Tuesday, June 4, 2013

What is a Species?

Here is a simple illustration that has come in handy when I explain natural selection.  I like to call it my Natural Selection Medicine Wheel, but in this diagram it looks more like a medicine rectangle.  

 I expect that the diagram will be self-explanatory to anyone who understands natural selection.  Consider a population of elk living, let us say, in Wind Cave National Park.  A heard of such elk blundering in the campsite my son and I set up last year.  The population phenotype indicates the observable animals, including their physical traits and behaviors.  From a few cases of observation, that means a bull and a lot of cows and maybe a few young male offspring. 
When the rut comes, all the healthy adults will seek to mate.  Pretty much all the cows will be successful.  The bulls will have to fight, and the biggest bull with the biggest antlers and attitude will succeed.  Most of the males will not be successful. 
That challenge will determine the makeup of the population genotype that will be carried by the cows and that will determine the next generation.  If indeed the largest, best armed males get almost all the cows, the next generation of sons will be similarly well-endowed. 
One of the most persistent questions in the philosophy of biology is what a species is.  Zoological classification looks for a number of identifying characteristics.  However useful that is, to say that a species is an essential bundle of characteristics is called essentialism and that is largely discredited. 
Another approach looks to phylogeny.  A species is one of the main branches on the tree of life.  The trunk forks here to produce deer and elk; there, to produce humans and chimpanzees.  That’s not bad, but it is rather arbitrary.  Which forks indicate a species and which a sub species? 
The best answer in the scholarship is that a species is a population of interbreeding organisms.  Of course, it raises the question whether two populations of organisms that could interbreed but presently do not are the same species or not? 
I am inclined to approach the problem in the spirit of classical philosophy.  Aristotle was torn between two answers to the basic question.  One is that the species is this here group of animals, pawing the turf and breathing mist into the air.  The second is that the species is the species form, for that is what persists over time.  I happen to think that Plato is the better guide on this question. 
I think that the species is indeed a form rather than a particular collection of meat and hoofs.  The form is precisely the wheel that I illustrate above.  The species is the dynamic by which some population phenotype produces a population genotype which… .  It may be said that this account works only for sexually reproducing organisms.  I reply that that is because only such creatures sort themselves into a species.  Common sense can tell us the difference between a wolf and a bear.  The dynamic of natural selection as I illustrate it above is the ultimate reality underlying that difference.  

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