Spontaneous species-wide transformation like that isn't evolution, its revolution. That shit doesn't happen naturally, it requires some sort of intentional technological catalyst.
I don't want to get into a spoiler tag discussion since it's not fair to Kirk, but I'll just say this: -
But yeah, individuals don't evolve in that way, especially not species-wide. It just doesn't work that way. Evolutionary changes happen across generations in response to environmental pressures, usually by way of random mutation filtered through the process of natural selection. It'd make about as much sense as a subterranean cave dwelling species spontaneously growing wings.
Additionally, the Vorlons *weren't* non-corporeal energy beings as a lot of people assume. They were highly evolved, very old and extremely powerful telekinetic beings who were nonetheless physical creatures, as were the Shadows. Remember Kosh is poisoned in the pilot movie by a chemical compound administered through the skin and Dr. Kyle is able to read his vital signs and treat him. More to the point, Ulkesh was killed by a combination of direct current from a very large fusion reactor, a barrage of ppg bullets and the last remnant of another Vorlon trying to rip it's head off.
Indeed, when we finally see a Kosh and Ulkesh's true forms...well, just the fact that they *have* forms proves they're not pure energy beings. Granted it's hard to make out the shape, but I like to think they're meant to look something like Lovecraft's Elder Things. For some reason the idea of Vorlons evolving from some bio-luminescent deep sea creature appeals to me.