The jury is still out on whether Neanderthals were a separate species or not. I think the evidence that most humans alive on Earth today carry DNA derived from their Neanderthal ancestors indicates they were a subspecies of Home sapiens.The standard dictionary definition not-withstanding, in science some different species can interbreed and make hybrids - some of them with apparently just fine fertility. Humans & Neanderthals, or different types of Macaws seem to do just fine out to at least three generations so far. And there's the Beefalo. With a bit less robustness in either habitat for the hybrid, the Carolina Chickadee and the Northern Chickadee have a narrow but very long range of interbreeding. And then there's the Liger and Tigon and Hinny and Mule where the female is sometimes fertile.
I would be flabbergasted (assuming elves were real) if any mammologist would classify elves (adulthood at 100 and lifespan of 750, darkvision, no sleep) and humans (adulthoodin late teens, lifespan under 100, regular vision, sleep) as the same species, even if the produce a hybrid (adulthood 20, lifespan 180 years, inferior darkvision, need sleep). Similar for orcs and humans. All in the same genus seems like a thing, perhaps as a ring species, if they actually had a common descent (which, for example, in Tolkien the elves and men wouldn't - what's the standard D&D origin?).
I think this points to the unsuitability of using the term species since magic in these settings has effectively made all humanoids a single species.That actually is untrue, depending on setting. In older settings elves were made to be able to bread with humans with a curse as revenge by the wife of the creator of elves to punish her husband. Orcs were given a blessing to bread with any humanoid and create half orc off spring in order to rapidly increase their numbers. Half Orcs breading with half orcs generate full orc off spring so the they can increase there numbers then purify their blood lines. So in those settings that fallow that lore they are different species that can't normally bread together but magic. Its kind of link a scientist adding frog DNA to dinosaurs because the can.
The jury is still out on whether Neanderthals were a separate species or not. I think the evidence that most humans alive on Earth today carry DNA derived from their Neanderthal ancestors indicates they were a subspecies of Home sapiens.
The origin of Men in Tolkien's legendarium is left unclear and mysterious. What is stated is that both Elves and Men are part of a single group called the Children of Illuvatar, and that Men (according to their own traditions) were formerly more like Elves, before being corrupted by Morgoth.
It has very little to do with being easily decided and everything to do with being decided by scientific consensus and there being vested interests in certain academic circles in classifying Neanderthals as a separate species. The Neanderthals have been classified as both Homo neanderthalensis and Homo sapiens neanderthalensis historically and continue to be as such today. As for what the determining factor is in what comprises a species, I'll refer you to the common dictionary definition: a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. That's the definition of the word I'm discussing. Also, I'm not talking about DNA derived from a common ancestor. I'm talking about so-called modern humans carrying DNA derived from their Neanderthal ancestors who interbred with modern humans with which they were contemporary. Literally everyone on Earth is descended from these pairings.If the jury is still out, and not easily decided on the side of being the same, then it still supports that ability to interbreed isn't a determining factor in what makes a species. (See also Macaws, Bonobos and Chimps, etc..) Both chimpanzees and humans carry a large number of common DNA from a shared ancestor - so a large amount of common DNA from an ancestor doesn't seem deciding either. Speciation seems a slide rather than a switch, unless each new species arrives together in at least one matched pair from the previous.
If the entire corpus of the Legendarium is used as deciding, then is much of anything sure in Middle Earth? The Ainulindale and Silmarillion seem fairly clear on them being two separate groups. The Gift of Men by Iluvatar vs. immortality for the elves seems a fairly important thing in the stories, with the Silmarillion as published finding some backing in LotR. "Luthien became mortal and was lost to Elven-kind" seems to fit pretty well there., as does the irrevocable choice given to Elrond and Elros. Does "they must reamain mortal, since the Valar were not permitted to take from them the Gift of Men" or letting Elrond's children choose it make any sense if death was merely something Morgoth gave to some elves he corrrupted? (As opposed to Morgoth and subsequently Sauron lying to the men about it). It feels as if the memory and wisdom of Galadriel and her kin (stretching back to Valinor before the Noldor rebelled) might be more trustworthy in such things than the legends of men.