Adaptations within species are extrapolated as a basis for assuming one species can become another.
At least not until the recent dumbing down of the definition of “species”.
The idea that mutations have occurred and prevailed in any given species, at the same time, in sufficient numbers to sustain the mutation, and have that trait become dominant, and result in a new species, is as ridiculous as the number of commas and conditions in this sentence.
They are readily apparent. But as I said, it takes time and effort to understand the specifics of evolutionary biology. Hard work pays off with understanding.
I have no idea why this is so mysterious to creationists. There is no expectation that physics, mathematics, chemistry, engineering, etc. be reducible to a sound bite and put on a post card. But there is this perpetual demand that evolutionary biology be explained in its entirety during a 30 second commercial break.
Spend some time studying the journal articles available here. Then critique the actual science, instead of the childish creationist caricatures of the science.
That's not a recent phenomenon. Darwin himself was self-confliced about 'species', and his "successor" George Romanes put forth a new improved darwinism with the added twist that there are no such things as species. Which makes you wonder why "Origin of Species" was necessary in the first place. But this point of view really got rolling with the modern synthesis, which defined species out of existence. Nowadays a darwinian is apt to ask you what he means by species whenever he asserts that 'species x transmogrified into species y' and you have the nerve - nay, the gall to challenge it.