you only need to SEE a Pokemon to add its entry to your pokedex. you do not actually need to capture every single Pokemon. so no, you don't need to evolve anything yourself. however, you will have to encounter the evolved forms in combat with another trainer or possibly in the wild at some point. in diamond, dialga appears on a sign (and in pearl, palkia) in a certain town specifically so that you can complete your 'dex
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It depends on the Pokemon you're trying to evolve. While some Pokemon can evolve without any problems due to being in the regional Pokedex, other Pokemon (Golbat, for instance) won't evolve without the National Pokedex. Note that this is only the case in FireRed and LeafGreen - in later games, Pokemon not in the regional Pokedex will evolve, but won't be recorded until you get the National Pokedex.
Parasect: Evolve from Paras.
Provided you have the national Pokedex. Yes.
You need the national pokedex 1st....
Platinum: Togetic: You must evolve Togepi.