Tribes, however are rarely isolated from one another and 'trade' would be spouses and arrange marriages. Also a tribe could not survive genetically with rampant incest in isolation. What is the rate of birth defect from incestuous reproduction? Would the offspring even survive?
Tribes could be
very isolationist in nature; that's sort of the whole point of a tribal society.
The risk of birth defects from an incestuous union is not terribly high; less, as has been mentioned, than for a woman over 40 giving birth via a non-incestuous union.
The royal families of Europe, throughout the classical and medieval eras and well into the renaissance era, were almost exclusively comprised of products of incest. In an effort to keep rulership of the entire Western World in the hands of the same few families, first cousins, uncles and nieces, and aunts and nephews were betrothed and married to one another with regularity. Such marriages were the norm for the time, rather than the exception to the rule.
Did they have a lot of birth defects, even after a dozen generations of such inbreeding?
Well, they weren't a terribly healthy bunch in general, not by today's standards... but few people were, at the time. There was no modern medicine; most of them- like all people of their time- died of illnesses that would be preventable or easily curable today.
The only possibly-incest-related effect that I can think of was a high incidence of hemophilia among the Royal Families; it was so common, in fact, that it was known at the time as "Kings' Disease".
I know there's a genetic component to hemophilia; I'm not sure if incest causes it to occur in a family at higher rates.
You can also look at the Ashkenazi Jews, who until this century didn't interbreed (at least not by choice) with gentiles, to the point that 40% of today's Ashkenazi Jews have recently been traced back to just four common female ancestors.
Yes, they do have a higher rate of certain genetic disorders; also a higher rate of certain cancer-causing genes. Nothing earth-shattering, though. They also have a (slightly) higher median IQ than any other ethnic group on earth, so maybe thousands of years of reproductive isolationism made them smarter. Who knows? :shrug:
Maybe they, as a culture, simply evolved or adapted to be smarter, because of their particular historical experiences.
It would be interesting if someone would do a similar study on the Hmong, who have been even more genetically isolated than the Jews, until even more recently. In fact, over 90% of the Hmong in the US derive from ten major clans, or families, and share the same surnames.
However, within the Hmong culture, there are strict cultural taboos/ prohibitions against marrying within one's clan.
That doesn't mean one can't marry somebody just one generation removed from one's own clan, however, and I'm sure they frequently do.
