The game itself will never tell you this. And reciting Macbeth backwards whilst standing on a hilltop with your pants around your ankles (possibly). To get yourself a Weavile, for example, you have to level up a Sneasel.
It's just too well hidden and too easy to miss. Pokémon can be very much a Guide Dang It series at the best of times - without a hook to a handy online resource, or a strategy book that could also be used to drop on planets you wish to crush, there's a whole heap of stuff you're unlikely to ever figure out for yourself.
