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Human Genetic Diversity

Cristian Wilckens cwilcken at ARAUCARIA.CEC.UCHILE.CL
Wed Aug 13 17:11:08 EST 1997

I was hoping someone more expert in population genetics than I am would
try to answer the question below from Guillermo Barron, but as no one
has I shall try to answer it myself (though I still hope someone who
knows more about it will contribute). If all postings from outsiders
were as intelligent and relevant as this one we wouldn't need to worry
about moderation. The question was as follows:

>I wondered if anyone on this group could answer a question from a
neophyte (graduate student in philosophy) whose understanding of
genetics does not penetrate much beyond reading Dawkins' *Blind
Watchmaker* and Dennett's *Darwin's Dangerous Idea?*

>A recent artcicle in Canada's *Globe and Mail* argued against incest on
the grounds that incestuous reproduction decreases genetic diversity
which many people count as a key factor in enhancing the survival of the
human species. The sociobiologist E.O. Wilson, for example, calls
genetic diversity a "cardinal value (1978)."

>Now I don't know if in fact unrestrained incestuous reproduction would
occur frequently enough to affect the human gene pool in any meaningful
way. But it does seem clear that heterogeneous human mating (if I can
call it that) *is* widespread. Up until very recently most humans mated
not only within their own race but within relatively narrow tribal and
geographical limits. This has of course changed quite radically within
the last century or so with the advent of global migration. My question
is: does mating between genetically different populations increase or
decrease diversity? In other words, would Laplanders mating with
Yanomani, and Tibetans mating with Cherokees increase or decrease human
genetic diversity?

My view is that any effect of incest on human genetic diversity will be
utterly trivial.

As far as I can remember from reading Crow & Kimura around 20 years ago
it is sufficient for two large populations to interbreed about once per
generation for any single gene to equilibrate between them. Thus
although many human populations may have been genetically isolated
before this century (e.g. the Highlands of New Guinea and the rest of
the world before the 1930s) there are probably no isolated human
populations now. In the present world, Laplanders mating with Yanomani
(etc.) will just speed up a process that is likely to be happening

Kimura's later book on the Neutral Theory of Evolution (Oxford
University Press around 1982, I think) is a much easier read than his
earlier one with Crow, but I don't remember if it has anything to say
about this sort of question. Unfortunately I can't check at this moment
as I am located several thousand kilometres away from my usual sources
of printed information.In any case, I am sure there are readers of this
news group who know far more about population genetics than I do.

Athel Cornish-Bowden
(Usual email, athel at ibsm.cnrs-mrs.fr; email until early September,
tiureta at abello.dic.uchile.cl)

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