Coloration has that odd dominance pattern; the bodypart appearance modifiers that are ranges are deterimined by some under-the-hood genetics inherited from the parents. Either could cluster into small subsets of the possible, but it still doesn't define a "race" in any way that is meaningful to the game's AI. An individual who joins another civ doesn't suffer any prejudice and his/her genetics pass down normally.
Even if you want to describe a "race" as the player's pattern recognition, I have not noticed any pattern of my Starting Sevens resembling each other any more than would be suggested by the raws. I'll admit it's possible that Toady spawns migrants based on the population of historical figures in that civ (maybe picking two at random as pseudo-parents), but that would still take many generations to form noticeable "races."
You appear to be either blind or not playing the latest version of the game. All you have to do is create an adventurerer in one dwarf civilization (in the latest version), compare how you look with how at least a dozen other folk look. And then do the same in another civilization and see what common traits tend to occur. Compare against the creatures created in object testing area if there is any doubt left.
There is no accident here, the civilization's 'races' do not happen by accident, instead each civilization has a default appearance that is created right at the start by the game. As is shown by the fact that extinct civilizations settlements are populated by racialised inhabitants automatically. I think that non-historical characters use the default appearance while historical characters use an actual system of genetics.
There's really no reason to assume fantasy species aren't close enough to interbreed. They certainly look like it.
My thoughts exactly.
Oh, like a ring species.
That's fine, I guess. If the more radically different morphs aren't either non-interfertile or severely isolated however, it becomes implausible that they wouldn't mix over time into one generic type.
Merely relative isolation has not (yet) caused all the real-world races to merge into one single uniform race so why would this happen in the Dwarf Fortress world?
Here's a possible explanation for why there could be interbreeding:
It could be that, lore-wise, some of the DF races are far enough off in appearance that they are on the verge of becoming species, but haven't yet done so completely. Having them referred to as different races, in this case, would be just a more extreme example of how society tends to use 'race' to compare different human ethnicity.
If the tag is how Dirst put it (which I would prefer for it doing the same thing as the interaction, but in a much more condensed code), then if people don't want their lore as such, it is just a simple tag removal.
Edit: Heck, if that tag is caste-level, then you could have it so that combinations of different castes results in only certain results rather than being random.
It is really not much need of an explanation as to why say humans and dwarves can interbreed. What is needed is an explanation as to why they cannot interbreed.
My idea is for the same to take all the existing castes of the same body-type (so two legs, one head, two arms etc) but different creature-type and then to randomly determine their relatedness to eachother. This would be determined based on a weighted number calculated by comparing how many characteristics they have in common. Egg laying and non-egg laying creatures can also never interbreed obviously.
The game then creates a proceedurely generated hybrid creature file, along with a suitable name and symbol. Based upon the number, there is a also a possibilty that the hybrid race simply cannot happen, in which case it is stamped with [DOES NOT EXIST] and acts as a placeholder.
Then there are a few rather nasty creature level complications like.
[ALWAYS_MISCARRIES]
[MISCARRIAGE_CHANCE]
[YEARLY_DEATH_PERCENTAGE]
These do what they say on the tin and represent creatures that can concieve fertile offspring but their biology does not combine well.
The fertility related tags would be decided at the caste level, the [INFERTILE] tag would mean that a particular caste of this hybrid creature cannot produce offspring.