If there were DnD Druids and Rangers and Elves, and etc... how much of this would be widely known everywhere at a technologically much earlier stage than IRL? What else would they know - germ theory? Would they tell it widely? Would they be believed? Or would those nature facts not be true in your campaign world (do barnacle geese come from barnacles)? Would the presence of magical origins confound it all (would regular birds not be distinguishable from regular ones)?
"..how much of
this would be widely known everywhere at a technologically much earlier stage..." (emphasis mine)
The first question in my mind is how much of "this" (meaning, the biological sciences as we know them) is even true in the fantasy world?
In our world, many of the facts of biology ultimately stem from evolution as a driving principle. The fact that we have "reptiles" and "mammals" and "birds" and so on is a result of evolution. But loads of our fantasy worlds have gods that are real and
created the fictional worlds. Those biological systems, then, are not the result of evolution, and only need to look like our biology on the larger scale, on the surface.
And trying to patch modern scientific reality onto fantasy runs afoul of the same issues when we try too hard to explain science fiction that isn't actually consistent with real-world physical law. Star Trek's warp drive (any FTL, actually) starts causing consistency and causality issues if you look too closely at it. Similarly, having modern real world "genetics" in your fantasy game world falls apart when you also have dragons that can breed with anything, and owlbears that are patched together owls and bears..
The real world has the benefit of the anthropic principle - it
has to be self-consistent, or we would not be in it to view it. Your fantasy world does not have that restriction, and you can, with very few steps, throw real-world science out the window. As soon as you concern yourself with how much "science" your fantasy world knows, you are opening yourself up to revealing all the inconsistencies in your fantasy.