family types and the selection for nepotistic altruism

it finally clicked in my head while thinking about polygamy what the importance of family types — nuclear vs. extended, etc. — might be in the selection for altruistic behavioral traits, especially nepotistic altruism or clannishness. i should’ve thought through polygamy sooner instead of putting it off, but hey — procrastination is heritable, too, so in the words of h. solo, it’s not my fault! (~_^)

the logic of the mating patterns/inbreeding-outbreeding theory goes that, given the right set of circumstances (i.e. certain sorts of social environments), selection for nepotistic altruism/clannishness ought to go quicker or be amplified by inbreeding (close cousin marriage or uncle-niece marriage) simply because there will be more copies of any nepotistic altruism genes (alleles) that happen to arise floating around in kin groups. in other words, inbreeding should facilitate the selection for clannishness…if clannish behaviors are being selected for in a population.

the thing is, though: the individuals carrying certain versions (alleles) of nepotistic altruism genes need to direct their nepotistic behaviors towards other individuals carrying those same alleles, otherwise their actions will be for naught. (yeah. kin selection.) if they direct their nepotistic actions towards people who don’t share the same alleles, then the actions will be “wasted” and the behavioral traits won’t be selected for — or at least not very strongly — and they might fizzle out altogether.

let’s take an imaginary society as an example: say everyone in our pretend population always marries their first cousins. their father’s brother’s daughters (fbd) even, so that we get a lot of double-first cousin marriage. h*ck! let’s throw in some uncle-niece marriages on top of it all. the inbreeding coefficients in such a society would be very high, and if clannishness was being selected for in our highly inbred population, the selection ought to move pretty quickly.

but suppose we separated all the kids at birth from their biological families and set them out for adoption by unrelated individuals — people with whom they likely did not share the same nepotistic altruism alleles. think: the janissary system, only on a population-wide scale. if we did that, there should be virtually no selection for clannishness despite all the inbreeding since pretty much no one’s nepotistic behaviors would be directed towards other individuals with the same nepotistic altruism genes. in this case, kin selection would just not be happening.

such a society does not exist, and i don’t think ever has. but there are societies out there with certain family types — namely nuclear families (or even post-nuclear family societies!) — which ought to have a similar dampening effect on any selection for clannishness.

northwestern “core” europe has had very low cousin marriage rates since around the 800s-1000s, but it has also, thanks to manorialism, had nuclear families of one form or another (absolute or stem) since the early medieval period — nuclear families are recorded in some of the earliest manor property records in the first part of the ninth century from northeastern france [see mitterauer, pg. 59]. on the other hand, eastern europeans, like the russians and greeks, while they also seem to have avoided very close cousin marriage for several hundreds of years (which is not as long as northwestern europeans, but is quite a while), have tended to live in extended family groupings. you would think that nepotistic altruism could be selected for, or maintained more readily, in populations where extended family members lived together and interacted with one another on a more regular basis than in societies of nuclear family members where individuals interact more with non-kin. societies comprised of nuclear families are more like my hypothetical janissary society above where the altruism genes that might’ve been selected for via kin selection instead fade away in the wash.

we have to be careful, though, in identifying nuclear family societies. the irish of today, for instance, are typically said to be a nuclear family society, but the extended family does still interact A LOT (i can tell you that from first-hand experience). same holds true for the greeks and, i suspect, the southern italians. i would say that these populations have residential nuclear families, but not fully atomized nuclear families which have infrequent contact with extended family (think: the english). the early anglo-saxons in england were also characterized by residential nuclear families — the extended family (the kindred) was still very important in that society. the individuals in a residential nuclear family society probably do interact with non-family more than individuals in a society structured around extended families or clans, but less so than a true nuclear family society.

the thought for the day then?: family types can also affect the selection for clannishness/nepotistic altruism.

that is all! (^_^)

previously: polygamy, family types, and the selection for clannishness and “l’explication de l’idéologie”

(note: comments do not require an email. irish nuclear family.)

just what i wanted to know!

geneticists will tell you that the percentage of their genomes that two first-cousins will probably share is 6.25% (standard deviation ±2.4% — i’ve been wondering that, too!). but that’s in a randomly mating population. i’ve been wondering what happens in a population where cousin-marriage is the norm and has occurred generation after generation — like in saudi arabia or pakistan. what percentage of their genomes are those kinds of first-cousins likely to share?

well, now i have an answer. (it might not be the answer ’cause these researchers were looking only at families with congenital disorders, so … you know … ascertainment bias and all that.)

in “Quantification of Homozygosity in Consanguineous Individuals with Autosomal Recessive Disease,” woods, et. aaaaallll. (there’s a LOT of them), found that, in populations with regular, long-term cousin-marriage over many generations — in saudi arabians and pakistanis, as a matter of fact — first-cousins shared, on average, 11% of their genomes in common. that’s approaching twice as much as first-cousins in a randomly mating population.

“In conclusion, we found that the amount of homozygosity is greater than expected (11% observed vs. 6% expected) in individuals with autosomal recessive disease whose parents are first cousins and who come from communities that frequently practice consanguineous marriage. First-cousin offspring had as much homozygosity as would have been expected for double–first cousin offspring.”

so, first-cousin marriages in places like pakistan and saudi arabia (and afghanistan, etc., etc.) are, on average, more like double-first-cousin marriages in places where mating is more random, like large segments of the u.s., europe and the west in general. maybe.

here, once again, are mr. light-blue and ms. pink and their offspring, mr. dark-blue, to illustrate the differences for you (click on images for a LARGER view — should open in new tab/window):

mr. light-blue and ms. pink randomly mating:

here are the proud parents with their offspring, mr. dark-blue, who inherited the family fortune half of his genome from his father, and the other half from his mother:

here are mr. light-blue and ms. pink again, only this time they are first-cousins and share (prolly) 6.25% of their genomes in common (the bits that overlap):

here they are with blue, jr., who has still inherited half of his genome from each of his parents, but he’s got two identical (by descent) copies of some of his genome — he is less unique than mr. dark-blue above (see how he’s narrower?):

here are mr. light-blue and ms. pink one last time — they are first-cousins once again, but they come from a society where cousin marriage is the norm and are the descendants of a line of cousin marriages. the “overlap” of their genomes (11%) is larger than the first cousins above:

and here’s the final incarnation mr. dark-blue who is even less unique than mr. dark blue ii because he inherited even greater numbers of genes that are identical by descent from his parents:

(note: comments do not require an email. mr. pink.)