Minimum number of breeding pairs for humans to repopulate the world?

I recently watched a pretty good post-apocalyptic short movie that ended with a man and woman finding themselves to quite possibly be the last humans alive. I assume that it would not be possible to repopulate the world with just two people, even assuming extraordinary luck in every other aspect the bloodline would quickly collapse through inbreeding.

So what would be the minimum viablle number? Assuming equal numbers of men and women (the whole one hundred women for one man thing often seems as much juvenile fantasy as anything else).

I do know that during the Toba eruption there is some evidence that a genetic bottleneck occurred and the total human population may have fallen as low as 1000 breeding pairs (what romantic terminology)

Thanks everyone, answer not needed fast :slight_smile:

I thought I heard something about the Toba bottleneck maybe being as few as 500 “breeding pairs”. :rolleyes: I agree there’s something lacking about that terminology (nevermind that humans are under no obligation to form just one monogamous pair-bond per lifetime, so just what counts as a “breeding pair” anyway?)

Another item seldom confronted in such scenarios is the required culling it takes to come back from such a bottleneck. Nor is it a frequently advertised aspect of rescuing moribund species like the California Condor. With such a limited gene pool you will get some terrible gene combinations and natural selection, if not human intellect, will be brutal in eliminating them. The long term upside - IF the species survives - is that you might actually eliminate some really bad traits. The downside is the appalling death rate that results.

A very few individuals will get gene combinations that are at least as good, or maybe even better, than the prior average and that’s what the future of the race will be built upon. Maybe.

So… how brutal are you willing to be here? How controlled the pairings? Because if you’re coming back from near-extinction successful reproduction and species survival might require you to reproduce with people you otherwise find awful. Or require every individual to reproduce with as many different others as possible to keep the gene pool as wide as possible - which will play merry hell with human interactions.

Whooping cranes- 15 adults in 1938, but have been brought back to "Endangered’ with maybe 400 of them.
Ca Condors- 22 in 1987, now over 400.

Virginia northern flying squirrel, 10 in 1985, 1100 now.

300 American Grey Wolves in 1960, now maybe 6000.

So, we dont* know* how few you need.

What would be the failure mode if there weren’t quite enough breeding pairs? All offspring die off because of genetic defects?

In animal husbandry the male to female ratio is about 1:25. Where you need to be curious is in how many human females would be necessary.

Only one pair would be necessary to repopulate the world. While reduction of genetic diversity would not be a good thing, it does not inevitably cause a “collapse” due to inbreeding. The real problem occurs when a population remains very small over many generations, or if certain deleterious alleles become fixed in a population.

Cheetahs at one time experienced a very severe bottleneck that severely reduced their genetic diversity. Today cheetahs are all so similar genetically that they can accept skin graft from one another without rejection. Despite this they were a highly successful species with an enormous range over most of Africa and south Asia before being reduced by hunting by humans.

The Black Robin of New Zealand’s Chatham Islands was reduced to a population of just 5, only one of which was a fertile female. It has now been brought back to 250 individuals.

This reminds me of a short story by Robert Reed.

I guess, with a bit of luck, one pair might suffice. But they would have to be young, very fertile and live for many years. In particular, the woman must not die early in childbirth. I think it would be vastly better if they were from different races that had been isolated a long time to maximize the differences. It would also help if the man knew something about helping with childbirth. All-in-all, a very chancy proposition. They should breed early and often.

To some extent, the minimum viable number depends on how good your genes are in the first place. A group of 100 who are randomly selected from a global population will have a lot more diversity than 1000 individuals who are all cousins. Also, a population riddled with bad recessive genes needs a much larger base than one with “perfect” genes.

And is cloning theory and practice approaching the point where one female sans male might be all you need?

I think the problem with inbreeding is not so much the birth defects, but the fact that if a population lacks genetic diversity, it is less able to respond to changes in the environment. A normal population will be diverse enough that should the environment change, some should be equipped to handle the change and survive. If the population lacks diversity, what kills one may kill all.

Grey wolves never got to that low of numbers. Maybe in the contiguous US, they did, but that’s not the only place the wolf lives. Note that the wolves reintroduced in the western US came from Canada.

Perhaps the best example to use would be the inhabitants of Easter Island. I don’t know how many they started with, but I assume it was one flotilla, so what - maybe 50 “breeding pairs”? Plus they would have been at the tail end of progressively more isolated populations on more and more remote islands. Before environmental collapse and/or European diseases came along, they were up to 15,000 and no indication of genetic issues.

True. But …

The animals still give birth to very close to a 50/50 mix of M & F. We achieve the 1:25 ratio by culling 24 of every 25 M and only letting one M breed. Given our god-like power over our domesticated animals this is easy to achieve.

We also have, after thousands of years of practice, a pretty good system for deciding which male is the lucky winner. We’re also selecting for very simple traits which manifest in the equivalent of early childhood and adolescence. Veal is eating the equivalent of a toddler. Full-up beef is eating the equivalent of a 10 year old human. Other domesticated animals are slaughtered at similar relative ages.

In an apocalypse scenario we could certainly manage by luck or skill to have the 1:25 ratio in the initial survivors. But what about the next generation? Who/what is going to exert that godlike power to prevent 96% of the males from breeding? And how is that agent to decide which one is the “good” one to permit to breed? Which traits are useful and when in the life cycle do they manifest?

There is no such thing as relative ages between humans and other animals. Many newborn animals can stand and walk immediately after birth, which humans can’t do until we’re a year or two old, but on the other hand, many animals are born blind and don’t open their eyes until weeks later, while humans do so immediately. Or you can compare the ages at which we’re able to reproduce (somewhere in the teens for humans, about a year for many other mammals), or the age at which we die (the better part of a century for humans, much shorter for everything else), or the age at which we’re able to talk (never, for most animals). And they’re not at all proportionate: For instance, it’s unremarkable for a cat to breed at a year old, and then to live to be 20, but the equivalent for a human who lives to 80 would be to breed at age 4. So is a 2-year-old cat the equivalent of a 30-year-old human (twice breeding age), or an 8-year-old human (a tenth of the way to death)?

I take your point about the skin grafting as evidence, but it made me smile: I’ll be disappointed if the research report did not cite Jeremiah 13:23. Then they could pass for/pass it on to their cousins.

Darn good points all. At the detail level some few species are loosely comparable, but most others certainly are not. The comparison doesn’t stand up to *quantitative *scrutiny. As you say.

I think however it is valid to point out that we slaughter livestock for food fairly early in their lives compared to their lifespan absent predation. The comparison to toddlers & teens is qualitatively valid as long as we don’t push it too hard.

The meta-point relevant to janeslogin’s comment being that it’s more practical to selectively breed for traits which manifest in juvenile animals with short natural lifespans than for traits which manifest mid to late in the lives of long-lived animals. The former being what matters for food livestock and the latter being what matters for rebuilding human society.

With the right circumstances, only a man and a woman is needed.

Breeding pair; defination

I suppose there’s a breeding menage a trois, and so on?

I don’t know the scientific answer, but there aren’t many genetic risks of first cousins procreating together. I’m not sure how that would affect the number of breeding pairs needed, but this also assumes the parents of the cousins were not related as far as I can tell.

First cousins share what, 12.5% of their DNA? I wonder what happens if two half siblings procreate. The dad has sex with two women, and those children procreate. They’d have 25% of their DNA in common. Not sure what he risks are from that.