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Half-Sibling Bigynandry Versus Double-Cousin Marriage As Multi-Generational Customs
Posted: Posted July 11th
Edited July 11th by chiarizio
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Imagine two societies, which I’ll call the Dairymen and the Beefeaters.

The Dairymen practice what I’ll call “Half-Sibling Bigynandry”. Each person marries, and has children with, two half-siblings of the opposite sex; one agnate (ie same father different mother) and one enate (ie uterine ie same mother different father).

The Beefeaters practice double-cousin marriage. Each person marries, and has children with, a cross-double-cousin of the opposite sex: for instance, a man marries the daughter of his father’s sister by his mother’s brother.

I will argue that, in the long view, the Beefeaters are much more inbred than the Dairymen.

In drawing representative family trees of individuals from these societies, one encounters different difficulties.



Compare a Dairymen child’s family tree to a Beefeater child’s family tree.
Going back one generation, the Dairyman has two parents, and the Beefeater also has two parents, just like every warm-blooded vertebrate on Earth.
Going back a second generation, the Dairyman has only three grandparents, while the Beefeater has four grandparents. Advantage Beefeaters.
Going back a third generation, the Dairyman has four greatgrandparents, and the Beefeater also has four greatgrandparents. Deuce.
Going back a fourth generation, the Dairyman has five greatgreatgrandparents, but the Beefeater has only four greatgreatgrandparents. Advantage Dairymen.
Going back a fifth generation, the Dairyman has six great^3-grandparents, but the Beefeater has only four great^3-grandparents. Game Dairymen.



Eventually we ascend to a generation in which there were only n/2 sexually-mature-and-active Dairyman men and/or only n/2 reproductively-fit Dairymen women of childbearing age. If this is the (n-1)st generation before our Dairyman propositus, this is the highest generation at which they could have had one more ancestor than the generation count; any further back there’d have had to be some repetition.

But the Beefeater kid was descended through a bottleneck of four-ancestors-per-generation from time immemorial. (Unless they can remember when double-cousin-marriage was instituted!)

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Comments?




There are 2 Replies

I think I remember someone claiming half-sibling bigynandry as the best way to retain genetic diversity in small populations that are cut off from other genetic pools without growth, or not much growth. It was brought up as a possible way to cope with situations on a generation ship, where there was little room if any for population growth and little or no way of exchanging genetics with outside groups, and genetic diversity would need to be retained as much as possible.

Posted July 12th by linguistcat
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@linguistcat:
I could be wrong, but I seem to remember four breeding females and four breeding males as the break-even point.
IIRC with five or more breeders of each sex the population will do better with a program of maximal outcrossing than with a program of half-sibling bigynandry.
But with three or fewer breeders of each sex if i recall correctly they’ll do better with half-sibling bigynandry than with maximal outcrossing.
If I recall correctly with four and four, both strategies work about as well as each other.

I could be wrong about where it is, but there must be a break-even point somewhere!

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Thank you for your comment!



Edited July 12th by chiarizio
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