Population size and stability:
I was initially surprised when my computer program showed that the population size that produced the most rapid growth was not the population size that demonstrated the lowest risk of going extinct over time.  It seemed intuitively obvious that extinction is prevented by offspring, so the more offspring the better.  As it turned out, of course, the program was a better predictor of reality than I was. 

I should not have been surprised.

Review what happens to the risk of extinction starting with very low population sizes and looking at progressively larger population size.  To begin with, populations restricted to a very small maximum size go extinct from inbreeding.  As the size increases, the rate of offspring rises very rapidly, but the risk of extinction falls more slowly.  Then the size reaches what I call the happy valley, where the potential for growth is still good but the population has a very low risk of extinction over protracted time.  At higher sizes, the number of offspring drops to below replacement level and the risk of extinction rises.  Then at large population sizes, extinction is again inevitable.

Consider the life of a male, the only kind of life I have ever known.  Start with an infant.  However endearing the creature is, he does not provide many surprises, certainly not nice ones.  All he can do is cry and perform the most basic biological survival activities.  He will not go anywhere unless he is carried. 

His behavior is stereotyped. 

With growth and development, the boy and youth are able to surprise one.  He can run swiftly and tirelessly and gets into places normal people couldn’t go if they wanted to and don’t want to, treetops, sewers, swamps, junkyards, barefoot in the snow, heavy surf, lake swimming at night.  But this amazing energy and spontaneity come at a cost.  He is unstable.  The very virtues that let him do so many things tempt him to do things that are unwise.  My younger brother has a phrase, “An intrinsically bad idea, like seeing how high you can shoot an arrow.”  I suppose just about every boy who has the chance does it once and only once.  The arrow goes straight up, stops, starts to come down backwards, flips over and speeds straight toward the – his lesson learned – rapidly fleeing boy. 

They say you should watch out for the boy who never gets into trouble.  That was once a fair approximation of me.  And part of the reason to worry is that he is not in trouble because he isn’t getting caught.  That leaves more time to get into more mischief. 

His life is characterized by high performance and low stability.

As a man, he is much less fun to watch.  As we used to say when watches were wound by the stem, and weren’t very accurate, you can set your watch by when he leaves for work.  At work he is stodgy and boring.  He gets the job done expertly enough, is in fact at the peak of his powers of productivity, but he isn’t ready for new ideas. 

His life has good performance and high stability.

Past his prime, he falls victim to the diseases of advancing years.  The first thing that goes wrong probably doesn’t kill him, but as more and more things go wrong with him, two things are happening.  He is getting slower and less enduring, and his efforts are being interfered with by his illnesses.  Eventually, although he may still be able to make a social contribution, he cannot or will not work enough to support himself.

His life has less than good performance and low stability.

And in the end is a period, brief if he is lucky, when he is as helpless as an infant, and not nearly as cute. 

His behavior is stereotyped. 

So populations have the same behaviors arranged in the same order as people.

It is not just people.  Suppose the youth we were watching takes an interest in cars.  He gets an old machine from a junk yard.  It does not run.  It is not going anywhere unless it is carried.

Its behavior is stereotyped. 

He spends some money, buys parts, installs a heavy engine, makes many improvements and winds up with an old fashioned muscle car.  The car accelerates dramatically, corners well, endures high speeds and is a joy to his heart.  But of course it only runs because he is constantly tinkering with it.

It has high performance with low stability.

He tires of the nuisance and replaces the full race cams, the racing tires, the carburetor, the clutch (this was a very old car) and the exhaust train with standard versions.  The car no longer thrills but it offers adequate performance and adequate stability.

It is still not as likely to be as good in those regards as a new modern car.

At last he sells it to someone who does not maintain it and ultimately sells it for junk.


I am sure there are other things that follow the same logic.  Usually things that work are a compromise between performance and stability and cost.  But it should have come as no surprise that the population with the fastest growth is not the population with the highest stability. 

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