Easier to Increase Diversity:
The average genetic constitution of a population can, of course, can change very fast.  It used to be said that within a couple of years of Haiti gaining it’s independence in 1804 under Jean-Jacques Dessalines, all the French slave holders and then all of the people with partly French ancestry were rounded up and executed.  The survivors were pretty much all descendents of African slaves.  The genetic diverstiy of the nation had diminished, let us say by half.  Whether the story is true or not, and I have not recently verified it, it would be possible in theory for some evil thing like that to happen. 

On the other hand, if one takes two villages and in one generation every one in one village that gets married marries someone from the other village, then the diversity arguably more or less doubles in a generation.

So which way do things trend?  Barring such extreme events as just mentioned, does genetic diversity increase or decrease?

The answer is that the genetic diversity of a population trends toward some value that is a function of the size of the population.  The bigger the population, at equilibrium the greater the diversity.  In other words in a small population the average person is more closely related to any other on average than in a big one.  This, of course, ignores the effect of the infertiliy produced by large population size itself.  That is only at equilibrium.  A population that is rapidly expanding by natural increase will have less diversity than and equal sized population at equilibrium, while a population that is in the throes of immigration from multiple sources will have more diversity than it would at equilibrium.

This is not a trivial issue.  We are in a position where our populations have become so diverse and effectively so large that fertilty is drastically reduced.  So how hard will it be to turn that around? 

The problem is, alas, cosmic.

The physical universe consists of matter, energy and information, encoded in matter or energy.  The rules for converting matter into energy and back and converting energy into information and back are fairly well established.  The fact is that energy tends to dissipate.  It has an “urge” to go where there is less energy.  Thus heat will flow from any object into any cooler object that is in contact.  You have heard many times the tired old doomsday scenario where the stars all eventually burn out, and without exploitable energy the structures on which life depends break down and life ceases.  It is the “warm death” piece of crepe. 

Of course if energy is dissipating, then information is getting lost, since they are forms of the same thing.  Also, the universe is expanding, which is to say that matter is dissipating.  If you follow that line of reasoning long enough you run into the idea that there are possibilities infinitely worse than the warm death.  But that is totally off topic.

The point is that structure breaks down because energy will always tend to disperse.

So consider a landscape with a host of little villages, each keeping to its own for most purposes including reproductive purposes.  Given the right size of each village, the situation is stable and there will be children who will laugh and play forever, or at least as long as the environment is benign.  The overall population has structure. 

Now move everyone into a single city.  Let the primary production in the fields be done by machinery operated by random members of the society.  Let everyone in the city choose mates at random.  Over the generations the genetic diversity of the city will rise until fertility collapses.  As that day approaches, how do you fix it?  That is precisely the predicament we are in. 

The first step seems fairly clear.  Looking at the Icelandic study, encourage people to marry third and fourth cousins.  Even that is not so simple.  That has only been proven to work only in Iceland.  And if the population persits for many generations on mating only with third and fourth cousins, then eventually the population will consist of a large number of separate entities each with a population size of a couple of dozen.

That is in all likelihood to small for comfort, but I do not know.

There is another problem.  Nobody has proven to my knowledge that marrying a fourth cousin is more likely to bring out genetic problems than marrying a 6th cousin.  But the numbers are fairly small.  If there are going to be hundreds of millions of such marriages, rare problems could turn up. 
So the first step is find the appropriate cousin and then get a genetics counselor who can provide assurance that this is not a bad thing to do.  That will be expensive, but weddings are already expensive.

So step one, find the cousin and seek council.  Final goal, create a host of separate stable communities.  Yes, there will have to be some sort of birth control, because these communities will be very fertile and will tend to grow.  Delayed marriages would work. 

But how do you then proceed after the first generation?  I really don’t know.  Remember that nature is not your friend here.  Left to itself, structure will deteriorate anyway.

I don’t even know whether the matter is urgent.  Can we afford to leave at least that problem to our children?  We certainly cannot leave taking the first step to them, and it might be quite wise to know what the next steps will be.

Of course it was utter madness for our ancestors to have led us here in the first place.  They knew perfectly well that the universe tends to disorder.  How could they trust such a treasure as the makeup of their offspring to a strategy that was unproven and that first principles told them would be difficult if possible at all to reverse?

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