Talkin’ ’bout their generations

According to the Decode results,  mothers contribute 15 mutations, regardless of age, while men contribute 25 + 2*(g-20) mutations, when g  is the average paternal age.  As I pointed out earlier, if g is the same in both sexes, the average number of mutations is just 2g,   which makes for 2 mutations per calendar year.  I’ve been checking out average maternal age: it doesn’t vary much.  The lowest I’ve seen was 26, the highest 30, so 28 is a reasonable number.  So far, in the data we’ve gathered, the population with the highest paternal age was in Gambia, with an average paternal age of 47.  If we assume that the average maternal age is 28 (which look about right from the graph: I haven’t digitized it yet) then the average kid would receive 94 new mutations (15 maternal, 79 paternal).  With an average generation length of 37.5 years (the average of 28 and 47), that makes for  2.5 mutations per calendar year: about 15% higher than you would see in most populations, where the gap between average maternal and paternal age is not nearly as large.

A Gambia-sized gap would result in a noticeably higher rate of neutral genetic divergence.  If it had existed long enough you might be able to notice it, but I think there’s a better chance of seeing this effect in  Australian Aborigines, who had high average paternal age and might have had it for a long time.   Other than the Australians, I would guess that all the old-dad societies are relatively recent.

The higher mutational load is not just a consequence of the higher per-year mutation rate in these old-dad societies – since generations are longer, there is less selection per calendar year (considering that most selection acts early in life).  The number of mutations per generation is probably the most important number.  I found some numbers for Polar Eskimos, hunter-gatherers  (they gather snow) in a tough environment: average maternal age was 27, average paternal age was 32,  for an average generation length of 29.5. They’d have 64 mutations a generation: the per-generation rate in Gambia  is 47% higher.

There are also qualitative differences in selection:  selection is weaker in childhood and stronger in midlife in an old-dad society, as Henry pointed out.  So that situation should select for longer life, except that’s hard to manage in the presence of higher-than-usual genetic load.

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8 Responses to Talkin’ ’bout their generations

  1. Murray Anderson says:

    That equation should probably be 25+2(g-20). At least that’s what you’re using for actual calculations.

  2. That Guy says:

    Greg,
    What about Pygmies who have a supposedly much lower age of puberty and childbearing for girls/women, and supposedly shorter generation spans and life spans?

  3. Steve Sailer says:

    I can recall reading an American genealogy hobbyist site on the topic of how, in their experience, male generation lengths for the ancestors of Americans tended to be longer than most expected. Unfortunately, I don’t recall the numbers.

  4. dave chamberlin says:

    It was traditional in many parts of europe for the eldest son to inherit the fathers estate and the younger sons to hit the road. Which plays directly into a long term breeding strategy of a higher survival rate for sons of younger fathers. Wouldn’t it be a strange quirk of history if this led to a gradual increase in average IQ in the populations that practices this long enough and was one of the crucial factors as to why certain parts of Europe kick started all the innovation that we now know came from this small section of the world.

  5. dave chamberlin says:

    I just got hit with an epiphany. If you are right in the assertion that age accumulated mutations lead to lower intelligence than it should become accepted and encouraged medical practice for men to freeze their sperm at a young age and then when they choose to have children for the mother of the child should be artificially inseminated with the previously stored sperm.

  6. Pingback: Chip Smith, "Rebel Girl: An Interview with HBD Chick" | Counter-Currents Publishing

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