July 15, 2015

Brian Charlesworth
Institute of Evolutionary Biology
School of Biological Sciences
University of Edinburgh
Edinburgh EH9 3FL
United Kingdom

Dear Brian Charlesworth
I read with interest your essay in the recent issue of Genetics, “What Use Is Population Genetics?” 
Genetics July 2015 200:667-669; doi:10.1534/genetics.115.178426
For years I have glimpsed population genetics from afar and wondered.  I read a book on population genetics that was an inch thick and came away with less than would fill a three by five card.  (Yes, we will use the old English system of measures here.  That’s a pretty big book and a pretty small card.)  What I learned was that if you know the rate at which some neutral mutation enters a population and know the prevalence of the population you can calculate and “effective population size.”  What I think it said was that a single population will vanish on average in a number of generations commensurate with the population size.

I had hoped to find a chart of a number of human populations and their effect population size.  I had no such luck.

The other thing I wondered was this: Suppose there is a population of 10,000 randomly mating and a certain neutral mutation appears in that population at a known rate such that the prevalence lets you calculate that 10,000.  All well and good and quite classical.  But suppose you have 100 populations of 100 each, and they are highly separated.  Individuals leave one of the hundreds and join another but very rarely.  Or suppose the hundreds occasionally divide into two sub populations that never rejoin and have little or no exchange of members.  Yes, that’s two questions I’m setting up.  So under those circumstances will the prevalence be identical with the prevalence under the earlier condition of panmyxia?  And if not, what is the effect on calculated population size; does it go up or down?

I’m sure this is quite basic and you know the answer off the top of your head, but cudgel my brains as I will I am not sure. 

It matters.  Earlier this year in the 19th chapter “Marry in or Die Out” of the textbook Handbook on Evolution and Society, the brilliant Robin Fox points out that this strategy of isolate-always-and-if-the-population-outgrows-a-certain-size-sudivide is universal and responsive to absolute need.  That of course means it is effectively universal bar our current massive urbanization with mating routinely outside of the tight little group of a traditional society, and the cost is unsustainable infertility. 

I’ll not burden you with my paper on fruit flies, their population size and fertility, nor my years of accumulated writings.  They are available on the web and I’ll direct you if you like, but Professor Fox is the best.  If you do not find his book to be readily available, let me know, and I’ll see what I can do.  It certainly makes no sense for you to plank down $200 to do me a favor.

All best wishes.


M. Linton Herbert MD

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By chance the professor sent me a note while I was in the very act of cleaning up after posting my own letter. If he encourages me to quote him I certainly shall. Otherwise I'll do my best to paraphrase it in a couple of days.