This is an idea I've been toying with for some time. My idea is a human race where FTL is relatively easy, so colonization is widespread. Its not entirely wild frontier. Habitable worlds are identified and a "Citadel" is constructed. Basically a building that is large enough to house the initial population, has the infrastructure to support them and can act as a seat of government when the colony begins expanding and the population has alternate housing.

Strict birth-control is enforced during the initial phase of colonization, because colonists must first expand their infrastructure and be able to live independent of the Citadel. After the colony is deemed stable enough, the birth control enforcement is lifted.

Now we come to the meat and potatoes. Assuming a fairly small initial population (say 200-400) where the workforce is supplemented with machinery, would it then be a good idea to ensure genetic diversity within the colony by also bringing a cargo of sperm- and egg cells? Basically couples who wish to have children may need to have one or both of their "genetic input" replaced with the "cargo gene pool".

  • $\begingroup$ How long must people stay in the Citadel ? Because it could be a good idea but it could also be necessary ! $\endgroup$ Commented Apr 10, 2017 at 6:56
  • $\begingroup$ The idea is that the Citadel acts as the initial housing of the colony. They stay there for however long it takes them to construct more permanent housing $\endgroup$ Commented Apr 10, 2017 at 7:06
  • $\begingroup$ I got this, but considering the technology of your world, how long does it take ? Two weeks ? One year ? Several hundred years ? $\endgroup$ Commented Apr 10, 2017 at 7:07
  • $\begingroup$ I would assume a few years at most. Colonization is largely monitored and controlled by the government, at least these colonies, and large companies are often contracted to help supply the effort. The colonies are supplied with sufficient materials and materiel to get a running start. By this point, starting colonies is a fairly formulaic procedure. $\endgroup$ Commented Apr 10, 2017 at 7:16
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    $\begingroup$ This is 98% the plot of Mass Effect Andromeda, the only difference is that the 'Citadel' is called the 'Nexus'. You may want to read up on this $\endgroup$
    – vanillagod
    Commented Apr 10, 2017 at 12:14

4 Answers 4


Genetic diversity is an issue, but may not be a deal breaker. There are several islands with populations that descend from a small number of founders: Pitcairn (6 women and 11 men) and Tristan da Cunha (7 women 8 men) have had to deal with much more severe inbreeding. You should screen your colonists for genetic defects. (Tristan da Cunha has a high prevalence of asthma, due to its founders). In general people 50 is enough for the short term, and 500 for the long term.

How easy is FTL travel in your universe? If there is a regular ferry service from other nearby colonies then the introduction of new arrivals can also increase diversity. However, having 200 individuals is on the small side, if there is to be no further influx of arrivals, whereas 400 is probably enough for there to be no inbreeding depression at all. Inbreeding only becomes a problem on the second generation (when recessive traits can start to appear) You may have 20 years to build up new arrivals.

If you are looking to expand the population then the limiting factor may be the number of wombs. If you are taking sperm with you, it is hard to justify the presence of men among the founders. Not having any men makes enforcing the initial birth control phase much easier, and then it makes the population growth phase quicker.

The effective population size is $$\frac{4N_mN_F}{N_m+N_f}$$ where $N_m$ and $N_f$ are the number of fertile men and women (counting vials of sperm as men here). You can achieve a healthy effective population of nearly 500 with just 125 women and a well-stocked sperm bank.

Further reading about this at this page, and at the sites and papers linked there.

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    $\begingroup$ What's Nm if there are no men? 0? 1? Infinity? Number of sperm donors? That formula is more confusing than anything else. $\endgroup$
    – DonQuiKong
    Commented Apr 10, 2017 at 10:32
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    $\begingroup$ No the formula works perfectly well. Nm could be tens of thousands with Nf set at 125. Diversity assured through large and well populated sperm bank (Nm = number of donors). Alternatively you could minimize the sperm bank to a lower number, play with the parameters and get the diversity you need. $\endgroup$ Commented Apr 10, 2017 at 10:59
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    $\begingroup$ Good answer. I would also point out that the inbreeding only becomes a risk on the second generation. So it's only really a concern if the colony is expected to be isolated for more than 20 or so years after its founding. $\endgroup$
    – user16107
    Commented Apr 10, 2017 at 11:34
  • $\begingroup$ @DonQuiKong The source for that formula is nature.com/scitable/topicpage/…. If there are no men then N_m is zero. If there are no men (counting vials of sperm as men here) then you have zero effective size (as you will not have any children being born). The purpose of the formula is to work with populations with unequal sex ratios. But read the source for the reasoning behind the formula. $\endgroup$
    – James K
    Commented Apr 10, 2017 at 13:38
  • $\begingroup$ @dan1111 Thanks, edited to include that. $\endgroup$
    – James K
    Commented Apr 10, 2017 at 13:44

A colony in a new found land is always an highly risky place, where death is more likely than in the colonists' homeland.

Taking for granted that the mortality rate will be higher, controlling reproduction is very likely to threaten the colony immediate future. You need to have as much offspring as possible to replace the deads.

You might consider loosening the family bonds, so that orphans are grown up by the community and have more chances to reach adulthood.

On top of this, having more genetic variability is a good idea in the beginning. Once the colonists have started adapting to the local conditions (it will take some generation) using homeland genetic material will be detrimental to the adaptation.

  • $\begingroup$ Yeah, birth-control doesn't make any sense. Colonists are the infrastructure. $\endgroup$
    – Agent_L
    Commented Apr 10, 2017 at 13:36

Would your cargo gene pool help? Of course it would. You're introducing genetic variety into your gene pool. That will help offset inbreeding. However it might not be the only solution to your problem.

travel is cheap

If FTL is plentiful, why not bring in more fresh 'breeders'? This can either be more colonists from Earth or colonists from nearby colonies that come of age. You could even enforce a policy that forbids mating with fellow colonists. Either the men or the women must go to other colonies.

That allows you a target for your education. You obviously teach engineering to the group that stays, that will maintain your colony. There is another reason why this is plausible. People tend to be not attracted to those they grow up with up to at least age 7. So anyone you saw daily up to age 7 tends to be unattractive to you. You treat them like a sibling. Obviously exceptions exist but that was about 5% I think.

Think of the stretchmarks

I'd argue if your civilization has both FTL and can make a colony with only a few hundred people and no extra imports from home, they can make artificial wombs. Why? Because they remove an important bottleneck.

If you don't have artificial wombs it makes sense to have more women then men. Fertilize the extra women with either cargo sperm or the existing men. Will require a cultural shift, especially the latter. Artificial wombs would allow even men to have children. All he needs is a cargo egg and time. That could create an interesting parent-child dynamic as you'd have a lot of single parents.


Numbers Not Required

If you are wanting to use artificial insemination, why not just use CRISPR, or something like it, to create artificial genetic diversity. CRISPR allows for the precise editing of genes. The unfertilized egg and/or zygote could have randomness injected into its genome to preserve diversity.

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    $\begingroup$ If you can do some minor gene editing, inbreeding is a non-issue. Any genetic abnormalities which occur get corrected through such medical science rather than reinforced through inbreeding. Even a single progenitor (albeit a purely female one artificially generating sperm through stimulation of stem cells) could successfully found an entire civilization if you can repair the mutations which develop now and then. $\endgroup$ Commented Apr 10, 2017 at 16:14
  • $\begingroup$ @pluckedkiwi Excellent point $\endgroup$ Commented Apr 10, 2017 at 16:16

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