Would the human race survive?
What I’m actually asking is: how many people would it take to keep our species going? I’ve read quite a few speculative and science fiction stories in which, due to some disaster, the breeding population falls to less than 20. I’ve always thought that, due to inbreeding, they would all be drooling idiots within a few generations, right?
PS: No cheating with frozen sperm or wierd mating rituals.
About the best number I’ve been able to find for the minimum viable population (MVP) for humans is 500.
Probably not right. The whole drooling idiot thing has been overstated. Inbreeding does not lead to idiocy. It leads to whatever recessive genes that happen to be in the gene pool coming to the fore. So it would depend on who the 20 people are. And what defective genes they might or might not have.
Of course as Darwin’s Finch’s post point out, that not the only fator
Since Darwin’s Finch’s link is to a paper published by University of California (Berkeley) it would seem only fair to counter that with a reference to the first two chapters of Genesis. 
[sup]Not that I believe that one either.[/sup]
The number of men survuving is very much less important than the number of women - but you’ve got to have enough men surviving to provide for all the women once the women are pregnant or mothers.
Well, yes and no. In a true Last Man on Earth scenario, even if all of the women are left, and the Last Man refrains from impregnating enough that there’ll be able-bodied single women to keep the infrastructure up, you’ve still got a genetic problem, in that everyone in the next generation is going to be half-siblings or closer. That can’t be good for the gene pool. I suppose one could dilute this some, if some of the male offspring mate with surviving females.
Recovering genetic diversity is pretty much a matter of having as many different combinations of offspring as possible. Inbreeding should be minimized to the extent practical… mainly in the form of prioritizing more distantly related mate pairings over, say, parental back-crossing.
It’s actually pretty easy to keep track of whether you’re maximizing genetic diversity or not. Assign letters to everyone in the original population… and the offspring are assigned the combinations of letters and an identifying number. (because AB1 will not have the same genes as AB2). The fewer letters in common are shared, the better the cross, generally speaking.
Supposing you start with three people, A B and C. They will have the offspring AB BC and AC. When the offspring reach sexual maturity it is actually better to cross AC with B, BC with A, or AB with C than to mate half-siblings or do a parental back-cross.
Ummm… what gender exactly are A, B, and C? If A’s a girl and B’s a guy, then C must be a girl to get BC… which means AC is the offspring of… zuh?
This whole alphabet thing would work with, say, elephant seals, but I’m not sure humans would be willing to put up with that arrangement.
And if there really was a disaster, things like artificial insemination may not be possible.
So we all die. It happens to all species.
Not much to ad, but I wanted to ask Susan to say “Hi!” to her Grandfather for me.
My participation in such a worthy cause would depend greatly on who we are talking about. Carmen E and J Lo, or Martha Stewart and Rosanne?
Assume the following scenerio: unspecified disaster kills every human male on Earth, but the infrastructure of civilization survives (like the comic series Y). A single male astronaut returns to Earth and starts siring children. Let’s be conservative and say that in the long term he could father 100 children a year, 50 male and 50 female, and that he can keep doing so for 40 years. As soon as the male children reach puberty at around thirteen, they begin fathering children. Since there will be non-related women still fertile up to forty years after the “event”, the first year’s batch of 50 sons could impregnate non-related females for 26 years (40 minus 14 [ 1 year sons were gestating and 13 years to reach puberty]). So… gawd, I’m getting dizzy trying to work out the formula but I think it’s something like 50 x 100 x 26 = 130,000 children the first year’s 50 sons could sire by unrelated women. The next year’s “class” of 50 sons would get a chance to sire 50 x 100 x 25 children by unrelated women, and so forth. So my math gives me a total of 1,565,000 children eventually born of an unrelated father and mother. At that point all 1,565,000 children have the same paternal grandfather (our intrepid astronaut) but no other shared grandparents. That doesn’t sound too narrow a gentic base, especially if the women involved were chosen for the broadest range possible.
Oh, duh, I forgot there would be a significant contribution from the next generation. By which I mean:
Year 0: the “Event”. Astronaut returns to Earth and begins his second career. Last generation of women unrelated to astronaut born.
Year 1: First generation of sons born. Succeeding yearly “classes” of sons born for next 39 years.
Year 14: First generation sons begin siring children.
Year 15: Second generation of sons born.
Year 28: Second generation sons reach puberty, begin siring children.
Year 40: Average age that last generation of women unrelated to astronaut reach menopause. All subsequent pregnancies by related women.
The math is beyond me at this point. Can someone come up with a formula? The variables are onset of menopause, gestation period, age of puberty, number of children that can be fathered per year, half of which will be male.
Shouldn’t one also take into account the proportion of each? The result of the original A mating with the offspring of BC would be half consanguinous with A and a quarter each with B and C, which one might represent by AABC. On the other hand, the result of B mating with AC would be ABBC, which would presumably be a better match for AABC than another AABC (but still pretty bad).
And, of course, the use of letters restricts us to an initial pool of 26 individuals, but I’m sure there’s some standard way of extending that.
Also, in Lumpy’s scenarioone must point out that there’s still only one distinct Y chromosone on the planet no matter what you do, which definitely means some loss of gene pool. But given how few genes there are on the Y, and that any recessive gene on the Y and not matched on the X is guaranteed to be expressed, I’m not sure that this is a big problem for the viablity of this big happy family.
I read recently that the average human carries around 3 or 4 recessive lethal alleles, genes for a fatal disease that are not expressed because there is another copy of that gene that does not cause disease and suppresses the lethal one. If someone (necessarily an embryo or fetus) has two copies of the lethal gene, though, the lethal characteristic will be expressed and will result in either a miscarriage or very early death.
So, instead of creating science-fiction mutants, inbreeding simply causes a higher rate of miscarriages and infant mortality. (Many, if not most, harmful mutations are sufficiently harmful to be lethal.) The reason for this is that the 3-4 lethal alleles each human has are likely to be for entirely different genes, while two closely related individuals may carry lethal alleles for the same gene. For each, there is a 50% chance that the lethal allele will be passed on, and a 50% chance that the healthy dominant allele will be passed on. Thus, if two individuals mate who carry lethal alleles for the same gene, 25% of the offspring will have two copies of the lethal allele and will not survive.
If you were the last man on Earth (and you carried 4 recessive lethal alleles) and you mated with the last 20 women on earth, there would be a 50% chance of passing on each of your lethal alleles to your offspring. There wouldn’t be any effect in the first generation, but later generations would suffer enormous infant mortality. (Some of your offspring would even have three or all four of your lethal alleles, and most of their offspring would die.) Note that I’m only considering alleles which are outright lethal and not those for genetic diseases.
Because of this, it would be much better if there were 10 last men on earth and 10 last women on earth rather than 1 last man and 20 last women. The birth rates would be higher with 20 women, but they would only be doubled, and the higher rate of infant mortality might cancel this out.
Another factor is that women under stress ovulate less frequently and have miscarriages more often under stressful conditions. Presumably, if some disaster happened that reduced the human population to around 20, the survivors would be under enormous stress.
One final issue to consider: After the first generation, female offspring would be unable to mate with anyone with whom they were not somehow genetically related. For the males, the 19 women who were not their mothers or half-sisters wouldn’t be genetically related, though they would technically be in-laws and still very disturbing. Genetically, though, if the 19 other Last Women hadn’t reached menopause before their male offspring reached maturity, that would be the choice of least inbreeding.
Opinion part: If I was the Last Man On Earth, I think that’s the end of humanity, because the genetic/moral consequences of it are too disturbing to be worth saving the species. Hopefully, if there is ever a Last Man On Earth, he won’t have taken a course in genetics, law, or religion.
Also, if I had the choice between squeezing out a baby every year for the rest of my reproductive life or letting the human race die, I’d probably say, “It was fun while it lasted, but let’s not do that again. Have fun ruling the world, cockroaches. You can’t do worse than we did.”