In the field of biology most people follow the lead of Ernst Mayer, who defined 'species' as "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." This works tolerably well for living populations of plants and animals, although there is always debate in biological circles as to the exact specifications. However, it is extremely hard to apply this definition to prehistoric populations. In these cases we go with Darwin's view how to decide whether a given form should be considered as a separate species -- "the opinion of naturalists having sound judgment and wide experience seems the only guide to follow."
You better believe this doesn't work.
This has led to a notable schism in the field of biology -- or more exactly, a never-ending series of schisms. It is most noticeable among those who deal with categorizing the fossil record; pretty much any paleontologist can be categorized into one of two categories:
A lumper is someone who tends to place new fossil discoveries into existing categories.
A splitter is someone who tends to give each new discovery its own specific name.
Splitters tend to be field workers, while lumpers tend be the ones who interpret the many fossil discoveries. (While it may or may not be a factor in any individual case, there is generally more glory in discovering a new species, or better, genus, than there is in discovering another Cro-Magnon).
Thanks to splitters, in the field of biological anthropology we have Paranthropus boisei, Paranthropus crassidens, Homo habilis, Homo ergaster, Homo heidelbergensis, and good ol' Homo sapiens sapiens. What's next? Well, for the newest model of humanity, people have suggested Homo ludis, Homo economicus, Homo symbolicus, etc. But, I on the other hand, am a lumper. I call them all Homo sapiens.
But at least we can have fun with it... I. Doolittle Wright has versified the eternal debate twixt lumpers and splitters in extended limerick form. I swear I'm not making this up. This is what anthropologists do with their spare time.
Consider the volatile splitter,
Excited and all of a twitter.
At the fun in the game
Of adding a name
For each little variant critter.
If you make up a different name,
For forms that just might be the same;
You can use the confusion,
To support the illusion
That the effort will add to you fame.
Combining a fragment of face,
With a jaw from a far-off place;
They say they are able
To warrant a label
Creating a whole new race.
But it's not always praxis that leads
To the multiplication of breeds;
Nor the glory of God,
But the conceit of the clod
With his ego-aggrandizing needs.
Now, a lumper's a different kind;
He thinks the world is designed,
Of similar bits
That wont addle the wits
In the limited scope of his mind
Specify! somebody flutters;
Simplify! one of them mutters;
The lumpers and the splitters
Will never be quitters,
So the field is a patchwork of splutters.
As names blossom forth anew,
There's much to be said for the view
That puts its store
In "less is more,"
And errs on the side of the few.
Much thanks to C. Loring Brace and his book The Stages of Human Evolution.