 people, as you suggest, I’d say go for it.

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  4. I saw this a while back in the context of the minimum crew size for a generation starship. The numbers ranged from as low as 150-300 to 10,000-40,000 if we want to be “safe”. Of course, in this specific case, you might be able to mitigate it with frozen embryos.

    In both cases, advanced gene editing, where you can just diversify the population as it gets larger, might be able to bring the number down to the point where social norms and dynamics become the limiting factor.

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    1. While I was researching this, I came across a bunch of different numbers in sort of the same ranges you describe. I also read one article that gave an oddly specific number, something like 218.

      But there doesn’t seem to be a whole lot of science to back these numbers up. Just guesswork. The 50/500 rule was the closest I could come, and even with that, it’s just a rule of thumb, not a hard and fast scientific law.

      Frozen embryos and gene editing would probably help a lot. I hadn’t considered those things.

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  5. I think it all depends on the species. As far as humans are concerned, unless you throw out conventional marriage laws, the 100/1000 rule would actually be a better bet because it would take generations to build up the population after a major environmental change. Now if you breed like rabbits then you are okay with a dozen 🙂

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    1. From what I read, major environmental changes are a huge factor in this. I gather that the larger number (whether it’s 500 or 1000) is sort of a buffer against the risk that some catastrophic event kills off a large percentage of the population.

      I ended up reading a little about “alternative mating strategies.” I’m not sure if I’m going to write a post about that, but it does seem to be an effective way to mix up the gene pool.

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  6. By the time we’re ready to send people to off-world colonies, we’ll probably have gotten to the point where we manipulate our DNA so much, we may not have to worry about inbreeding.

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  7. I think some polynesian islands where originally settled by single ships with maybe 10 couples. Perhaps such groups later got into contact with other groups, but starter poplulations might have been quite small in some cases.

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    1. Makes sense to me. I think so long as you can have fresh people coming in, this isn’t as big of a concern.

      For example, I was originally going to write this in relation to a Mars colony, but a Mars colony would likely have new colonists arriving from Earth every twenty-six months.

      On the other hand, a colony in Alpha Centauri or beyond might have to wait a few dozen generations before the next wave of colonists arrive. For them, I think this could become an issue.

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      1. Perhaps one could carry a large supply of frozen human eggs and sperm along. If it is possible to protect it from radiation damage, this could be used to prevent inbreeding even if the actual population is small at any time. Eggs and sperm of the living people would also have to be kept for later generations. This way, there would be a larger virtual population while the actual population could be relatively small.
        I don’t actually believe in space colonies, but it is an interesting idea.

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      2. Somebody else mentioned the frozen eggs idea. I hadn’t really thought about that, but it makes sense. If the time ever does come when we’re sending colonies out into the galaxy, that may well be how we’ll get around this particular problem.

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  8. Would it be OK if I cross-posted this article to WriterBeat.com? I’ll be sure to give you complete credit as the author. There is no fee, I’m simply trying to add more content diversity for our community and I liked wha5t you wrote. If “OK” please let me know via email.

    Autumn
    AutumnCote@WriterBeat.com

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  9. Sorry to be jumping into this thread late, but I wanted to mention that one of the major determinants in calculating a number like this is the genetic diversity in the population you’re starting with. A Japanese colony would need a much bigger number than a US or UN colony.

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    1. That’s an interesting point. Now that I’m thinking about it, I remember reading awhile back that Iceland in particular has a dangerous lack of genetic diversity among its native inhabitants. I wasn’t aware of it being an issue in Japan, but I guess it could be a concern for any island nation.

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