That should read:
"A horse can still produce a mule..."
I'm not sure if this is scientifically correct, but the principle involved is the "differentiation" of the species of horse and donkey..
At some point, horses and donkeys split into two different breeds or species..
For a very long time, there was plenty of interbreeding, but over eons, the two species became less and less alike..
Such matings became "accidental" until man came along and started interbreeding on purpose..
Without human interference, ( and maybe in spite of it, ) horse and donkey will eventually be unable to interbreed at all..
Result,.. No more Mules..
While still closely "related" enough to produce offspring, they are generally infertile.
Occassionally a horse/jenny or donkey/mare offspring will be fertile and able to produce offspring..
What am I missing?
I think what is missing is Time.
The Salamanders have not remained distinct for a long enough period to completely become two different "species", if that's the right word..
Something similar was reported recently concerning birds that split their migration around the Himalayan mountain range.. ( National Geographic? )
Those that went North and East can interbreed...
Those that went North and West can interbreed..
Those IN the Northeast and Northwest cannot..
The two branches even have developed different coloration, as well as mating calls and habits..
Given time, the salamanders will become distinct branches of their species, with more than mating habits to differentiate them.. physical characteristics will eventually develop as well, to make identification of qualified mating material easier to identify..