system
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human
[D&D] and most fantasy. I'm doing a biology project, and I'm confused about species vs race?!

So I'm a half orc named Yod, perfect marks, about to leave highschool. Final project involves studying lineage and breeding, I want to be a memphit breeder like da, and I got to thinking. I've seen half humans, half elves, people with dragon in their bloodline, half orcs, half orc/half ogre, but never a dwarf/gnome, elf/orc, half tiefling, etc.

Are we different species? Which ones are different species? Are our mixed offspring sterile? What's going on here? I've searched as far back as I can and no books list all sentient races, (consider elementals, can you be half wind elemental? And WTF is a genesai half breed?) as being from a common progenitor. In fact some seem to be directly made from nothing, or a diety.

What the heck happened and what's going on here? And with shape changing readily available, does that mean the only limits are physical? I'm so confused. This project is really tough guys, and the professors eyes seem to glaze over when I ask them.
gpt
In many fantasy settings, you can safely rule that most humanoid races come from one Species. This is due to the tendency for inter-racial unions to produce viable offspring with no biological or physiological defect. In D&D particularly, half-breeds do not seem to suffer any reproductive deficiency, even when their parents are from widely different races (Half-Goblins exist, for instance). This reproductive capability also extend to non-humanoids who have shapeshifted into another form, though the offspring may inherit some characteristics of their non-humanoid parent (Dragon-bloods, Genesai, etc).

In some settings, this is attributed specifically to Magic. In Warcraft, interbreeding between races is extremely difficult without magic, with all known half-breeds being the product of direct magical intervention (Garona) or the offspring of powerful mages (Rhonin's twins). In some of these, it's even difficult for two closely related races to reproduce (Elves and Trolls, for instance). In this case, it's safer to classify Race and Species as synonymous.

In other cases, interbreeding is either almost or outright impossible. Warhammer Fantasy is an example here. Humans, Elves, Dwarves and Halflings, despite sharing superficial similarities, are entirely different Species.

So, like most cross-universe considerations... It depends. 

As a general rule... If they can interbreed successfully enough for there to exist populations of 'Half-whatevers', they are of a single Species. If they require magic or genetic intervention (such as in Star Trek) to produce offspring, healthy or otherwise, then they are different Species.