As well all know, in order for mammals to produce an off-spring, there needs to be sufficient hereditary (genomic and genetic) match. Pretty much, supposed the germ cells of two different species indeed did fuse (which may or may not happen, should a breeding attempt these species between occur), the resulting genetic code would be gibberish, causing a fatal development somewhere along the line, and the embryo / haphazard fetus would be ejected. In fact, the very distinction used to separate species from each others are the ability to produce offspring that in turn can produce offsprings. Mules, for instance, are infertile, thus, donkeys and horses are separate species, despite their ability to interbreed.
Now, realize that the realms are filled to the brim with magic, so magical creatures of all kinds are created, thus "impossible" matches can be done. Note that this is not scientifically impossible. Enjoy here, the image of a geep, that I teleported in from another world, supposedly more technologically advanced than ours;
Geep are created, not by fusing cells, but by taking embryonic cells of the two species and placing them next to each others; thus, half roughly half the cells are sheep, half are goat. The two species are similar enough to allow comprehensive cellular signaling, thus, a viable adult is possible. Note, that should this thing breed, the offspring would be either sheep or goat, as the germ cells would, perhaps randomly, be from one of the both composite species. Thus, the geep is not a species of it's own, as even supposed it could breed, the result would not be another geep. No, a geep is a chimera.
Plants and insects sometimes have crazy genetic setups, rather than fusing genomes, they are simply added on top, yet become full grown individuals. Can mammals be fused in a similar fashion? I imagine, in theory, that they could, but mammals are more sensitive. For instance, a tree can have leaves on the trunk and not die. A humanoid cannot have his heart on the outside of the body and survive. Of course, given enough time for liches or crazed mages to research, why not? And note that such magically created creatures would be able to breed.
However, in the realms, ASFAIK we do NOT see half orcs half elves, while both half human half orcs and half human half elves are common, suggesting something in the orc and elf genome simply do not match. We also (save flying gnomes that may or may not be canon) do not see half halfling half humans, half humans half dwarves, and half halflings half dwarves. Gnomes, meanwhile, are described as distant cousins to dwarves.
Therefore, I suggest that, among the common humanoid species, there are three distinct liineages:
1. Humans - from whence both orcs and elves evolved. That humans are the originator species explain why orcs and elves cannot interbreed - they took such different evolutional paths, that elf germ cells do not recognize orc germ cells as the same species, while human and elf and human and orc germ cells do.
2. Dwarves, from whence gnomes evolved.
One interesting observation can be drawn from this: despite their close cultural association, goblins and orcs are NOT related!
Further, nymphs, mermaids and sirens etc are magical constructs, using humans as a base. Interestingly, nymphs, mermaids and sirens etc breed with humans, however, can only produce female off-spring! This makes nymphs etc rather difficult to classify biologically. I think the best classification would be a symbiotic race of humans.
My final observation would be that, yes, I imagine that half-orc half-elves could exist, but the cause would either be:
1) magical fusion
2) selective or random breeding of half-elves and half-orcs, where a high genetic component of elf and orc is kept, but continuous breeding with humans eventually removes the genomic mismatch that exist between elves and orcs and an offspring would be produced, with enough distinguishing elf and orc features phenotypes to for all intents and purposes be a half-orc half-elf mixture! Certainly, the creation of such a creature would be a crowning achievement any biologist could be proud of!
Please, I therefore need your help in advancing this important science. I am searching for volunteer half-orcs and half-elves to interbreed, as specified by my algorithms following a quick medical exam. You will be compensated by 250 gold, and you get the added benefit that you will not need to hunt down a mate, I will assign you one and legally marry you (I was the captain of a ship in my younger days). Further, your offspring can expect the same easy gold once they reach of age. I am especially interested in second, third, or even fourth generations of half-elves or half-orcs.
I am also interesting in any slave-collared half-elves or half-orcs, especially, as mentioned, several generation ones. I will pay 1000 gold a piece, or more, if you can produce a pedigree, proving the half breed is of several generations.
I also reach out for financial institutes interested in this very important research. Imagine, for instance, what a benefit this rare breed could bring for any standing army: recruits of supreme strength and dexterity both, with the elves naturally refined senses and magical propensity.
/// Lord DrakeICN