Different species can't breed. That's like, the definition of a species. Which is seemingly what a separate creature entry is in the RAWs from all the other examples there.
Races are referred to as "castes" in the RAWs (along with genders), not separate entries.
Castes are not races. Castes are genetic varieties possible WITHIN a species, so male and female are castes because they are genetic varieties within a species.
Races are actually what are known in the files as Entities. Each entity has it's own unique genetics so you can often determine what specific civilization an individual originally comes from by looking at what they look like.
As things are at the moment, elf, dwarf, human, goblin etc are actually species because they cannot reproduce. But the whole marraige/reproduction system is obviously incomplete and it is not in line with fantasy conventions for them to be different species. (plus there is the Threetoes story)
Although also, it wouldn't snowball for the same reason that elves themselves don't snowball:
1) Arbitrary magical population cap the game puts on a civilization (which should maybe be removed, but otherwise would be replaced with starvation limiting at some point)
2) Incessant wars and unfortunate accidents.
I am talking about the snowballing of the immortality
trait not the snowballing of elves per-se. The caps on population actually facilitate the process because only when a person dies does a new person get to be born. In the same manner that the historical population cap leads to goblin nobles rising to power even when they are negligable amount of the general population, the general population pap will lead to immortality snowballing.
Part of me thinks so what? Is it just not natural selection at work, eliminating the inferior genes and propogating the superior genes within a species? As the immortal being is genetically superior so it's immortality trait is destined to propogate if it does not come with a trade-off.
I'm not sure we should struggle with biological realism in a world with dragons, hydras, etc. I support DF's true-to-fact approach on most things but it seems better to suspend our disbelief here.
Re: immortal offspring, perhaps the immortality of elves and goblins has to do with their magical nature rather than their biology. If you're not full-elven or full-goblin, you don't share in their nature and so aren't immortal.
Also, intermarriage/shenanigans (when we get those) could be partially based on ethics or values: most people wouldn't breed with goblins 'cause they don't like goblins. It'd depend on the individual's personality, how tolerant they were to other races. This would just serve to cut down on the number of hybrids you'd find - if it was a coin-toss you'd trip over the damn things walking down the street; they ought to be fairly rare.
Is the magical nature genetic? Obviously it is or else someone other than elves and goblins would have achieved immortality due to cultural factors.
People like (individual) goblins in the game. They like them a lot, so much that they keep electing them to positions of power over and over again. We have a world devoid of racial/species prejudice despite ample opportunity for it in biological terms.
Maybe even if e.g. the dwarf-goblin child didn't inherit immortality per se, it could have greatly prolonged lifespan, like having a max age 3-10x higher than normal dwarves.
That was the solution that occured to me initially may or not make sense depending upon we envison immortality to work. If we percieve it as working magically it will be an either/or thing for certain. If it works biologically true immortality is also likely to be an either/or thing as well.
Immortality (rather than extremely long life) would theoretically be possible because it already exists at the species levels. Species do not grow old and die, meaning that individuals that are old do have the ability to produce youth in a new being but for some reason not in themselves.
If you kept cloning additional offspring and using their STEM cells to replace your own organs indefinately, including the reproductive tissues used to create the cloned offspring then your organism could perhaps actually never grow old. The whole thing is kind of strange because as I understand this the body already clones it's own cells to replace tissues.
It could then be that STEM cells in their original embryonic forms are actually immortal and the 'problem' is that we replace our tissues with replicas of adult cells rather than with STEM cells replicated from an unbroken line of such cells. Elves and goblins could simply be mantaining an internal line of embryonic STEM cells and then formating them to create brand new tissues forever.