I don’t know if I want to do a whole thing on it, just because it’s not really a question about Pokémon at all – similar or identical systems are extremely common in RPGs and even appear in other genres. It’s just one of the basic commonplaces of reducing combat to an abstraction that can be portrayed by a video game (some games actually do try to explain experience points in a way that makes sense in the context of the story, like the Star Wars RPG Knights of the Old Republic II, but it’s often more trouble than it’s worth for the writers). The word “experience” seems to demonstrate that the basic assumption behind it is pretty simple: the more you do something – such as fighting – the better you get at it. I think it’s highly unlikely that all the growth we associate with levelling is related to the Pokémon getting physically stronger, tougher or faster, although some of it probably is. They’re learning to apply their strength more effectively, endure pain with greater focus, and move more precisely with better reaction times. These are all things people can do too.
Evolution’s the weird part, and I’ve always thought that the way the anime portrays it is our best hope for making sense of that; it regularly seems to be a result of diffusing psychological blocks of one sort or another, of strong desire or desperation to overcome a specific obstacle, or of achieving some new sense of self-awareness (my further thoughts on this in some of my anime commentaries, particularly to The Problem With Paras). Fighting often means using their powers often, and using their powers often means Pokémon better appreciate their applications and limits, and become more in touch with the forces that allow them to function. I don’t believe we’re supposed to be able to figure out exactly how it works, particularly not biologically – I’m pretty sure on some level it’s basically magic – but I think we’re meant to see evolution as part of a Pokémon’s realisation of its potential, and battles and experience as one of the fastest ways to understand that potential.
