There's a Game Theory that similar Pokemon from diffeent regions evolved to their region. For example, in Kanto the local bird Pokemon Pidgey has a short beak. Kanto has few trees and flat land, so Pidgey evolved its short, stubby beak to dig up bugs to eat. In Hoenn, the bird Pokemon has giant wings, which were evolved to cross the vast oceans present in Hoenn. In Unova, the bird Pokemon Pidove has evolved to be cute so it would be fed by humans like city pidgeons. I don't remember everything but it is a pretty good explanation.