May 19, 2014

Rasmus Nielsen

Dear Professor:
I read about your work in Polar Bear Evolution – Fast and Furious SCIENCE vol. 509 no. 7499 May 8, 2014 page 565 and quickly found Shiping Liu et al. Population Genomics Reveal Recent Speciation and Rapid Evolutionary Adaptation in Polar Bears Cell 157, 785–794, May 8, 2014  Elsevier Inc. page 785.  I am fascinated. 

My first reaction was, “Uh oh.  This spells trouble.  500,000 years may seem like a short time to others, but that’s kind of a long time for my own purposes.”  But looking closer I have seen that the estimate is not time for speciation but time since speciation.  In my panic I ran across estimates of species ages in Himalayan songbirds.  (Arne O. Moores Supply and Demand, NATURE vol. 509 no. 7499 May 8, 2014 page 171 and Trevor Price et al. Niche Filling Slows the Diversification of Himalayan Songbirds page 222 in the same issue.)  Now it was millions of years.  But again, this is time since speciation or between speciation events, which the article convincingly points out must await the appearance of a new niche.

My best guess for time for speciation has long been about 2,000 generations based on evidence of mice in the Azores, rabbits in the Canary Islands and domesticated camels in Africa evolving from Bactrian to dromedary.  Until further notice, I shall keep my estimate.  I see you include in the paper an estimate of generation times.  Thank you, thank you, thank you.  I tend to think of generation times rather than years, but I find most people report years.

I am intrigued by the fact that you have found that the greatest evolutionary selective pressure has been on genes having to do with the heart.  My first reaction was, “Ah yes.  Cold climates need stout hearts.  But I now take it that the relevant mutations have more to do with handling the fat in their diets rather than preparing them for their arduous lives.  (I still doubt I could swim as far through cold water as a polar bear, but at least nature has provided me with a disinclination to try.)

So thanks for an intriguing study.  As you suggest, maybe the study of those genes will turn up some way humans can survive our current fat and lazy live style.

Now to the part you can skip.  Why in the world should I care about time for speciation?  Who cares?

We all should.  Alfred Russel Wallace pointed out, which obvious fact was never apparent to Charles Darwin, that evolution is a race and thus speciation is a race; once that niche opens, the faster speciating species (don’t you hate my prose style?) gains an advantage.  But there is always a countervailing pressure in biology. 

Consider a population that is divided by some natural event, say a glacier advances.  The barrier remains for 2,000 generations.  Then it goes away.  Rapid speciation means that the two populations cannot have a significant number of offspring.  Now consider some piece of DNA crucial to speciation.  Let a random mating population be 1,000.  Homologous segments of DNA shared by siblings mix in a pool or 2,000 copies taking on average about 2,000 generations to get back together.  When they do so, they cannot work together.  Speciation has occurred.  Since this is true of all DNA, the species must die out.

Evolution must fix this, and does so by doing a rough census each generation.  My best guess is that methylation patterns, which change each generation, must match sufficiently or there is a procreative penalty.  The random mating population of a thousand is eliminated, eliminating a threat to the whole species.  This takes about ten generations.  In humans the generation time is about 30 years, so any large random mating human population such as the administrative class of any civilization must die out in that time.  Sure enough civilizations tend to collapse by the time they reach their 300th birthday.  Ours appear to be on the way out.  You can check my evidence at

Sorry to introduce a jarring note, but it has become my whole life.  And for a moment I thought you had evidence to suggest (don’t I wish) that my assessment is wrong. 


M. Linton Herbert MD

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