Through the course of the Star Trek franchise, some species have undergone significant changes in their appearance.
For example, here is a Klingon as seen in The Original Series.
And, here is one from The Next Generation.
Quite interestingly, if we go further back into Star Trek universe history, we find that Klingons from before the time of TOS looked much like they did in TNG. Here's one from Enterprise.
Klingons weren't the only ones who went through this, and it didn't just happen in the transition from TOS to the TNG/DS9/VOY era. Here's a Trill from an episode in TNG.
And, this is one from DS9.
I'm sure these aren't the only species to have undergone changes like this. Have there ever been in-universe explanations for these, or others?