Hybrid breakdown:
Many things about this subject trouble me.  If a population is to survive, there must be adequate consanguinity.  It seems simple enough.  You can have too much or too little of anything vital.

Pretty much the information I have been able to lay a hold of seems to be consistent.  But there is an issue that nags me.

Pretty much it seems that if you let a population of mammals grow according to the principle of Malthus, it will not expand to infinity but will have a fertility crisis at about five or six generations and a worse one after another four or five.  Let’s call it five. 

If you divide a population of mammals and separate them for two thousand generations then when they get back together they have undergone speciation and cannot have offspring that are able to survive and reproduce indefinitely.  That would be roughly sixty thousand years for humans, and sure enough pretty much all of us were sitting around the same campfire that long ago.  There might be some outliers, but let’s not worry about them just now.  For our present purpose we are all the same species.  And if you were to brow beat me you could probably get me to admit that three thousand generations to speciation might be possible even though I don’t believe it is the case.  But don’t try to go to ten thousand.  We would have to go back and look at some numbers. 

Either way, speciation effects do not exist for humans.

But the effect of lack of consanguinity is everywhere from ancient civilizations that collapse with such dismaying regularity to fat pigeons in the park that refuse to outgrow their food supply in spite of the fact that only a finite amount of food can be brought to them.  (All right.  Pigeons are not humans, or even mammals for that matter, but you get my drift.)

So there are two different processes. 

Now if you look at the chromosomes of different species with a microscope, by an large they have different patterns with the number varying and the number of any one approximate size varying.  Because of the mechanics of cell division, an egg and a sperm of very different karyotype as they call it are not able to produce a viable offspring.  Since the species must begin as one individual, this must be a late change.  All right, maybe you can come up with a cunning arrangement by which the germ cell line of some individual has a different kayrotype from the rest of the individual and this results in two offspring that are able to mate with each other.  But don’t try to tell me there are going to be a lot of changes as a first step.  I’ll stick with my guns and say kayrotype changes tend to be late and accumulate by small degrees. 

Now speciation has if I read it right somewhere about twenty definitions.  That means as a species divides there will be about twenty days when somebody is going to say, “Today they are different species.”  But for us they are different when they reach the point where they cannot cross and have offspring that can reproduce indefinitely. 

If you cross a Bactrian two humped camel with a dromedary one humped camel you will get a one humped camel with a depression in the middle of the hump or a camel with two partly fused humps depending on how you look at it.  And if you follow it for a few generations the line will die out.  Camels express hybrid breakdown and are just barely different species.

But how many is a “few?”  I have never found an answer.  I don’t know whether it is a fixed number in one type of cross or it varies.  I don’t know if it is the same for one type of cross as for another.

But I have the eerie feeling that it may be five. 

If so, it is quite a coincidence.  We would have the same number of generations with two entirely different mechanisms.

There seem to be four possibilities:

  1. The number of generations does vary.
  2. The number is not five.
  3. It is a coincidence.
  4. There is some subtle underlying principle beyond my own ken that picks out five generations.

Numbers 1 and 2 can in principle be ruled out by real world evidence.  In fact somebody may already know.  Number 3 would not make me happy.

And if I ever come up with a reason for number 4 I shall let you know.

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