What variables of an evolving Pokémon remain the same?

Solution 1:

Looks like it's just the amount of experience needed to level up to 100 and the number of egg cycles needed to hatch.

  • Capture rate can change
  • Gender ratio can change (most obviously, in gender-specific evolutions like Gallade and Vespiquen. But also for Azurill and Marill)
  • Base happiness can change
  • Egg groups can change (for baby Pokémon like Pichu)

Everything else, like types, stats, and abilities, obviously changes in many cases.