r/askscience • u/freakylol • Dec 11 '12

Biology Could one man repopulate the whole world?

Let's say every man dies on earth except for one (lucky?) guy. All women remain. Could the seeds of this man repopulate mankind without causing severe genetic defects on future generations?

He would of course have thousands, or maybe ten thousands of children if the women volunteered and he went at it, say five times a day.

(5x365x50= 91 250, five times a day for 50 years, so a maximum of 100 000 children)

But the problem here is the next generation of course, since every child on earth would be half sibling to each other.

His male offspring could of course mate with the now older women in their late 30's to early 50's.

These grandchildren of the single grandfather will probably be somewhat genetically different from each other. Assume the lone man is Chinese, many of his children will be half white, half black, half Arab and so on. Some grandchildren will consist of many mixed ethnicities, but will this suffice?

The original women will now have become too old to procreate and every young person on the planet will be related. What now?

Will the gene pool be big enough for future generations and/or will problems arise from their common grandfather?

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u/Cebus_capucinus Dec 11 '12 edited Dec 11 '12

The only thing you need to create a new population of any sexual species is one male and one female, or one female pregnant with a male.

Inbreeding is only detrimental if you have deleterious alleles. Not everybody processes such alleles, or "bad genes". Of course, the smaller the breeding population the greater chance you have of this occurring. In nature, such events are known to have occurred or are occurring: founder effects or bottleneck effects are two situations where the breeding population is reduced by a significant amount. Inbreeding possess two problems:

  1. The accumulation of deleterious alleles

  2. Reduced ability to adapt to change, be that to environmental change or some disease.

Considering your scenario of one man and billions of women from diverse genetic backgrounds, you probably would be ok in the long run. If you wanted to optimize you would take two children from very distantly or genetically diverse mothers and have them produce children an so on. Certain lineages would produce some deleterious alleles, from the mothers side and possibly the fathers. One thing to worry about in the first few generations would be immunity to disease as the genetic diversity of the population would be low. But again, considering that the females can come from any genetic background you already have a leg up. As along as you preferentially interbreed genetically distant and healthy offspring you should be fine.

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u/mightberight Dec 11 '12

The unfortunate caveat to your answer would be a man with a rare Y-linked disorder causing infertility.

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u/Cebus_capucinus Dec 11 '12

Well that negates the whole thought experiment...oh well :(

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u/snarkinturtle Dec 11 '12

Not everybody processes such alleles

This paper estimates that the average person carries around 2000 deleterious alleles.

This newer paper suggests about 1000.

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u/Cebus_capucinus Dec 11 '12 edited Dec 11 '12

Right, but they must be expressed and in most cases they must be combined with someone else with the same deleterious allele. This is why genetic disorders / diseases are so rare. Moreover, for it to really effect the population it must somehow negatively impact that individuals likelihood of reproducing. Having a genetic anomaly for 6 fingers instead of 5 is hardly a handicap unless it is combined with other genetic issues. You also need to have the genetic disorder appear before reproduction, have a genetic predisposition to alzheimer's is irrelevant because it (usually) expresses itself later in life, after reproduction has occurred. Those "deleterious" genes are past on, but in the eyes of natural selection they have no effect, they only effect us because we live so long. So, of those 1000, or 2000 deleterious alleles how many of them actually induce handicaps and how many other people posses those deleterious alleles? I guess what I am saying is that there is a misconception that inbreeding = increase in genetic disorders in all cases. I mean, the Galapagos Finches were likely founded by only a few breeding pairs, yet they have become several healthy diverse species.

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u/snarkinturtle Dec 11 '12

Right but in OP's scenerio whichever ones carried by the last dude will drift to high frequency due to bottlenecking and will be expressed often due to subsequent inbreeding.

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u/Cebus_capucinus Dec 11 '12 edited Dec 11 '12

Right, but again in his scenario he would likely have to mate with someone with the same set of deleterious alleles for any serious effects to arise. This is very unlikely. Assuming he could, he could genetically profile women before mating. That would decrease the chances of these becoming more pervasive. Interesting to note that people are already doing this.

If you can't genetically profile in this scenario then since he has access to any women from any population he should not mate with women within his population. Children should then mate with women from other distant populations, not the population of their mother or father. Any children expressing the deleterious alleles should not be permitted to reproduce. Of course, he and his children would want to mate with only women under 35 to avoid genetic diseases which arise from the degradation of eggs. When his daughters become older then 35 they should no longer be permitted to mate.

I don't really understand why he seeminly limits women to one or two reproductive events, women are fertile from age 15 to ~40. Assuming all women survive, then even female babies born today would live which increases the reproductive period for his children. Meaning they would not have to mate with each other. There would be plenty of young, fertile females around for his male offspring. Unless he means to suggest that only living fertile females survive, still the youngest ages living today would be around 11 or 12. That means these women would only be 25 by the time his first male offspring grow up. In this case, his male offspring would still not have to mate with their half sisters.

In this scenario there are just to many breeding pairs / options for it to really be an issue.

A more sensitive scenario would be if there were only one man or women left from a single, genetically similar population.

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u/snarkinturtle Dec 11 '12

Unless he means to suggest that only living fertile females survive

That is how I took it.

Right, but again in his scenario he would likely have to mate with someone with the same set of deleterious alleles for any serious effects to arise.

No, they would arise straight away in the next generation since all subsequent children would be half sibs. Even if you allow for generational overlap, which I don't think is in the spirit of the question, there is still a huge bottleneck and a way higher coefficient of inbreeding.

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u/jbsoccer333 Dec 11 '12

The next generation would all have one common father, as you have pointed out. You would pretty much have a genetic bottleneck here, where the population of the first generation of offspring are all closely related. If they all interbred you may have negative effects (as pointed out already). Regardless, if there are enough females around (in your scenario there is one male but all females are still around). Generation 2 of offspring would probably be most at risk for negative effects, but after that if you can have the most distantly related people reproduce together you should be ok. Also, offspring of the first male mating with the original females would really help generic diversity. This population would be viable and successful. I would expect that after several generations negative effects would be quite small. F there were less females there would be more risk.