Coincidence? I think so!

The Decode study on mutation rate and paternal age found that mothers contribute 15 new mutations, regardless of age, while  men contribute (25 + 2*(g -20), when g is the average paternal age [assumed to be greater or equal to  20].  Obviously, the number of mutations per generation goes up  as average paternal age increases.  What about the average number of mutations per year, which is a useful number for neutral theory?

If the generation length for both sexes is equal (to g), then the average number of mutations per year is (1/g)(15 + 25+2(g-20)) = 2, independent of  generation length. Very convenient for neutral-theory calculations.  But this cancellation is a coincidence: if  number of mutations contributed by women were higher or lower, the per-year rate would change as the generation length changed.   A big difference between average paternal age and average maternal age can also change the  per-year mutation rate:  if the average paternal age was 42 and the average maternal age 28, the per-year mutation rate would be 2.179.

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6 Responses to Coincidence? I think so!

1. Dienekes is leaning towards revising the generational mutation all the way down to ~1.20 *10^8 per generation, but that’s presuming avg male age around 30. Have you found yourself leaning in any particular direction on this, Cochran? My own intuition is that 30 is perhaps the oldest avg I could imagine for males for the time after our split with fx. the Neanderthals; but I don’t know enough about our 10K+ BP history to be sanguine.

• Mean age of biological fathers among Herero is 41.

• Agree, there are many semi-modern societies of this type. Do you think that they are paleotypical, though?

Compare to chimps, fx:
http://news.sciencemag.org/sciencenow/2012/08/generation-gaps-suggest-ancient.html

“As they report today in the Proceedings of the National Academy of Sciences, chimpanzee mothers ranged in age from 11.7 to 45.4 years at the birth of their offspring. The average age of reproduction was 25 years for females and 24 years for males, giving them an average generation time of about 25 years.”

• Greying Wanderer says:

“My own intuition is that 30 is perhaps the oldest avg I could imagine for males for the time after our split with fx. the Neanderthals”

I wonder if there patterns in the averages e.g. between differences in the averages of h-gs, pastoralists and farmers or latitudinal differences?

2. Steve Sailer says:

Genealogists sometimes calculate the average length of a generation over the last, say, 500 years.

3. Luke Lea says:

Is that supposed to be 2 x (g-20) or 2 raised to the g-20 power? Gotta be the former.