Boogieman:
"If you simply define 'species' so loosely as to be able to claim 'new' species based on simple accidents of circumstance, of course you will be able to cite 'new' species being created." Sorry, but I need to ask if you grasp the basic concepts of biological classifications, which start with breeds, races & varieties at the bottom end of the scale and advance up to phylums, kingdoms and domains at the top end?
By definition, separate species (i.e., horses & donkeys) are populations which don't naturally interbreed, whereas sub-species, breeds and varieties of the same species can & do interbreed (example: human "races").
And, separate species of the same genus can sometimes be forced to interbreed (for example, in captivity, producing mules).
However, different genera in the same biological family normally cannot interbreed under any circumstances (i.e., Indian & African elephants).
My point is: populations' ability, or inability, to interbreed is one basis for all such distinctions as breed, sub-species, species, genus, family, etc.
Boogieman: "However, that doesn’t prove Darwin’s central thesis that species are endlessly malleable and can transform from single celled organisms to the variety we see today merely through the mechanisms of evolution."
Of course, Darwin's theory says nothing about "endlessly malleable">
On the contrary, it insists that every evolutionary step must be a "baby-step", of small incremental changes each one of which must make the organism more favorable to natural selection.
So the appearance of "malleability" only results after untold millions & even billions of years of accumulating very small changes.
“By definition, separate species (i.e., horses & donkeys) are populations which don’t naturally interbreed...”
Certainly, that’s the case now, but if you know anything about the history of taxonomy and its relation to this theory, then you should know species wasn’t always defined that way. The main proponent of using the more modern, looser definition of “species” was, of course, Charles Darwin. So to use his definition as a confirmation of his theory which led him to that definition is really just circular reasoning.
“Of course, Darwin’s theory says nothing about “endlessly malleable””
Oh it certainly does. This notion of endless malleability is a direct logical consequence of common descent. If all the diversity of species we observe today sprang from a common ancestor, then there must be no practical limit to
how far an organism can be modified.
“On the contrary, it insists that every evolutionary step must be a “baby-step”, of small incremental changes each one of which must make the organism more favorable to natural selection.”
This does nothing to detract from the notion that evolution requires no limit to the malleability of an organism or it would not work. It doesn’t matter how small the steps are since evolution posits constant variation and accumulation of those variations.