Re-populating the species

Just wondering … If there was a disaster and I mean a DISASTER on that wipes out 99.9% + of the human species. What is the smallest number of (fertile)survivors required to re-populate the Earth with enough bio-diversity that you wouldn’t be wading at the shallow end of the gene pool for all time.

10 ?

50 ?

100 ?

Keep in mind that we dont want offspring with 3 arms and 5 legs…

5 legs might be a survival advantage after whatever disaster befell the bipeds.

[hijack]There’s more than breeding and genetics to consider too; for example it would be quite difficult for a lone couple to survive without any support from a community (yes, I know there are people that live in remote log cabins, but do they manufacture all their own clothes, tools, food, toiletries etc?).[/hijack]

Sorry, carry on.

Humans cannot survive without others around them. We have evolved as social animals and since about the time we were Homo habilis, we’ve always lived in communities. Given that various studies (no cites I can think of, all from memory I’m afraid) have indicated that humans optimally live in groups of 50-200, I would suggest that a minimum of 50, preferably with survival skills. (They’ll have to make their own clothers, grow or hunt their own food, make their own cars, brew their own booze and style their own hair.)

Well what if 99.9%+ of the population were just rendered infertile? And assume there were enough people around to keep the fertile few socially surrounded for two generations or so. How many would we need then?

I’ll WAG that two would be enough, if they were young and started breeding like rabbits. Sure, some of the grandkids will have genetic problems, but not necessarily fatally so.

How many fatal recessive genes does the average person carry? Lets call it R. For each pair of kids breeding, there’s a 25 percent chance both carry it, and then a 25 percent chance that their kid will get two copies, and die. So you’re looking at 93.75 percent survival rate for each bad gene. The total survival rate for the grandkids is then 0.9375[sup]2R[/sup] If R = 5, that’s a 52 percent survival rate for the grandkids. For R = 10, it’s still a 27.5 percent survival rate. Assume sixteen kids, and you get about a doubling to quadrupling of the number of people for each generation following the first sixteen kids, so the population will recover. The average number of fatal recessives will drop as they get selected against, so the survival rate will slowly improve in succeding generations, and we’ll make our way out of the shallow end with time.

The harder question is whether some other species will start encroaching on our niche before we’ve recovered in number and genetic quality.

On the other hand, if R is more like 50, we’re screwed.