The Goldilocks Zone Between Inbreeding and Outbreeding by Peter Frost in Aporia. I have liked him for years, as he doesn't mind being unpopular.
The more closely related a man and a woman are to each other, the higher their risk of producing abnormal embryos. The risk is highest if they are siblings, lower if they are first cousins, and lower still if they are second cousins. This is well known. But did you know that the same risk then starts to increase progressively with further decreases in relatedness? The sweet spot for having healthy children seems to be marriage between third or fourth cousins.
Farther along it is related to the decrease in sperm quality, which The Studies Show discussed and I linked to three weeks ago, Decreasing Sperm Count.
The epidemiologist Michael Joffe has argued that the increase in outbreeding explains a century-long decline in sperm quality and a corresponding increase in testicular cancer (Joffe, 2009).
Joffe rejects the usual explanation — the rising level of estrogenic compounds in the environment, like dioxin, DDT, PCBs, PBBs, phthalates, and others. This putative cause fails to explain why the decline in sperm quality varies so much between different geographic regions, often within the same country. Why, for instance, has it been steep in Paris and nonexistent in Toulouse? Why is it nonexistent in domestic animals that are no less exposed to estrogenic compounds? And why did the decline in sperm quality begin before the commercial production of most estrogenic compounds?
The phrase "fails to explain" is often key in understanding a theory. Sometimes an idea looks so tempting, but someone points out "Okay, but absolutely none of the Asian countries follow this pattern." Oops.
That's an issue that is very gingerly talked about in genetic research, and a small number of careful studies have looked at, in specific, hopefully non-controversial situations. The question at the heart of it is, are there human subspecies distinct enough that there are hybrid issues, like sterility and/or reduced fitness of subsequent generations? There are a great many large chromosomal rearrangements across different ethnicities, which cause no problems within that group, because nothing is lost. However, in theory, a cross between people with different rearrangements could result in gaps or duplications in following generations. The best place to look at this would probably be within Africa, as there is more genetic diversity across African ethnicities than across the rest of the world
ReplyDeleteI hadn't even thought of Africa, and of course you are correct. If one were to divide the population into two groups, it would be the San and everyone else. If three groups, the San, the Khoi, and everyone else. And this pattern continues for I think a dozen more iterations before there is finally a break outside of Africa.
ReplyDeleteIn our family, my son from Romania married a woman from the Philippines, populations that have been separated for what, 40,000+ years? They have two daughters.