What is a Species?

One of the difficulties encountered by those grappling with the implications of evolution is the attendant slipperiness the concept of a species.  It seems to us that we have no difficulty in recognizing that there are different kinds of things in the world:  here are trees, these are fish, over there are some potatoes.  But difficulties emerge when we zoom in and try to provide a more fine-grained analysis, because there appear to be some gray areas or borderline cases.  And the narrative arc of evolution maintains that things are subtly changing all the time, so that the fuzziness of defining a species occurs not only across similar organisms we find right now (for example, the Eastern meadowlark and the Western meadowlark), but also along the development of a family line through time (for example, those hominid fossils they keep digging up in South Africa)

A common way of giving a more precise definition of a species is to say that only members of the same species can mate and produce viable offspring which themselves can reproduce.  So horses and donkeys are not of the same species, because their offspring--the mule--is sterile.  But such a definition precludes asexual "species" like bacteria, and seems to miss what we intuitively feel are different species of plants that hybridize to form new species.  The young earth creationists prefer to use the term "kind" ripped out of its historical context and forced into scientific service, claiming that there is lots of adaptation and (micro)evolution within kinds (for example, all the present day dogs and wolves are descended from an original pair of canine-like ancestors on Noah's Ark).  But we have no operational definition of a kind that helps in identifying different kinds today.  Another approach might be to signify a certain range of variation in the DNA, and say that any organism whose DNA is outside that range is a different species.  But defining that range seems to be completely arbitrary (why not allow one more difference in base pairs??).

This discussion could go much deeper into the topic of evolution.  And perhaps one day I'll get bold enough to go there in these pages.  My interest in the discussion of species today is related to the philosophical topic of realism and nominalism.  That is to say, are there (objectively) certain kinds of beings in the world, or do we group individuals together using subjective conventions and call them a kind (when we might have grouped them in different ways)?  I like teaching about this topic in Intro to Philosophy, and I was reminded of it this morning when I stumbled across a dialogue written on a blog about the topic.  Here is an excerpt of it, with G being the realist and C the nominalist (warning: this is not Philosophy 101--more like 201):

G: I mean that man is inherently capable of reasoning. That that's the kind of being that he is.
C: I see. Well to me this entire “kind of being” notion seems like an an abstract invention. An intellectual trompe l'oiel.
G: Catullus, everyone believes in “kinds of beings.” You believe that an orange is different from an aardvark. That they're different kinds of things. By nature.
C: By convention. They are both basically assemblages of carbon atoms and H2O with a little of this and a little of that mixed in to give some local colour. The carbon and water and what-not are basically just wavelengths of a primordial energy that we call light. Our minds look upon the light and form the impression of a four legged eater of ants. Then we invent a word to group together similar impressions and thus, presto chango, the category of aardvark is produced.
G: Well in that case a human being is the kind of rational subjectivity that is capable of deducing aardvarks from the primordial light. He's still, by nature, a rational being.
C: So the objective truth is that a human person is an absolute subjectivity who uses his reason to produce reality and then imagines that his, or her, productions are objective. Yes, all right. I think I can agree to that.

 When I teach this topic in Intro to Philosophy, I'll illustrate the nominalist impulse by having students put ten American cities into two equal groups according to some logical criterion.  The cities are:  Atlanta, Boston, Cincinnati, Detroit, Kansas City, Los Angeles, San Diego, San Francisco, St. Louis, and Tampa Bay.  How would you group them?  Is there a correct way?  The obvious answer is that there is no one correct way to group them.  Is that the way everything works?

Nietzsche believed that any time we used a common noun like "leaf" we were destroying the individuality of things.  All the individual leaves or snow flakes are different (but imagine how long it would take me to say, "I'm going out to rake the leaves" if I couldn't lump all those individuals into one "kind"!!).

This isn't just some clever philosopher problem.  There are important implications to how we answer this question.  Since this post is already longer than most people will read, I'll leave it to you, dear reader, to tease out those implications.