Parasites of trees and why there are any trees left:
The same kind tree expert who looked over my data on the different appearance of tropical and northern forests without being swept away also put this question to me.  “Since trees live fore decades and can evolve only over decades, while their parasites live months to years and can evolve over months to years, why do not the parasites evolve so fast that they destroy all the trees?”

My answer is simple.  Trees are big.  They are bigger than they need to be for obvious purposes.  It is useful for a tree to grow until its crown is out of reach of browsing animals, but once that has been done, there is not much point in more growth.  Browsing animals can strip bark and munch saplings no matter what the height of the adult tree.  It is useful to grow to the same height as other trees so as to get a share of the sunlight.  And growing taller would draw lightning.  But the balance could usually be struck at a much lower level.  But trees get big.

Imagine a parasite on a tree.  It limits its population size just like other animals.  But then it develops a mutation that lets it exploit the tree very well.  The number of parasites skyrockets and then collapses as it gets too large for its genes.  It takes many generations for it to develop a way to control its mating pool size, by which time the tree species has had time to develop some kind of resistance.  Sure there are diseases that wipe out trees like the royal palm, the American chestnut and the European elm and chestnut.  But there are still lots of trees.  The fact that they are so large lets them support a lot a parasites before they die, and thus lure the lucky seeming parasite to its destruction. 

Why is that?

I will go over the logic again and see how easy I can make it.  Imagine an animal in some landscape.  Imagine one individual animal.  That animal has chromosomes.  Consider two of its chromosomes.  We will call them Bill 1 and Fred 1.  When the two chromosomes are inherited in the next generation we will call them Bill 2 and Fred 2 and so forth.  Obviously Bill 2 and Fred 2 cannot be together, but Bill 3 and Fred 3 might and any subsequent generation could bring them together.  Now Bill 1 and Fred 1 work together like a yoke of oxen very effectively.  So would, say, Bill 5 and Fred 5.  But that is not going to happen, because in our story the animal that carries Bill 2 wanders out west for a while, and while it is there a glacier comes in and cuts the landscape into 2 parts.  Nothing can cross the glacier.  On both sides of the glacier there are good times and bad times.  Mutation happens.  Selection happens.  Genetic drift happens.  Then after a million generations the glacier melts and Bill 1,000,000 can find itself in the same animal as Fred 1,000,000.  Now they cannot function.  They are too different from each other.  Over that many generations any two mutually isolated animal populations will become different species.  They cannot make offspring that are fertile for an indefinite number of generations.  That is called speciation, and it will happen as sure as death. 

Also in the landscape is an animal of a different species.  There are two chromosomes in this animal, Lynn 1 and Sarah 1.  This time, while the glacier is hanging out, the species that contains Lynn 1 and Sarah 1 and later Lynn 2 and Sarah 2, has a very lucky break.  The population rises to 1,000,000.  There are good times and bad times.  Mutation happens.  Selection happens.  Genetic drift happens.  And eventually descendants of Lynn and Sarah wind up in the same animal.  How long will it take?  Well on average, assuming no genetic drift, it takes 2,000,000 generations on average.  That is just about enough time for genetic drift to be effective.  But suppose we look at generation 1,000,000.  Now we can pretty much ignore drift.  So Lynn 1,000,000 and Sarah 1,000,000 wind up in the same animal.

Guess what.  Just like Bill 1,000,000 and Fred 1,000,000 could not work together, Lynn 1,000,000 and Sarah 1,000,000 cannot work together.  The sheer size of the population has done exactly the same thing that the glacier did.  Bill and Fred and their kind never interacted with the glacier.  It did not affect them except by keeping them apart.  And the Lynn and Sarah have been separated just as long under just the same circumstances. 

Sorry, you can’t have a random mating pool of a million individuals of any animal without an eventual population crash.  Got it? 

Sure, there’s more to say.  I have said more.  I shall say more, but it is sort of like the man who asked a starlet if she would marry a man she didn’t love for a million dollars.  She said she thought she would.  Then he asked her to have sex with him for $40.  She demanded to know just what he thought she was.  He answered, “We have already established that.  We are now just haggling over the details.” 

Everything else is just a matter of the details.  The suicidal population size is a lot less than 1,000,000, but just how much less?  How long does it take to get into trouble?  Once the birth rate begins to fall, can the population be saved?  How late is it possible to save it?  Taken all together, these make up the second most important issue of our time.  The most important issue, of course, is whether we even care enough to talk to each other about it. 

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