Minimum viable human population

Discussion in 'Biology & Genetics' started by alanwc, Nov 23, 2010.

  1. alanwc Registered Member

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    In other threads people have talked about the minimum number of people for a generation starship or to repopulate the earth. Here are a couple of real world examples:

    Tristan da Cunha (current population 264) was populated from eight men and seven women.

    Pitcairn (current population 50 after extensive emmigration) did the same with 15 men and 12 women.

    Both islands are extremely isolated and survived for 200 years. Only Tristan da Cunha had problems with inbreeding and they were relatively mild.
     
  2. jmpet Valued Senior Member

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    Um... okay- thanks for that info.
     
  3. Fraggle Rocker Moderator

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    The figure I've seen is 600. Two centuries isn't enough time to guarantee that the problems of inbreeding will manifest.
     
  4. Mr MacGillivray Banned

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    The first post contradicts your statement. It said that the problems with inbreeding manifested themselves on Tristan da Cunha within 200 years.
     
  5. alanwc Registered Member

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    More info

    I've done some more checking. The genetic "abnormality" on Tristan da Cunha is asthma. Three of the original settlers had it.

    200 years is 10 generations. It's hard to believe inbreeding problems wouldn't have shown up by then.
     
  6. alanwc Registered Member

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    600?

    Where did the figure of 600 come from?
     
  7. Skeptical Registered Senior Member

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    The minimum number is a variable.
    It depends on who the original people are - how genetically fit. More importantly, it depends on how much they are supported by the environment they live in. If the environment is salubrious, then quite sickly people can survive. If it is a rough environment, then the unfit are removed quite rapidly.

    In theory, one man and one woman could repopulate the Earth, under the right conditions. There would be horrendous mortality in the first few generations, when more than half of those offspring from brother/sister matings come up defective. However, the silver lining is that defective genes would be removed from the population very quickly also. The resulting human population would have limited genetic diversity, but humans have not relied on that for survival for a long time. We adapt our culture and technology to survive - not our genes.
     
    Last edited: Nov 26, 2010
  8. Dinosaur Rational Skeptic Valued Senior Member

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    50% mortality?
    What is the source of that statistic?

    A lethal gene in both parents would have an expectation of a 25% mortality rate in the first generation. The expectation in the second generation would be about 17%

    The above would be for a 100% lethal recessive.

    Incest is only harmful if both parents have the same harmful recessives.

    Cleopatra was the result of several (perhaps 4-6) generations of brother/sister marriages. The royal family of Egypt at that time did not consider anybody worthy of marrying into the family.
     
  9. Hercules Rockefeller voltage gated ion channel Moderator

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    Yes, but only a small percentage of alleles follow the classic Mendelian dominant/recessive mode of phenotypic inheritance. There are very few genuine Mendelian traits in humans. The majority of alleles contribute to phenotype in an additive and/or co-dominant fashion.

    Having said that, it’s the potential ‘uncovering’ of lethal recessive alleles that poses the short-term risk of inbreeding. But it’s quite unlikely that will happen; there isn’t much risk to inbreeding in the first few generations. The risk in the longer term lies with the fixation of allele frequencies in small in-breeding populations, and the resulting loss of fecundity, embryo viability and developmental defects that always arise as a result.

    As for the minimum human population required for a generational starship, I don’t know. :shrug: I’ve seen many different figures offered for this question. Does anyone have a scientific reference to support a particular figure?
     
  10. Skeptical Registered Senior Member

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    To Dinosaur

    If we accept the premise that one man and one woman are trying to repopulate the world, they will not have the support of an industrialised civilisation. Life is tough when you are in that situation. Thus, non fatal defects will still prove fatal, because you do not have the support structures to ensure survival.

    The other point is that, for a few generations, defects accumulate. A brother/sister mating results in offspring with about a 50% chance of serious - not necessarily fatal - defect. However, if brother and sister in the next generation also produce offspring, the percentage rises. Over many generations, the percentage of defective offspring continues to rise until natural selection dominates, and most of those genes are eliminated.
     
  11. Hercules Rockefeller voltage gated ion channel Moderator

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    No, it doesn't. Why do you think that?
     
  12. Skeptical Registered Senior Member

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    http://en.wikipedia.org/wiki/Incest#Inbreeding

    I quote :


    "A study of a group of 21 made up of brother-sister or father-daughter offspring found that 12 had abnormalities with 9 of which were classed as severe."

    OK, I rounded this to 50%. 12 out of 21 is more than 50%. 9 out of 21 is less. Either way, we are talking of serious consequences.
     
  13. Mr MacGillivray Banned

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    The literature suggests that the minimum viable population is entirely dependent on the environment. The more marginal the environment the larger the viable minimum population needs to be.
     
  14. jmpet Valued Senior Member

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    The more the merrier. 600 people will yield 50 viable mates- the rest being same sex (50%), too old, too young or simply not desirable.

    600 is a good starting point. Of course the generational ship will have to accomidatate 2,000+ people over time which I think is the pop max for a space ship.

    A genetic mix of 600-2000 is enough to overcome mutations and ensure an indefinite specie.
     
  15. Fraggle Rocker Moderator

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    But it also said that it did not manifest on the other island--yet.
    Oh I have no idea. But I'd be surprised if it's been revised by a whole order of magnitude.

    So your scenario is not a carefully planned crew for a generation starship, but just the tiny bunch of humans who happened to survive the disaster that wiped out everybody else? If you're recruiting people you'd check their DNA for hereditary problems. And you wouldn't take too many really old people because you need them to keep things running, not just for breeding, so you don't want them to die two years into the mission. You would also take a bunch of frozen sperm and ova to chlorinate your gene pool even better.
    I'm sure that figure was derived by arm-waving. Obviously the more the merrier when you're talking about trying to found a new civilization.

    Can we even envision a way to build a ship that will hold 600 people? For starters, it has to be built in orbit because we'd never be able to launch something that massive out of our gravity well, but still, all the individual parts will have to be carried up there unless we can find the raw materials on the moon. How much fuel will it need to carry? Even if it uses a Bussard ramjet (which another member recently advised me is not nearly as practical as sci-fi authors make it seem), accelerating to Bussard velocity will take a while, and during that time we'll be using conventional propulsion.
     
  16. John99 Banned

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    The minimum viable human population is two men and two women and they would be coupled 2 and 2. The 4 are fertile and able to reproduce, obviously.
     
  17. John99 Banned

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    Here is the sequence start:
    ||-||
     
  18. Hercules Rockefeller voltage gated ion channel Moderator

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    I have a developmental biology background; I performed genetic research using zebrafish and Drosophila. Two of the cornerstones of genetics are sibling crosses and back crosses (ie. breeding siblings with parents). The output from these crosses enable you to determine the inheritance patterns of given traits of interest, be they natural or genetically engineered.

    I have performed more sibling and back crosses in zebrafish (a vertebrate) and Drosophila (an invertebrate) than I could possibly remember, and I can categorically tell you that it isn’t the case that 50% of the progeny from such crosses are severely abnormal (unless, of course, they are a specific line carrying a disease allele). Somehow I think I (and the countless other developmental biologists worldwide) would have noticed if that was the case!

    Of course, genetic research with model organisms utilises inbred strains where many deleterious recessive alleles have been bred out, thus increasing the chances of normal progeny from such crosses. The instances of abnormalities from inbreeding crosses of outbred lines does increase, but you don’t go from zero to ~50% when comparing progeny from inbred and outbred lines.

    As for your wikipedia example, the devil is in the detail, as it is with all scientific studies. Without reading the precise methodology of the study I can’t comment on it. The simple explanation for those results is that the families chosen contain alleles for genetic disorders and are not, as such, “wildtype” matings. Indeed, the preceding sentence emphasised that abnormalities from inbreeding "...varies depending on the number and quality of inherited congenital disorders of which the family members may be carriers."
     
  19. Hercules Rockefeller voltage gated ion channel Moderator

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    In the long term it almost certainly isn't.
     
  20. Skeptical Registered Senior Member

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    Hercules

    Without meaning to be nasty, I have to say that crossing Drosophila and zebrafish is not exactly a definitive guide. Humans are different.

    We have had a social taboo on incest since prehistory, and for good reason. Significant and substantial defects in the offspring of incest has been known since biblical times.

    Actually, the worst defects tend not to happen, simply because the result instead is miscarriage. Incestuous relationships lead to a higher than normal rate of miscarriage, because doubling harmful recessive genes often means the child dies in utero.

    The other point worth noting is that the worst effects of incest tend to happen over several generations. For example : if a man and his cousin marry, their children will mostly be OK. But if one of those children then marries his/her cousin, then the chance of defective children rises substantially.
     

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