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Old 09-24-2012, 06:29 AM
Blake Blake is offline
Join Date: Mar 2001
Posts: 11,049
Originally Posted by foolsguinea View Post
Because people often marry within their ethnic group, even to the point of preferring to marry first and second cousins in some tribal cultures, the MRCA model cited (being invented by relatively exogamous Yanks, I suspect) is useless.
It isn't, because you seem to be assuming that you need 50% exogamous marriages to produce a common ancestor. You don't. Even in populations of millions, just a single exogamous marriage each generation will pretty much guarantee a common ancestor with the rest of humanity within the past few thousand years. See President Johnny Gentle's post about pedigree collapse for why this is the case.

Also note that while many groups will prefer that women marry within group few, if any, care if men sire children with outsiders, And common ancestry works just as well through the male line as the female. Just a single man each generation fathering a child with an outsider will lead to recent common ancestry.

The actual degree of exogamy isn't really important. In fact within small populations, when endogamy is the rule but exogamy occurs regularly, that will actually speed the pedigree collapse because once a line in in the pool, it can only be excised by physical extinction, it can never "marry out".

As a result, exogamous Yanks where a million people annually engage in exogamous marriages aren't going to be much more nodal than a group where just one individual every twenty years reproduces exogamously.

For some isolated groups it could be thousands or myriads of years ago.
Could yo name one of these groups, with some evidence for this claim?

Even excluding various isolated aboriginal groups and the whole Amerindian set
It's a virtual certainty that 100% of Amerindians have both European and West African Ancestry within the past 400 years.

Quote: have several genetically distinct groups in sub-Saharan Africa that may not have a particularly recent common ancestor with outsiders.
Do you have a name in mind? Because groups like the San, Hadza and Mbuti are the most genetically distinct groups on the continent, but they all engage in regular intermarriage with their nieghbours and always have done.

I can't really think of any groups in Africa that are even approximately isolated.