I mean, certainly in current and previous editions, Orcs are close enough to humans to reliably produce fertile offspring (which would, theoretically, indicate they were different expressions of the same species), so if you're doing amateur zoology perhaps that is indeed the right way to go?
I think it is, but the question is whether orcs and humans belong to different subspecies of Homo sapiens.
Looking at Wikipedia, it states that subspecies
is "used for populations that live in different areas and vary in size, shape, or other physical characteristics (morphology), but that can successfully interbreed" which seems to fit the orc/goblinoid/kobold group quite well with its distinct morphology from the human/elf group which has a more anatomically modern appearance. Wikipedia also stats that "In the wild, subspecies do not interbreed due to geographic isolation or sexual selection." I think this also favors orcs as a separate subspecies because while elves and humans might choose to mate with one another, they would generally not choose to mate with orcs, but of course that means that I would need an epithet to form the name of the subspecies.
I didn't realize you were going for Elves as night-adapted - your text upthread seems to suggest their difference are more a matter of magical influence rather than adaptation (and their night vision could be seen as an expression of their keen senses generally).
Well, the adaptation could be magical. I wasn't intending to imply a necessarily biological adaptation, nor did I really mean it as the result of a process of adaptation taking place over time. I suppose I just meant it in the not quite correct adjectival sense of "suited to".
I think of elven darkvision as linked to their origin in a time of starlight and their continued preference for operating at night under the light of the moon, as opposed to humans who came with the rising of the sun and are associated with great light. Whether their darkvision is an adaptation (perhaps cultural) to the conditions in which they found themselves or if it was a gift from their creator in anticipation of those conditions, I'm not entirely sure.
Elven keen senses I'm inclined to think of as part of a way of life that's closer to nature, much of which was learned under the tutelage of the Archfey.