You can evolve any Pokemon once they reach the right level or conditions. A message would appear and they will evolve. Starters won't evolve though. I assume you have never played a mystery dungeon game before, but you have to at least beat the main storyline and perhaps complete certain story quests after that before you can evolve. But be grateful. In explorers of sky, I had to level up to level 60-70, recruit palkia and dialga, and recruit manaphy before I could evolve. Be glad that it's much easier in the new games.