That is not always the case. There are actually a surprising number of species that can produce fertile offspring.Quite true. However, it's accurate to say that if two populations
cannotbreed and produce fertile offspring in wild or artificial settings (your guppies), then they are
not the same species. In theory, if it were not for chronological spans,
Homo sapiens,
Homo neanderthalensis (also classified as
Homo sapiens neanderthalensis) and
Homo erectus could very likely interbreed and produce fertile offspring. Comparison to the Neaderthal genome suggests there was little interbreeding. But was this due to courtship behaviors as in guppies or fruit flies?
More likely due to other types of sexual selection, for example, appearance or speech and expression, e.g., women find men who are funny and/or can dance well to be more attractive than those who cannot -- and yes, there have been studies to this effect, FWIW.
H. neaderthalensis may not have been all that attractive to archaic
H. sapiens. Note that sexual dimorphism in facial features is more pronounced in
H. sapiens. Like the guppies, sexual selection is a huge evolutionary driver, but for humans, there are some "baseline" values, I think, that cut across cultural divides.
For example, I keep tropical fish. Poecilia wingei, the Endler's Livebearer, should not be kept with Poecilia reticulata, the Guppy, because they will breed under captive conditions and produce fertile offspring.
In the wild these species usually don't interbreed where they overlap due to differences in courtship behaviour. Like
Canis lupus domesticus or
Canus domesticus. Sort of. But wolves and dogs, although considered by some taxonomists to be "different species" do not have courtship barriers. They do have big "cultural" ones though.
It's always being revised and it is quite annoying to keep track of. That's because species is largely a human construct and still is being fiddled about with due to increasing sophistication in molecular genetics, bioinformatics and cladistics.
Perhaps we could say elves are borderline for speciation - certainly in the same genus but only questionably of the same species. But very, very close even if borderline if one is to go by interference and evidence, notwithstanding JRRT's explicit statements on the two kindred. E.g.,
Behaviors -- the elves of the Silmarillion certainly demonstrate the most fundamental of human behaviors -- speech, craft and abstract thought. Social structure. They demonstrate human emotions: envy, anger, love, altruism, empathy. These suggest behaviors
extremely similar to mortals. Such behaviors and emotions of course have cultural contexts, but they also have strong biological/neurocognitive components. Their behaviors also suggest the complex interplay of androgens and estrogens with behavior and in a very human manner.
Appearance -- Turin is considered "elven-fair" and even given a moniker to reflect that. Same with his mother. His sister Lalaith, too, I believe. The Númenóreans are considered very much like the Eldar in appearance. When Aldarion first sees Erendis, he thinks she may be one of the Eldar. If one is to buy into LaCE, then it's difficult to distinguish young mortals and elf-children. So elvish and mortal morphology also sounds similar, probably unlike archaic
H. sapiens and
H. neaderthalensis.So a difference in species could be possible, but much like the wolf and dog and maybe throw coyotes into the mix, too. Different in some ways, but with common behaviors, too. There are plenty of arguments in favor of coyotes, wolves and the domestic dog for being the same species, but different sub-species based not only on genomic identity, but behavior commonality, too. Similar arguments could readily be applied to
H. sapiens firiens and
H. sapiens eldarensis or
H. firiens and
H. eldarensis. The speciation borderline becomes a
very dim one.
Given the lack of interbreeding I would say that if they aren't species by the time of the War of the Ring they soon will be. Genetic drift alone would do it, given time.The characteristic of indefinite longevity would seem to be such a huge difference as to separate the two races/sub-species so even with common geographical borders, there might be little intermingling while species or sub-species remain barely distinguishable at a genomic level.
Again, I'll cite Tolkien's conceit that his legendarium comprises a
history or collection of
histories. If he's going to pull that card, then one can state that not all intermingling is documented. Or alternatively, if one points to the limited number of unions -- Beren and Lúthien, Tuor and Idril, Imrazor and Mithrellas, Aragorn and Arwen -- then it seems reasonable to acknowledge what he wrote in Letter 153, i.e., even if these were rare events, Elves and Men are "biologically akin." Then there's that "imaginary history of our own world" thing. The concept of Elves as something anything other than human (but with that something extra) rips me out his world of a green sun almost as fast as a flat earth with astronomical bodies created from fruit -- a lovely Mannish myth.
But really, given the mythological underpinnings to Middle Earth the whole question of elvish and human placementErrr, "elvish and human" is sorta like saying "chimpanzee and human" even if one accepts speciation between the two groups. They're still arguably
Homo something or
Homo sapiens something. As Cro-magnon is to Neaderthal so Elf is to Mortal.
...on the genetic tree is rather academic. Of course the question is academic. That's why it's fun!