Species aren’t really as well defined as we often like to think. Now and then, you come across something that reminds you that the idea is mostly a human-created tool in order to make categorization easier.
One of the classic definitions of species goes along the lines of “a population whose members can interbreed with each other to produce viable offspring.” Keyword being “viable”, which is why horses and donkeys are considered different species despite being able to make (sterile) mules. That definition doesn’t always work right, though.
Probably one of the best examples of this is what has come to be called the Ring Species.
Say you have a species of salamander that lives in the streams of a certain mountain range. But the tops of the mountains themselves are too high for the salamander to cross. Instead, the population slowly spreads around the range, until it finally spreads far enough to reach a pass low enough for them to live in and spread through.
Eventually, the population has managed to encircle the entire mountain range and meet up with the other end of the population, which spread in the other direction around the mountains. The only problem is that the species took so long as it spread, that the two ends have evolved far enough apart genetically that they can’t reproduce.
They are effectively two different species at each of the ends. But here’s the kicker:
There’s still a viable chain of population groups between the two, each of which can still reproduce viable offspring with the ones next to it. As far as any salamander in the middle of the chain is concerned, they’re the same species as their neighbors. But drop them elsewhere in the chain, and they might not be.
That salamander, by the way, is Ensatina eschscholtzii. Better examples include the Larus Gull, which instead of reproducing in a ring around a mountain range, reproduce in a ring around the North Pole.
To make sure I’m being clear, in this image, gulls at point 1 can breed with gulls at point 2, which can breed with gulls at both points 1 and 3, etc. But the gulls at 1 and 7 can’t breed with each other.
Dogs almost have a similar thing going on, with how much we’ve changed them through domestication. They may be genetically similar, but I doubt you’d see the crossbreed of a Great Dane and a Chihuahua anytime soon. (I apologize for that mental image)
Now that you get the basic concept, let’s take a closer look at what we could do with it!
It doesn’t really require that a Ring Species have only two “ends”. That’s simply the example most likely to occur in nature, where Ring Species are pretty rare anyway. But there’s nothing stopping you from creating gradients of species that have multiple splits due to geography or other phenomena, resulting in multiple ends.
You might even be able to use this with the stereotypical player races. Humans, Elves, Dwarves, Orcs, Gnomes, Halflings, they could all be variant population groups in one giant Ring Species, making them actually work as “races”. Humans could be situated between Orcs and Elves in that continuum, explaining where Half-Orcs and Half-Elves come from, while also explaining why you don’t see any half-Human-half-Dwarves walking around. (Maybe the cross between a Dwarf and an Elf is a Gnome?)
Or the Morlocks and Eloi as branches off of humanity.
The genetic variation could also express itself however you like. You could have ring species separated not by physical obstacles but by things like breeding seasons, or mate selection.
I could even see a plot point or two where the center sections of a Ring Species are destroyed in a climactic disaster, and the two ends pick up the pieces and continue on as two new-ish unique species.
It likely won’t be something you use regularly, but I think it’s still a neat concept you can keep in your back pocket for an interesting scenario or two when you need it.