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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