How are they related?

[brian 333]

Humanoids come in a broad array of sizes, shapes, and attitudes. Some can breed outside their race, while others apparently cannot.

Accepting that this is how it is because that's how it is is the standard answer, but that's not how we roll!

Let's begin with the giants. The standard giants are varieties of humans, mostly, right? But what if we look deeper? Hill giants may be orcish giants, ogres goblins, fire giants are definitely dwarf giants. Hmm.

What about gnomes? Magical cousins of dwarves? Halflings as elf hybrids? There is a lot of explaining to do here.

So, let's begin with the concept of created races versus bred races. As a point to begin the discussion, let's assume four created races, created by the same process which created the world, and assume all others were bred. We may change our minds later.

Elves, dwarves, humans, and goblins fill my needs of the moment. Now we begin cross-breeding and hybridizing.

Elf + dwarf = gnome
Elf + human = half-elf
Elf + goblin = halfling
Dwarf + human = ?
Dwarf + goblin = ?
Human + goblin = orc

We seem to be missing two races. Perhaps dwarves don't cross breed all that well, but elves do. Or perhaps we have rare or slow breeding races somewhere that fill in the blanks. Whatever the case, we now have all but one of the PC races and the potential for human-orc and goblin-orc hybrid.

Now giants come back into the picture. Giants tend to be, but are not always, elementally aligned. Frost giants, fire giants, cloud giants, etc. So, let's postulate that giants are the result of outsiders breeding with mortals.

Elf + air elemental = cloud giant
Dwarf + fire elemental = fire giant
Human + water elemental = frost giant
Goblin + earth elemental = stone giant

Okay, the basic giants further hybridize with outsiders and mortals, creating a bewildering variety of giant breeds: the

Stone giants + goblins = ogres
Stone giants + humans = hill giants
Cloud giants + titans = storm giants
And so on and so forth.

So, there is a lot of room for debate over which races breed how to make what, but the idea is to begin with a small number of created humanoids, and from there breed out to the diversity seen in game. A sort of evolution by magical selection.

[PhoenixPhyre]

Humanoids come in a broad array of sizes, shapes, and attitudes. Some can breed outside their race, while others apparently cannot.

Accepting that this is how it is because that's how it is is the standard answer, but that's not how we roll!

Let's begin with the giants. The standard giants are varieties of humans, mostly, right? But what if we look deeper? Hill giants may be orcish giants, ogres goblins, fire giants are definitely dwarf giants. Hmm.

What about gnomes? Magical cousins of dwarves? Halflings as elf hybrids? There is a lot of explaining to do here.

So, let's begin with the concept of created races versus bred races. As a point to begin the discussion, let's assume four created races, created by the same process which created the world, and assume all others were bred. We may change our minds later.

Elves, dwarves, humans, and goblins fill my needs of the moment. Now we begin cross-breeding and hybridizing.

Elf + dwarf = gnome
Elf + human = half-elf
Elf + goblin = halfling
Dwarf + human = ?
Dwarf + goblin = ?
Human + goblin = orc

We seem to be missing two races. Perhaps dwarves don't cross breed all that well, but elves do. Or perhaps we have rare or slow breeding races somewhere that fill in the blanks. Whatever the case, we now have all but one of the PC races and the potential for human-orc and goblin-orc hybrid.

Now giants come back into the picture. Giants tend to be, but are not always, elementally aligned. Frost giants, fire giants, cloud giants, etc. So, let's postulate that giants are the result of outsiders breeding with mortals.

Elf + air elemental = cloud giant
Dwarf + fire elemental = fire giant
Human + water elemental = frost giant
Goblin + earth elemental = stone giant

Okay, the basic giants further hybridize with outsiders and mortals, creating a bewildering variety of giant breeds: the

Stone giants + goblins = ogres
Stone giants + humans = hill giants
Cloud giants + titans = storm giants
And so on and so forth.

So, there is a lot of room for debate over which races breed how to make what, but the idea is to begin with a small number of created humanoids, and from there breed out to the diversity seen in game. A sort of evolution by magical selection.

This is somewhat similar (although differing in the details) from how I've approached things:

Three ancestral lines: Aelvar, Titans, Proteans.

Aelvar differentiate based on selective breeding and self-experimentation into the varieties of elves.

Titans degenerate (due to a really bad decision) into dwarves (who remain basically unchanged to this day) and jazuu (goliaths). Goliaths, via rune magic that no one fully understands, can be transformed into giants (as individuals). Different ranks of giants come from surviving further into the rewriting process. Giant-kin come from failing at certain stages. Giants don't breed (they're sterile), but giant-kin do (due to a quirk of genetics). Giant-kin have throwbacks, which is where "half-ogres" and the like come from (not actual interbreeding).

Proteans became goblinoids (all 3 of which are actually one species[1]). From these arise basically all of the other races. From goblinoids were bred (via mixing bloodlines with both elves and animals) humans and orcs. Halflings arose due to rampant "magical radiation" causing mutations in goblins. Most of the anthropomorphic animal "races" came due to elven genetic magic.

One big cross-cutting category are the "Touched". This is where an individual or group lives around the influence of any of
a) fiends --> tieflings (fiend-touched)
b) large amounts of faith --> aasimar (gods-touched)
c) various elemental energies --> genasi + a whole slew of variants of other races
d) fey forces --> eladrin and gnomes
e) powerful dragons --> "dragon-touched" (including kobolds and some dragonborn[2])
These tend not to breed true if the influence is removed, and often show up as "sports" in a larger population. So there aren't many "tiefling civilizations"--there are just tieflings popping up. If a pair of fiend-pact people have children, they're more likely to be fiend-touched. Etc. They and the gods-touched are often seen as "special".

[1] the base is goblins, who have an unconscious shared tribal memory and are deeply connected at the tribe level. When a tribe gets large enough or is put under stress, they unconsciously (and consciously) pour their energy into a few individuals, turning them into hobgoblins (who serve as the "sanity" of the tribe, as they're more disconnected from the froth of the tribal memory) and bugbears (who serve as more militant/heavy-lift elements). But this transformation is temporary--without the tribe, they revert back to goblins.

[2] there is a group of "true dragonborn" whose ancestors were created artificially by infusing dragon soul fragments into unborn children. They breed true and are slightly more "stable" than most dragon-touched. But they only exist in one small nation in one small area on one continent.

[brian 333]

I agree that there is more than one way to breed a humanoid! Where you begin is a question of how you envision how the original creation of your world. That may impact potential crossbreeds, too. For example, humans may be derived from breeding elves and dwarves, and a different sub-race of elves may be responsible for gnomes. Or something different.