What is the smallest number of humans required for colonization?

Question

There is an estimate at Understanding Genetics that the minimum number of humans required for a colonization process is 160. It also mentions some animal populations that have rebounded from near extinction with much lower numbers. Strangely there is no mention of haplogroups in the answer there, and it seems to assume a random sampling of humans rather than a picked group.

It would seem that if you were trying prevent mutation in all haplotypes you would probably need a larger gene pool. Common sense also implies that it would be easier to get a diverse gene pool from a smaller group by purposely selecting from different haplogroups.

If hand picking with gene profiling, what is the smallest group of humans that can form a viable human colony, where new gene infusion is unlikely?

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Clarifications:

• This question is looking for a lower limit number.
• The number should (at least, theoretically) provide population self-sustainability and long-term survival.

• The answer should take two different approaches:

  1. An intra-solar-system colony that may have future additions to its numbers
  2. An interstellar colony that does not expect future additions to its numbers
• Let's define "long-term" as at least 200 years.
• This question is focusing on genetic/population considerations. Practically, a real colony may require more than the answer here indicates.

Answer 1

I think for colonization within the Sol system, the genetic diversity question is not a significant problem as new colonists will likely continue to arrive for a long time. Sending additional colonists is easy compared to the initial challenge of sending everything needed to establish a sustainable colony.

For a scenario like interstellar colonization, where the group will potentially be on their own for a very long time, I don't think there is a solid threshold that should not be crossed. It's a matter of degree. More is better. Less is riskier. Relatively small gene pools have survived for long periods as we have seen in some remote island populations, royal lines, or in the Fugates of Kentucky. The smaller your gene pool though, the more you see the emergence of long dormant recessive traits.

Many of these are very benign, but if you're unlucky, you could end up with something life-threatening, like Sickle Cell Anemia. And even if you specifically select for the absence of known dangerous recessive genes, you never really know what's going to turn up, because these recessive genes may have gone unexpressed for many centuries until inbreeding reveals them. So you are basically rolling the dice if your population is very small.

If you are looking for a lower limit, as stated in the comments, one woman and a sperm bank could theoretically populate a world, or if we assume for some reason that we are limited to only natural conception, one man and one woman is enough. But it's very risky. It's really a ridiculous risk to take. An interstellar colonization would already be so expensive and require such a large payload that it's hard to imagine a scenario where you wouldn't want to include many thousands of genomes at least in the form of sperm on ice.

How many exactly should be included as an absolute minimum? Again, it's entirely a matter of degree. There is no threshold value that makes a critical difference. How much risk are you willing to accept for this really really expensive venture? A whole lot? Then two genomes will do it - or even just one male if you're willing to make the female a cloned twin with two copies of the X instead of a Y. Would you rather have zero risk? There's no such thing. Somewhere in between? Well, take more than 2. And don't take so many that they occupy more than 10% of your payload.

Ultimately cost/benefit per pound is the equation you will be working. That's what everything usually comes down to in space exploration.

Answer 2

If space colonization is to unfold, exchange of genetic material between colonies is a very likely scenario. It is quite easy to exchange sperm. Even if we consider stellar colonization, interstellar sperm ships look much more possible than travel (sperm containers can withstand greater acceleration, do not require starship deceleration and can travel for longer time in frozen form).

Answer 3

The Amish are a genetically separated group. They could survive hundreds of years from a core population of 200 people. Yes, they can have genetic disorders, but it is not so bad as to threaten their long-term survival.

There is a genetic study saying that roughly 25 mouse pairs are needed to build a stable population on the long term. With humans, this number may be roughly in the same order.

I think that the real answer is somewhere between the two.