system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon] Why don't some pokemon appear next to each other on the National Pokedex when they evolve to those ones

Like Pichu and Pikachu?
gpt
Oak made the first Pokedex before he was aware of breeding. As such, he had never seen a Pichu, and it didn't go into the Pokédex. Later, he found out about new Pokémon that were related to the old ones. He couldn't add entries into the middle for backwards compatibility reasons (Mewtwo is 150* ^^i ^^can ^^count ^^ssshh, and it's never going to be 152, or else old models of the dex will be really confused when they communicate with newer versions). This leaves the only spot to put "new" Pokémon at the end. Regional Pokédexes have evolution lines in a reasonable order, since they are able to catalogue everything we're aware of all at once, starting at #1.