Another topic elsewhere got me thinking about this question.
Assume that you had a group of humans in some kind of hypothetical situation that they could never leave.
With the assumptions that:
None of them initially were related to each other at all (as unrelated as human beings can be, no common ancestors for 40 or more generations)
They would submit to a pre-determined plan about who they were to have children with
There were no hazards in this hypothetical Eden that would kill them except through old age, and that it could support an infinite population . . .
What is the minimum number of people that you’d have to start with in order to grow a society of potentially infinite size without running into the dangers of inbreeding?
Two, pretty much. Inbreeding is a problem mostly only if the population remains small for several generations. Then a good bit of diverisity can be lost simply by random chance.
If you have a male and a female starting out who are not closely related, and they don’t by chance have too many detrimental recessive genes between them, and they have large numbers of offspring, who interbreed with each other but also have a large number of offspring, then the dangers of inbreeding will be minimized. While not ideal, the population should not suffer enough from genetic defects to interfere with its further increase. If you had 10 individuals, you would be fine.
All, or almost all, of the pet hamsters in captivity are descended from a single female captured in 1930. You can find a great variety of color morphs and other types in domestic breeds. Of course, hamsters are much more prolific than humans, but the general principles apply.
We have done variations on this before. I say that just because I remember some relevant info. The dangers of inbreeding aren’t generally all that strong. Procreation between first cousins isn’t a good idea (but not unheard of) but the effects are basically gone once you get to second cousins. Therefore, you could have a stable and quickly growing population as long as you had at least as many second cousin and greater pairings as the generation before. I can’t do the math partially because I think you would need to figure out the optimal number of children per pair of people and how fast you wanted things to grow. It seems like your conditions would only require a fairly small group however.
I think it’s because the golden hamster was thought to be extinct, but then a pregnant female was caught in Jordan and taken into captivity. That’s if I remember correctly, any way.
Just to expand a bit on Colibri’s post, the dangers of inbreeding, while potentially problematic, pale in comparison to other dangers that would wipe out a small population of H. sapiens. Assuming that they were technological primitive, a small group would be much more vulnerable to disease, dangerous weather or other natural disasters, or just bad luck in hunting.
I may be wrong, but aren’t there numerous examples of successful island species that have evolved from very small initial populations? Wouldn’t Galapagos finches be an example?
It’s not that simple. Procreation between second cousins is basically safe, so long as they come from an otherwise-diverse population. But if every generation is mating with their second cousins, for many generations, then a second-cousin pair will effectively be much more closely related than a second-cousin pair from a genetically diverse population, and the risks from inbreeding will be correspondingly greater.
Really, though, the question depends on what level of genetic disorders are considered acceptable. Siblings mating will result in a fair number of genetic defects (any child from such a union will have a 1 in 4 chance of exhibiting any recessive genetic trait for which either grandparent is a carrier, and a 1 in 2 chance of carrying the gene without exhibiting it), but they will also result in some healthy children, who can then continue the lineage. Plus, the offspring who survive will have a lower incidence of the problematic genes, so with time, the population will grow progressively healthier.
Does it matter if half of your kids die at birth, if you just have twice as many to compensate? Ethically, it probably does matter, if we’re talking about humans. But biologically, assuming that there are plenty of resources available to spend on childbearing, either is equivalent.
Are you sure you didn’t mean to say “both” instead of “either”? If not, can you show your math?
Assume grandpa is AA and grandma is Aa. That will produce equal number of AA and Aa offspring in the first generation. Let’s assume that we have an AA boy and girl and an Aa boy and girl. If we assume random matings, that will produce pairings of AAxAA, AAxAa, AaxAA, and AaxAa. Only 1/16 of the kids produced from those pairings will be aa (ie, 1/4 of the kids from the last pairing).
I’ve actually just written a Staff Report on this exact subject, which will probably be posted in a few months. The short version is that the Golden, or Syrian Hamster, has a very small range near Aleppo in Syria. (Recent research suggests it is not uncommon in that range.) A female and her litter was captured in 1930, and was used to establish a laboratory population for medical research in Jerusalam. They proved to be so suitable and so prolific that descendents were sent to other labs elsewhere, and the species eventually entered the pet trade. There was really no need to go out and catch any more from the wild, although a few more were found from the 1970s on. It is uncertain if any of these latter have descendents in present captive populations, but if so they must be relatively few compared to the descendents of the original female.
Don’t think so. Unborn children aren’t normally counted in population figures. I was assuming that Ms Eve would have been pregnant before coming to wherever this is all taking place.
In terms of genetics, which is what we are talking about, it is an effective population size of three. Breeding obviously won’t take place until after the children are born and are sexually mature, so it’s irrelevant that you started with a single female. Also, if the twins have different fathers, that’s also an effective breeding population size of three.
Fraternal twins, which come from different eggs, can have different fathers if the mother has intercourse with two different men in a short period of time and the eggs are fertilized by sperm from each of them. Rare, but it does happen.
Identical twins, which come from a single fertilized egg, of course must have the same father.
I’m sure I meant “either”, but I screwed up on the math. The correct probability, for one grandparent a carrier, is 1/16, as you note. Of course, this is independantly for each recessive trait carried by either of the parents, so the odds for getting any recessive nastiness might be greater.
