So why are you asking that question? How is "species" defined. Does it really have a definition? Does it matter? Isn't "species" just a modern reaction to the biblical term "kinds"Now that I've taken a few minutes to think about it, I'm starting to remember why I was dodging that question. I could write a long, rambling discourse on the topic, but in all honesty the best I can do for a definition of "species" is to paraphrase Justice Stewart's concurring opinion in Jacobellis v. Ohio: I might not ever be able to intelligently define the term, but I know it when I see it.
If you ask working biologists to give you a definition for "species," most will provide you with some version or another of Ernst Mayr's Biological Species Concept:
"species are groups of interbreeding natural populations that are reproductively isolated from other such groups."In practice, however, most working biologists tend toward the "Justice Stewart" species concept, especially when their own preferred organisms are involved. It's a simple (and common) problem, really. Mayr's definition looks absolutely fantastic on paper, but it has this nasty habit of falling all to pieces when confronted by reality. We keep using Mayr's definition when asked to give one because (to paraphrase another famous guy) it is the worst possible definition for "species" - except for all the other ones that people have tried.
The next question, of course, is why we keep trying - why we think that it is important to try - if "species" is such a hard concept to define. Opinions differ on this, and I can only give you my own. It matters because we are trying to describe something that really exists. Some people do disagree, and believe that what we call "species" are simply human groupings, but I think they're wrong. I think that species really do have a biological existance, independent of our own intellectual activities.
The reason that we have a hard time coming up with a definition of species that works for everything is that there isn't one. Different types of organism keep themselves sorted out in groups in different ways. Mayr's definition works (at least in theory) for organims that are sexually reproducing. It goes all to hell when confronted with asexual organisms, and it gets worse when you toss plants into the mix - they do the wierdist damn things as they evolve.(Go read John Wilkins' paper on the topic, since he does a much better job explaining this.) Because evolution is an active, ongoing process, you will also find - no matter what definition works best for your organisms - some groups that are pushing the boundaries between species. Mayr's definition explicitly lumps together anything that can successfully interbreed, but what do you do when two populations successfully interbreed only 0.001% of the time in the lab, and never in nature? What do you do when they can be hybridized in the lab 100% of the time, but when they consistently refuse to interbreed with each other in the wild? What do you do when two populations will only produce sterile males when crossed, but always produce fertile females?
One of the best examples of this problem involves Lions and Tigers. Female Tigons (the hybrid offspring of a male tiger and a female lion) are fertile, and potentially could serve as a means of moving genes from tiger populations into lion populations or vice versa. This means that tigers and lions are potentially interfertile, and under a strict application of the biological species concept would be considered to be a single species. Of course, we "know" that they are not. Tigers and lions do not interbreed in the wild, are very different in appearance, and have very different behaviors. If they are not different species, the term becomes essentially meaningless.
So, to sum it up, species are real, and there are definitions for "species." The best definition will depend on the group of organisms that you are looking at, and no matter how you define "species," you will probably still find some group or another that is at a point in evolution where it doesn't quite fit the definition you're using. To sum up the summary, life is pretty damn messy.
But it does matter. When we look at living things (as our most distant human ancestors did) we see species. We can tell that living things are divided into groups, and that each individual is more similar to the other members of the group than it is to any other group. (Usually, anyway.) It matters because it is the way nature behaves, and because that is what we are trying to describe and explain.
It's a tired analogy, I know, but dealing with "species" really is a lot like dealing with the concepts of "child" and "adult." We know that there are differences between children and adults. It takes no real insight to look at the two and identify significant distinctions between the groups. The problem comes when you try to come up with definitions of "child" and "adult" that are capable of unequivocally distinguishing the two. It can't be done - there is no sharp distinction between the two there to identify. The same could be said for other age-based distinctions, like "adolescent" and "senior citizen." We recognize those as "real" groups, sharing numerous common features, even though we can only come up with arbitrary methods for distinguishing them from the neighboring class. The same is true for "species." We can see that they are real in nature, and that organisms really do form distinct groups. We just can't come up with a definition that will always allow us to distinguish a single species with very distinct populations from two species that sometimes interbreed.