Many species in the Star Trek universe have "bumpy foreheads"--most famously, the Klingons.
What are the likely factors that influenced the evolution of Klingon cranial ridges?
Could it be a protective measure against some environmental factor like radiation, or does it serve some other purpose?
It's a sagittal crest. Their cranium is encased in an exoskeleton, which has a tricipital lobe. Perhaps their skull forms from both sides, and the ridges are where the bone fuses?
Check the TNG series for the "Genesis" episode where a synthetic T-cell becomes a viral infection that causes the crew to "de-evolve". Worf's proto-self is not a creature to be toyed with, and the cranial ridge is much more pronounced. As Teknophilia said in another answer, there's an extra skeletal shell around their skull.
Klingons are built to survive, as was mentioned in another TNG episode - "Ethics" (the one where Worf becomes paralyzed from the waist down). The visiting doctor who is working on an experimental paralysis surgery is reviewing Klingon anatomy and remarks offhand that whoever designed them certainly believed in spare parts.
I think there is some thinking going on behind the make-up stying in ST.
As a rule, the more aggressive beings tend to have heavy ridged foreheads, which I have always presumed reflects thicker and heavier bones - the ridging would allow for overall thickness without excessive weight. For beings who like to hit each other a lot, it makes sense that they have developed something there to give some protection against frontal attacks. They thicker skull in other places is probably hidden behind their hair.
Those species who have developed a more Vulcan approach to life seem to have smoother heads, to better fit their larger brain capacity.
It is also significant that heavy forehead makeup tends to make someone look agressive, and smooth features the opposite, probably reflecting out own past developing from Neandethal through to Homo Sapiens.
In a DS-9 episode where Commander Sisko and Worf travel back in time to the original Enterprise, original series footage of Klingons brawling with crew members is shown. They have no prominent cranial ridges. To the question "Those are Klingons? What happened?", Worf answers, looking rather embarrassed, "It's a long story". This implies that the cranial ridges were of recent origin, at least for Klingons (although from a scriptwriter's standpoint, it was clearly an attempt to address a discrepancy in a scene).
