“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.
You appear to argue that modern biological classifications are somehow less valid than those before or at the time of Darwin.
And this is because today's more accurately reflect the latest data on morphology & DNA of related species than did the older classifications?
Boogieman: "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."
But of course, there are huge restrictions and limitations to evolution, beginning with the fact that there have been no new land phyla to evolve since the Cambrian Explosion over 500 million years ago.
Within the Cordates, mammals are the last class to evolve, some 225 million years ago, and within mammals, the first primate-like creatures didn't appear until 65+ million years ago.
Point is: there is not unlimited malleability.
Yes, ancient carnivors of 60+ million years ago did speciate into cats, wolves, bears & wolverines.
However, cats didn't become dogs, monkeys didn't become frogs, mammals didn't become fish -- those are evolutionarily impossible.
Boogieman: "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."
But natural selection limits all speciation to those modifications which work within their eco-system.
So, some fish fly, but they are not birds, and some birds swim, but they are not fish.
There are serious limits to what evolution can produce, limits placed by the nature and laws of the Universe itself.