So a spaceship is on its way to a new planet. The crew needs to survive during 6000 years on board. They will not get any help and they need to survive on their own. How many persons would need to be on board for the crew to survive genetically?
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2$\begingroup$ Almost any number is fine really, my guess is smaller than what you will need to operate your ship (within most stories, assuming it's not fully automated). The downsides of incest are not as dramatic as one might think. Historically earth was populated by very small groups of people. Some so-called "lost tribes" have been surviving for much longer than that with incredibly small group sizes at times. Also, that far into the future, you can assume we can do some genetic engineering. Perhaps you want to ask instead about other things, e.g. what would be socially acceptable. $\endgroup$– Raditz_35Commented May 8, 2018 at 10:06
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$\begingroup$ Welcome to worldbuilding. Please note that as it is now your question is too broad and simply unanswerable. We need to know which technologies are present on the ship to make a call. With Space Shuttle or LEM like tech the entire population of Earth would not be enough, as they would be quickly dead. $\endgroup$– L.Dutch ♦Commented May 8, 2018 at 10:13
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1$\begingroup$ @Raditz_35 You should post that comment as an answer. If survavility is the goal, the problems with endogamy would not be that severe to risk it. A small group of a few dozens would make it just fine, even if all their descendants are lactose-intolerant or have a strong tendency to became bald before 40. $\endgroup$– RekesoftCommented May 8, 2018 at 10:39
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1$\begingroup$ @Raditz_35 Upfff! I upvoted your answer without realizing it was you. :p $\endgroup$– RekesoftCommented May 8, 2018 at 10:47
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1$\begingroup$ Possible duplicate of What is the minimum human population necessary for a sustainable colony? $\endgroup$– AshCommented May 8, 2018 at 12:00
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3 Answers
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First off lets make some assumptions to reach a baseline, this would then be the absolute minimum number of people needed.
1: All adult couples produce 2 offspring: therefore there is no decline in population, and all survive at least until offspring are produced
2: All babies born will grow up and form a couple with a member of the opposite sex
3: That all couples produce 1 male and 1 female offspring
4: That all members start off genetically unrelated
5: There is no artificial means or technology to assist genetic variation
6: That these rule are abided by by all members for the entire journey
With these rules we know that 2 produce 2 produces 2 produces 2 etc, therefore there is no decline in population but also no unsustainable rise, GP=Grandparent , P=Parent, C=Child, NG=Next Gen. for ease, lets suggest odd numbers are male, even female
GP1+GP2, produces P1 and P2... P1 + P4 = C1 & C2... C1 + C6 = NG1 & NG2
GP3+GP4, produces P3 and P4... P3 + P2 = C3 & C4... C3 + C8 = NG3 & NG4
GP5+GP6, produces P5 and P6... P5 + P8 = C5 & C6... C5 + C2 = NG5 & NG6
GP7+GP8, produces P7 and P8... P7 + P6 = C7 & C8... C7 + C3 = NG7 & NG8
The human race (as a whole) averages 4 generations per century, for each century that passes past this first 1 to ensure that genes never reproduce with brothers sisters or cousins, the initial pool will need to double, however anything past cousins is genetically safe, even cousins are generally genetic safe, in the above example above NG1, is related to all of the above however they are second cousins to NG2 7 and 8, closer for everyone else, therefore genetically safe.
So if the rules were always followed then "in theory" the species would survive for the full duration, however a single early death would scupper the entire venture. and no one knows what genetic effects 6000 years of second cousins breeding would cause in the long run, so a safer number would be 32, this gives 4 times the success than the above model, however that is only if the initial rules are followed. when you realise that these rules could never be truly enforced in a practical sense and some parents would want more than 2 kids but others would want only 1 or maybe even none, then the more the better. i would say a practical baseline would be 256, this gives 32 times the chance of success and genetic diversity.
This is however just a baseline and is assuming there is no genetic "Tampering" or as Raditz_35 suggested, artificial wombs and a stock of genetic material, as a rule... the more the merrier, but also if the story requires a little genetic tension, then maybe try and enforce the rules and see where the story goes...
answered May 8, 2018 at 11:58
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1$\begingroup$ "and no one knows what genetic effects 6000 years of second cousins breeding would cause in the long run" Maybe thats how a bunch of monkeys evolved into (mostly) hairless, pale/not-so-pale, tail-less creatures with short toes and the ability to talk. 6000 years is pretty short on the evolutionary scale, so probably won't get super-humans unfortunately. $\endgroup$ Commented May 8, 2018 at 14:31
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$\begingroup$ @Ron Beyer: Indeed, if you look at the various European Monarchies, they've been breeding with cousins and second cousins fairly consistently for about 800 years in one way or another and it seems the only genetic side effect has been bigger ears. but then again, they occasionally had a small influx of alternate DNA, which could make all the difference... $\endgroup$ Commented May 8, 2018 at 14:41
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1$\begingroup$ @BladeWraith there are actually several major side effects of genetic bottlenecks. The first one that springs to mind is hemophalia. Other royal exampls are the Habsburg Jaw (mandibular prognathism), scoliosis, porphyria etc. Other diseases common in genetic quagmires are obesity, depression, cleft foot, epilepsy, fumarase deficiency, colonic polyposis, Huntington's chorea, Gaucher's & Tay-Sachs, thalassaemia, skeletal dysplasias, diabetes, various vitaman deficiencies & probably colour blindness (if included in the starting breedstock). So yeah, more than just big ears :) $\endgroup$ Commented May 8, 2018 at 19:32
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The answer depends: Do you have artificial wombs? If this is so, the answer is 0, otherwise 1, female.
All you need is a freezer with some eggs and sperm in it, or, since we are talking about the future, just a databank and a machine that can produce that stuff. There is no reason to bring extra crew just for genetic diversity, that would be super uneconomical. Such things will eat your resources. Also, such a journey is incredibly boring, who will want to sit that one out with the 3 video games they brought with? It's much more efficient to create humans once you're there. While you might want more in your story, you can have basically as many as you like if you consider that you can just bring genetic diversity with you.
Just in case you want to go with the 1 single female option (and some daughters/granddaughters at times), consider perhaps at all times spatially separated backups in case there is an accident. But that's not the point of my answer. My point is: If you have any humans on board, don't worry about that stuff.
If you wonder about the requirement "based on present day technology", see the discussion in the comments (I don't want to make the answer longer than necessary), but I think I gave many options to account for different levels of development, from a fully automated wombs to a freezer.
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$\begingroup$ @antweg glad you found an answer that works for you. We do generally advise to wait 24 hours to accept answers. This is because 1) it gives users in other timezones a chance to answer and 2) some users do not answer questions that have a green accepted tick, even if they do have another solution. Don't worry, if they really want to, they will. You just might be loosing out on potential answers. Welcome to the site. Hope you have fun here :) $\endgroup$ Commented May 8, 2018 at 10:58
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$\begingroup$ @EveryBitHelps Thank you for the information! As you guessed I'm new to the site and I'm thankful for your kind help $\endgroup$– antwegCommented May 8, 2018 at 11:53
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$\begingroup$ This is a good, creative answer but I'm having trouble rectifying this against the OP's comment of "present day technology" since we arguably don't have the tech today to support this (caching genetics and generating humans at will, via artificial womb - or even via a female, at least not reliably and consistently enough to make "1" a viable answer). $\endgroup$– dwizumCommented May 8, 2018 at 13:02
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$\begingroup$ @dwizum "The story is based on today's technology", I think so is my answer. I don't read it as "exclusively present day technology". This wouldn't make any sense either because we don't have the tech to build that ship. While we might not be able to implement my approach currently, I would argue that we are certainly working on the basics. As I stated, an artificial womb is optional, but I don't think it's far-fetched super advanced technology that would be absolutely unbelievable in a story set say 30 years in the future (even though one could argue that it might take longer). $\endgroup$ Commented May 8, 2018 at 13:08
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$\begingroup$ That makes sense. Contrasting your answer to the "generation ship" concept really puts the OP in the position of having to clarify requirements in order to choose. $\endgroup$– dwizumCommented May 8, 2018 at 13:12
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You should look into Population Viability Analysis, and Minimum Population Size. For conservation purposes PVA tries to model non-extinction to 1000 years.
This study suggests conventional estimates for MPS - at least 500 adults - are off by at least an order of magnitude (so suggesting a minimum population of 5000 to have a reasonable chance of survival to 1000 years).
The problem with PVA is that is requires multiple assumptions, and the further forwards you project, the less reliable your point estimates.
I'd suggest either really getting into PVA, or picking some suitably large ball-park, at least in the tens of thousands.
That's going to be some large space ship. Either that, or a smaller population, cryogenically frozen (if possible).
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$\begingroup$ Have you read your "study"? It doesn't apply in this scenario. They account for many things that would not be an issue on a build-for-humans-and-long-term-survival spacecraft $\endgroup$ Commented May 8, 2018 at 10:10
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$\begingroup$ Sorry. Disagree. Question asks "how many persons on board to survive genetically' for 6000 years. My answer provides pointers to using real science to address the question. $\endgroup$– NickCommented May 8, 2018 at 10:13
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$\begingroup$ So you are just answering the title without considering the body? just because someone stated some numbers in a scientific sounding article talking about a completely different problem, you can't apply them to everything. That's unscientific, so no, you are not using real science. $\endgroup$ Commented May 8, 2018 at 10:15
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$\begingroup$ No, I am addressing the question as asked, in bold, in the body of the text. One interpretation is that the crew live on a ship for 6000 years; there is nothing in the question that suggests otherwise, and if that's what the questioner has in mind (some kind of huge generation ship) then my answer will be useful. $\endgroup$– NickCommented May 8, 2018 at 10:18
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$\begingroup$ If this is so, you are using numbers that don't apply to the scenario. $\endgroup$ Commented May 8, 2018 at 10:32