Wednesday, 8 July 2015

Cousin marriage and inbreeding revisited

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I wrote about this a few years ago:

http://charltonteaching.blogspot.co.uk/2011/03/first-cousin-marriage-good-bad-or.html

I noticed a recent discussion which quoted a recent paper claiming significant damage to intelligence from cousin marriage as it is practised under modern conditions.

https://westhunt.wordpress.com/2015/07/03/inbreeding/

What has to be said is that cousin marriage practised under modern conditions where child mortality rates are just a few percent; is a completely different situation from cousin marriage practised under the situation prevalent among almost everybody throughout most of human history - when child mortality rates exceeded fifty percent.

When most children died, there was an extremely tough selective sieve, which would be expected to remove most children with genetic damage - more would be removed by differential deaths of young adults (especially of men) and sexual selection with assortative mating.

So, most of the problems of inbreeding, including cousin marriage - are likely to be an artefact of the almost-complete-removal of the most powerful selective mechanism operative on humans. 

Conversely, the advantages of cousin marriage would have been relatively much greater - these advantages including better knowledge of partners, higher rates of social cooperation, and also the mutation-purging benefits of assortative mating - in that the lineages with the 'best' genes would tend to marry; while the left-over lineages, in which genetic damage was concentrated, would also marry and their damaged genes would tend to be eliminated from the gene pool. 

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3 comments:

Paul Warkin said...

"and also the mutation-purging benefits of assortative mating"

And over time this would result in a population that has fewer problems when inbreeding. So it doesn't make as much sense to talk about inbreeding in general--the effects are different for different populations.

ajb said...

Here's a general question I have about the mutational meltdown theory (MMT) you are proposing in regards to recent human populations, although perhaps OT.

If mutational load -> decreased reproduction, would those reproducing less - on average - have higher mutational load?

If so, is MMT to an extent self-correcting? (Mutants might live to adulthood, but have decreased reproduction.)

Bruce Charlton said...

@PW - Yes - but new mutations occur with ever generation, and in humans apparently at a high rate; so the need for selection will remain.

@ajb - I don't think that would necessarily follow. Among modern humans the least reproducing include those who have the least mutations (eg those of high intelligence, because intelligence is a 'fitness' measure of 'good genes') - probably because the cultural reasons for ultra low fertility overwhelm the fertility reducing effects of mutations.

MMT may be self correcting, or else the population may go into a 'mutational meltdown' (word search that on the IQ personality genius blog) - when the reducing population reduces the probability that the population can evolve-out-of-trouble.