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Imagine humans were to colonize a distant planet and it was a single one-way trip. How many people would they need to bring?

Obviously 2 is the minimum, but that would result in a lot of inbreeding.

So what number is the minimum number of people you can have in an isolated community and still maintain a healthy diversity?

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Without doing any digging, my understanding is that first cousins can really interbreed without too much risk. So a first estimate that 2 completely unrelated mating pairs from distant parts of the human genetic tree might make a minimal gene pool candidate. I can see an argument being made for 8 too. –  shigeta Dec 18 '12 at 2:08
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2 Answers 2

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Actually it is a very important question for laboratory animals (and, I imagine, endangered species) and was calculated to be 25 couples.

With any number of animals (including humans), there is always some inbreeding happening, but you can reduce it with the number of breeding pairs and careful pairing. When you get to 25 pairs (50 animals) and have complete control over pairing, you can sustain the genetic diversity practically infinitely (especially if you take into account spontaneous mutations).

Of course, such control over who can have children with who (plus whether one is at all allowed to procreate and what will be the sex of their children!) would be questionable morally, so in case of populating a distant planet, we would need a larger group, to provide for sexual preferences, fertility problems etc.

Some information on laboratory outbred stocks.

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nice answer - it makes me realize though that there are problems keeping some dog breeds viable - they tend to accumulate too many diseases (link below). Yet there must be thousands of animals in most breeds that are in trouble. Is this because controlled breeding is used differently (emphasizing breeding traits vs diversity), because there was few original dogs in the pool, or why? nytimes.com/2011/11/27/magazine/… –  shigeta Dec 18 '12 at 18:59
    
I am no expert on dogs, but did learn a bit about in- and outreeding. Each dog breed is either open or closed. In a closed breed, only offspring of animals already belonging to this race belong to the race. In an open breed any animal that looks similar enough (or has other important traits, characteristic for the breed) can be a parent of a "breed" litter. Under certain circumstances (e.g. too few animals in the breed, too bad overall health of the breed) a closed breed can be opened and after the problem is solved, closed again. –  jkadlubowska Dec 19 '12 at 9:34
    
How did people arrive at this number? The linked article doesn't seem to explain it. –  ymar Nov 10 '13 at 21:43
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It's just mathematics. If you pick two outbred mice/rats etc. and mate them, the offspring is about 0,5-1% more inbred than their parents (meaning both their copies of any gene are a tiny bit more likely to be inherited from one ancestor mouse). If you have enough animals, this effect is smaller than the effect of other phenomena, such as genetic drift (compare with the assumptions behind Hardy-Weinberg equilibrium). I don't know the exact equasions behind those numbers, I just know, that this was calculated and such-and-such numbers are used. –  jkadlubowska Dec 4 '13 at 19:28
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Inbreeding isn't negative at all, so one couple would suffice for colonization.

Inbreeding fixes recessive traits and the ones displaying unwanted traits can be culled. Actually, inbreeding is one of the most potent weapons of evolution, it speeds things up greatly. We went through a major bottleneck event ourselves and lived to tell the tale.

The exact number of individuals needed for a healthy species that will survive X number of generations depends on the species though. To maintain healthy genetic diversity and establish enough different alleles to allow for sustainability of the species. I don't remember the exact number but I think for humans it was something between 1000 or 10000, and if you get below that number the species will likely go extinct by natural causes.

Of course, everything is completely different when you have full control over who mates with who. But still, in the above example of 1 couple, they can successfully start a seemingly healthy population, but due to low genetic diversity they - as a species - won't be able to respond to, say, increased radiation, changes in atmosphere, a virus, bacteria, shortage of food, etc. That's why 1 couple is basically enough, but to ensure longetivity of the species 25 couples is far from enough.

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And if the undesirable trait is recessive, how do you cull it? Queen Victoria had the allele for hemophila, and after a few generations, it was in all the European royal houses, with multiple people suffering from the disease. –  swbarnes2 Jul 9 at 21:24
    
You cull a trait by simply not procreating the specimens who have that trait. But, first of all, some genetic diseases are not noticeable before old age so you can't know about them unless you study the genome, and second of all it isn't ethical (or desirable, but that's a different story) to cull humans. Especially if it's the Queen. In case of haemophilia, a female can be the carrier but rarely has the disease. It just shows up with certain males down the blood line and in that day and age they didn't know why. –  Dan Horvat Jul 11 at 13:21
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