exogamy, kinship, and heterozygosity

Gerold Firl (geroldf@sdd.hp.com)
1 Oct 1996 19:57:37 GMT

This is an amnplification of a brief note to daniel maltz about how
culture-specific rules of exogamy and kinship exert an influence on
levels of heterozygosity. Many in the soft sciences who loudly profess
their fear and loathing for sociobiology seem to believe that they are
struggling to oppose the encroachment of biological determinism into
the study of culture and the arts; that is a misapprehension. I will
try to explain how incest rules influence human genetics, specifically
the level of heterozygosity.

An organism is homozygous at a particular gene locus when identical
alleles (varients of a particular gene) were inherited from both
parents. A heterozygous site would have different alleles, as in the
case where you inherit the gene for blue eyes from one parent, and
brown eyes from the other.

different organisms have different levels of heterozygosity, ranging
from quite low (say, 10%) to very high (in the 80's for some insect
species). Generally, species (or subspecies, for that matter) which
inhabit more *variable* environments will have higher levels of
heterozygosity; closely related strains of _drosophila_ can exhibit
very different levels of heterozygosity if one population lives in an
environment with different levels of variation in temperature, for
example. One way to think of it is to view all the recessive genes
carried by the population as alternative morphologies, held in reserve
for times when rapid adaptation is required and you don't have _time_
to mutate. %^)

In the case of humans, I propose a different adaptive value for high
levels of heterozygosity: our unique cultural adaptation for the
specialized division of labor. "It takes all kinds to make a world",
as the folk saying goes; leakey and lewin, in _the people of the
lake_, have proposed that the human division of labor should be
viewed as one of our primary adaptations, along with our ability for
cerebral abstraction and speech (and, I would add, our evaporative
cooling system). Carrying a higher genetic "load" (to use a somewhat
misleading term) of recessives provides for more variety within a
population, thereby providing a greater span of abilities, talents,
and proclivities within a society.

I am assuming that homozygosity for recessives incurs some kind of
fitness penalty; I don't know how valid this assumption is.

Prediction: Cultures inhabiting environments with greater variation in
conditions (either due to climate, geography, or competition) will
have exogamy/incest rules promoting a greater genetic distance between
parents, and will have greater levels of heterozygosity in the

An example of the latter environment might be europe around 900-1100
ad, when the catholic church greatly increased the scope of incest
prohibitions. This was also a time of enormous turmoil and warfare,
and a period where economic expansion and differentiation was
proceeding rapidly.

Here's an example I'd like to find: cultures which have sort of a
kula-ring exogamy rule, where genes are passed from clan to clan for
several generations before returning. This would be especially
valuable in areas where recessives which are dangerous when homozygous
(such as the sickle-cell gene) are fitness enhancers when

Disclaimer claims dat de claims claimed in dis are de claims of meself,
me, and me alone, so sue us god. I won't tell Bill & Dave if you won't.
=-=-=-=-=-=-=-=-=-=-=-=-=-=---- Gerold Firl @ ..hplabs!hp-sdd!geroldf