June 7, 2013
to be posted on nobabies.net

Cameron M. Smith

Dear Cameron Smith:
I have read Cameron M. Smith Starship Humanity SCIENTIFIC AMERICAN vol. 308 no. 1 January 2013 page 38 with great interest.  You make a number of interesting points, and I was repeatedly thinking, “Yes, and …,” but I shall confine myself to a single one, which is my abiding obsession as you can tell from the name of my web site.

You point out that every society has its “ultimate sacred postulates.”  In Western societies one powerful one is: Genetic diversity is sacred.  Genetic diversity is great.  If your diversity is limited evolution gets most irate. 

In Western culture we have people who have conflicting opinions on how best to care for our greatest treasure, which is of course our genetic constitution.  Our chimpanzee cousins would have little use for our other treasures once they had finished raiding our refrigerators.  The dominant line of reasoning is that bad genes act up if they are paired with similar bad genes so you must never ever marry even moderately distant kin.  The other line is the discredited “eugenics” teaching that you ought not marry anyone t-o-o-o distant, but absolutely never even moderately distant kin.  I understand that there are parts of the world that still encourage marrying cousins, but I don’t really have good recent numbers on that.

Recent work with plants (Elizabeth Pennisi Epigenetics Linked to Inbreeding Depression SCIENCE vol. 333 no. 6049 September 16, 2011 page 1563 reviewing work by a team led by Philippine Vergeer, Hugens Building, Room HG 01.132, Radboud University Nijmegen, Molecular Ecology, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands p.vergeer@science.ru.nl) has shown that for plants anyway inbreeding depression is completely reversed by raising plants from seeds in an environment that strip methyl groups off the DNA and thus cancels out certain epigenetic effects without changing the base pair sequence at all.  Serious oops here.  Our sacred postulate is simply wrong.  Matching genes in a small population just isn’t the problem.  Of course it couldn’t be.  A small population would rapidly eliminate recessive deleterious genes.  Funny thing about sacred things; people don’t give them much thought.

Of course inbreeding depression is real.  It’s just epigenetic.  That means that the mechanism can cause changes far faster than any genetic mechanism can.

And thereby hangs a tale.  (I’m sure Shakespeare intended the pun.)  The weight of the evidence is that if you take a population and let it increase thousand or more and assure random mating that population will die out in about 10 generations because of steadily accumulating mismatch of epigenetic markers.  And if you get serious about it and don’t let anybody marry anything closer than ninth cousin you probably lose the whole population in about five, maybe six.  In other words your intrepid star farers given a very diverse population to begin with and systematically forbidding cousin matches will arrive at their new home absolutely sterile. 

Yeah.  I know.  That’s not what everybody else you ever heard from says.  But I do have evidence.  Here’s a link:
I shall attach a paper I was involved with that has more evidence.

Of course it’s really not a problem.  The technical fix is simply to include only a moderately diverse population and then to subdivide it into … oh I don’t know … groups of maybe a hundred couples.  They don’t have to be air tight.  In any one generation a group might admit a transfer from one of the other groups.  But pretty much you have to split them up if they are to survive.

There’s not even any hurry.  You might be able to do it after launch if you have not gone too far with the diversity thing to begin with. 

Let me concede that I might be wrong.  The evidence is very strong and the number of lines of evidence is remarkable.  But anybody can be wrong.  In fact on this one I think just about everybody is wrong. 

But if I am right, then civilization has to reconsider our sacred postulate and start encouraging going back to marrying moderately distant cousins; when we did that before there were plenty of babies.  If we cling to our superstition, then I fear we will become the stuff of folklore long before that deep space mission can be launched, if indeed humans are lucky enough even to survive. 

What do you think?


M. Linton Herbert MD

Cameron Smith came back with the following wonderful reply:

Hi, Herbert, I think this is fascinating. I just this AM read, incidentally, about this same issue of diversity in plants that are closely inbred; I am still trying to understand it. I am currently expanding my treatment of the population genetics issues; I'm starting with a review of about 50 studies of inbreeding issues, effective and census population sizes and so on. I am getting a better handle on things. I agree that reproductive freedoms we take fior granted today will be different on starships, and I'm addressing that in a n article specifically on the cultural adaptations necessary for such voyages. Thanks very much for the heads-up; I will be reading your items in the Am over early coffee :) I will CC you when I have published my article on this technical issue of population. All told, it is looking to me more that on the 5-generation timescale that I am working with the Icarus Interstellar planners, genetic issues are of less concern than demographic factors. That is, with a bit of care for the genetic founding population, the most threatening issue is occasional catastrophe that could put the population into a demographic 'extinction vortex' if the age / sex ratio is sufficiently disturbed. For these reasons I'm leaning towards a population for the first interstellar craft of 18,000-60,000 people, with 40k of a properly age and sex-structure depopulations being a reasonably safe and reasonable number. There is a whole new issue of population growth once you reach a new planet (which is the goal) and that is coming up on my horizon as well :)
Thanks very much for your interest!

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