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.