Do you think Beforans and Alternians are biologically identical?
Depends. I’d say the core functions of Alternians and Beforans has to be identical as they’re from the same race - almost all cat-like creatures have whiskers, ability to hide claws (except cheetah) and see good in the dark. At the same time there are minuscule differences that can be spotted between Alternian and Beforan trolls that cannot be linked only to cultural differences. For example:
Latula and Terezi have completely different sets of teeth
…something post-scratch!Latula doesn’t share with pre-scratch!Latula, as Redglare seems to have identical teeth with Terezi.
Amporas have different sets of fins, which is either a major mistake from sprite artist (fins cannot be different if we’re talking about same line of family, because it can mean completely different species in genotype - unless xenobilogically seadwellers can be differentiated by other traits) or a showcase of different biology.
The only picture of Dualscar doesn’t clearify much here.
Now, I’m not a big fan of differentiating Beforus trolls on sprites that were made by guest artists, cause they have major differences from walksprites and thus, aren’t really that canon. On the other hand, you can’t tell much about biology of Beforus trolls basing on walksprites alone.
There’s the thing with Beforus and Alternian trolls tho: they both have lived in completely different environments (Beforans were peaceful and Alternians extremely aggressive) and this surely had to influence some of their biological traits. If you consider that Beforus trolls become ancestors post-scratch, then it has to mean that:
a) Scratch does something to trolls that changes some of their biological features;
b) Scratch doesn’t do anything but years of evolutions down the generations do, as f.e. Latula’s lack of sharp teeth would eventually evolve into sharp fangs (through genetical mixture) which Terezi has;
At the same time you have to remember that Beta trolls we know were once ancestors to Alpha trolls, so in the end it’s all a bit like a pendulum going back and forth in a loop - aggressive traits vanish in non-aggressive environments as they don’t have to be used and they reappear once the individuals are tossed into environment that requires for them to be there (during the given span of time of generations, process of elimination and so on).