how have small tribes ensured or maintained genetic diversity?
In: Biology
A lot of them haven’t, at least not to what we’d consider to be an optimal level. And that’s not always guaranteed to be a problem.
While a population can eventually reach a point where inbreeding has created so many birth defects that children rarely live to adulthood, it takes a very long time for that to happen unless there was already a strong predisposition for those defects present within the gene pool. In the short term, a loss of genetic diversity will just make a population more vulnerable to being wiped out by external factors like a new pathogen, sudden climate shifts, or the depletion of some vital resource. Humans tend to be uniquely talented at anticipating and avoiding these risks in our day to day lives compared to most species living on the planet, meaning it’s a lot easier for us to endure even with that heightened vulnerability.
There is extremely few cases of small tribes surviving completely isolated from everyone else. Some of the most isolated tribes might only interact with their neighboring tribes once or twice a year. And in doing so they will often have marriages between members of the tribes and therefore get some genetic diversity.
They normally intermingle with neighboring tribes, which in turn mingle with other tribes, some island isolates tribes do have some issues with genetic diversity.
They don’t really. cousin marriage is traditional in lots of places. I don’t think anyone really likes incest, it’s not good, but the risks of it are exaggerated (because it’s gross). It doesn’t generate new defects out of nothing. So small populations kinda just end up with the problems they end up with generation after generation and if they aren’t fatal the first time they continue to be not fatal going forward.
Answer: they often have elaborate kinship systems that dictate who’s allowed to marry who. Often those extend outside the tribe to other tribes with whom they have some traditional association. This can result in a much larger network of genetic exchange.
OP’s premise is flawed. These tribes often didnt (need) to keep their genepool diverse.
Inbreeding is generally considered bad because it heightens the selective pressure for recessive genes. In short term this means diseases with recessive inheritence are seen more often which causes the population size to shrink. In the long term these faulty genes will die out and the population will often recover until it reaches a new equilibrium.
Another reason why inbreeding is bad is that it leaves a population vulnerable to be wiped out completely by illnesses. Diverse populations will have a higher chance of a subpopulation surviving. But this is kind of where op has it backwards: if the tribes dont interact with the outside world pathogens wont even be transmitted.
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