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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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  • $\begingroup$ 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. $\endgroup$ – shigeta Dec 18 '12 at 2:08
  • $\begingroup$ It depends on how you select the people, if they are carefully selected for genetic diversity you might be able to get away with a hundred, but you have problems if even a few people die, which is not unlikely in a completely new environment. So even with careful selection and breeding controls you still want a few hundred. the more other things like skills matter the more people you need becasue you can't just select for genetics which brings you closer to 500 individuals. If people are selected more or less at random (genetically) you want more than a thousand. $\endgroup$ – John Nov 27 '16 at 21:23
  • $\begingroup$ The big problem is NO new genetic material is going to be introduced (aside from mutation), and the population is going to be expanding, dying, and need to continue into perpetuity. So you need a a lot of initial diversity. This isn't a lab study where you only need to maintain for a few generations. $\endgroup$ – John Nov 27 '16 at 21:25
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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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    $\begingroup$ 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/… $\endgroup$ – shigeta Dec 18 '12 at 18:59
  • $\begingroup$ 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. $\endgroup$ – jkadlubowska Dec 19 '12 at 9:34
  • $\begingroup$ How did people arrive at this number? The linked article doesn't seem to explain it. $\endgroup$ – ymar Nov 10 '13 at 21:43
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    $\begingroup$ 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. $\endgroup$ – jkadlubowska Dec 4 '13 at 19:28
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    $\begingroup$ Have you considered women's point of view? That in this scenario somebody decides to use a woman's body to generate some number of children and you cannot decline, you are literally forced into pregnancy? Pregnancy and childbirth, which, even with modern medical advancements, are an important cause of death for women? I guess the people who board such a ship can make this kind of a decision, but their children would not be given any choice with regard to bearing a second generation. $\endgroup$ – jkadlubowska Oct 19 '15 at 20:16
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I'm not an expert, but after reading more about it I would say between 500 - 1500 couples without trying to control the pairing.

I don't know where Dan Horvat took these assertion:

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.

but it seems to be supported by the evidence of past bottlenecks in human population, like the Toba Cathastrofe Theory. The human population is supposed to had a maximum of 3,000–10,000 surviving individuals. I am very optimistic and take the minimum: 1500 couples. It left to match that amount with Dan's source.

The exact answer would come by the designer of the life and DNA. ;-) That info seems to have been leaked by the creator's son in a conference. Thomas has taken note from his words:

I shall choose you, one out of a thousand, and two out of ten thousand, and they shall stand as a single one.

It seems to me that it is talking about genetic diversity, and the minimal proportions are those: 1/1000. That's why I take 500 couples as minimun.

By the way, in the previuos versicle it seems to says that that selection would be in the moment when the human race has finally ended evolving to the intended social and morphological status (very rare indeed):

When you make the two one, and when you make the inside like the outside and the outside like the inside, and the above like the below, and when you make the male and the female one and the same, so that the male not be male nor the female female; and when you fashion eyes in the place of an eye, and a hand in place of a hand, and a foot in place of a foot, and a likeness in place of a likeness; then will you enter the kingdom.

Going back to science, it seems that not to much effort should be made to control the pairing, because we seem to have a subconcient attraction for those individuals of the other sex that present better histocompatibility with us. In the case of humans given by odor and appeareance (mostly simetry).

Also the genetic diversity is supposed to be reached faster, if the environment is more aggresive. Normally a big mutation and adaptation happen after hostile and persistent environment over near 40 generations, something observed with the Peppered moth evolution

So I suppose that if you use well cared laboratory animals, the number can be 25 couples. But if you use a natural hostile enviroment the genetic diversity could be reached sooner.

It might be that if you build a nice human space station in another planet, maybe you need 25 couples, but if you let them nude with no technology and fig leaves you need fewer couples in order the species to survive more adapted (Although you might got many failed attempts)

Another point to consider, is how valuable are those animals for genetic studies?, how much is the study affected by the captivy of their population?, resembles their growing places the natural environment?, how hostile is these environment? resembles this environment the natural population dynamics ? how many generations ago their ancestors had a natural environment ?

Laboratory animals might have less genetic diversity and might need more couples.

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  • $\begingroup$ The distinction between this being the correct answer and the one chosen, has to do with the level of genetic preselection. I agree that a minimum of 1000 "randomly chosen" people would be required, as one must not forget the impacts of homosexuality and varying levels of "fidelity" in mating pairs. $\endgroup$ – ZacWolf Mar 30 '17 at 16:09
  • $\begingroup$ I guess than homosexuality and fidelity might have also to do with epigenetics and the needed dynamics of the population. In some fishes like: "Lythrypnus dalli" , even the genre of the individuals changes at a morphological level according to the necessities of the population. (sciencedirect.com/science/article/pii/S002209810400680X) $\endgroup$ – yucer Jan 12 '18 at 0:33
  • $\begingroup$ Also it might be that less individuals and good conditions for expanding the population generates less "fidelity". $\endgroup$ – yucer Jan 12 '18 at 0:41
  • $\begingroup$ At the contrary, an excess of genetic diversity in a population (if such concept exists) might generate more fidelity. $\endgroup$ – yucer Jan 12 '18 at 0:41
  • $\begingroup$ And also... the homosexuality could be the response of big populations to with a low amount of genetic diversity and a deficit of natural selection. $\endgroup$ – yucer Jan 12 '18 at 0:44
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I think the issue of epigenetics throws this answer into debate once again. This reminds me of my biggest issue with the book The Transhumanist Wager, which was centered around a very Libertarian tenant of "worth".

Without knowledge of the way in which every aspect of human genetics (including epigenetics) works, how can we possibly establish the "worth" of a given set of base allele, and thus determine minimum viable population? If such a bottleneck were forced (through disaster or "choice") would such a resultant population still be considered "human" by today's standards?

I would say that Humanity must still be defined by more than the sum of our current knowledge of genetics.

I believe these questions are something that are very often missing from the debate around The Singularity [establishing an artificial intelligence beyond the intelligence of humanity]. Most argue that such an event would be the end of humanity, but I disagree; as any intelligence created by humanity would not be so stupid as to destroy the diversity of intelligence that essentially established its baseline. In this line of thinking, personally I believe it would be more along the lines of the ideas presented in the movie Her, in which such Intelligence(s) would establish a way to remove themselves from direct human control, but ultimately observe/compare/contrast humanity as "wild stock" useful for error-correction, and a seperate line of modeling.

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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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    $\begingroup$ 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. $\endgroup$ – swbarnes2 Jul 9 '14 at 21:24
  • $\begingroup$ 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. $\endgroup$ – Dan Horvat Jul 11 '14 at 13:21
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    $\begingroup$ Actually, that's a much more complicated case. The short answer is: you can't be completely certain you've culled the recessive allele. You can diminish it's frequency and this has been done in a neat way in the Mediterranean with thalassaemias (a group of diseases related to sickle cell anemia) - young couples with non-negligible risk were tested and if both carried the gene, they were advised against having children or given access to abortion if the fetus turned to be sick. Given stricter control, reduction could be achieved fasted, but when the allele becomes rare, it gets more difficult. $\endgroup$ – jkadlubowska Oct 19 '15 at 20:30
  • $\begingroup$ Dan Horvat's scenario is only theoretically feasible. People have, on average, 15 damaged genes that may lead to a deadly disease. The first couple (and next generations, too) would have to produce lots of children in order to give them a good chance of findinf couples who can have healthy children. Much more likely this scenario would result in a population with uncommon genetic diseases being common, like in Finland (which artificially created a series of bottlenecks from the 17th century, a fascinating story), read more here: cs.sjsu.edu/~khuri/Aalto_2010/Norio_2003.pdf $\endgroup$ – jkadlubowska Oct 19 '15 at 20:38
  • $\begingroup$ Hey, Dan. Where did you got these numbers from: "1000 or 10000" ? $\endgroup$ – yucer Nov 27 '16 at 14:45
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You only need a single female. The space ship could carry a database of genetic material in the form of frozen sperm and eggs. The first female would impregnate herself to breed and raise as many daughters as she can, using as much genetic variety as possible. The daughters would do the same once they can.

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  • $\begingroup$ Welcome to Biology.se. You should always add references for your claims. $\endgroup$ – have fun Aug 11 '18 at 14:37
  • $\begingroup$ I didn't mean to convey the scientific insight that the OP was presumable looking for; I was hoping that would be clear. This answer points out a logically valid alternative interpretation of the question. $\endgroup$ – A. Hennink Aug 13 '18 at 4:48
  • $\begingroup$ In this case yours is probably more suitable as a comment $\endgroup$ – have fun Aug 13 '18 at 17:06

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