Transferring PokemonThere's one thing all Trainers must do in Pokemon GO, and that's transfer any unwanted or extra Pokemon. There's really no need to keep multiples of Pokemon - unless saving to try to evolve as many as possible with a lucky egg - or even a lower stage evolution when you already have a second or third stage of the same evolution species. You only have 250 Slots for Pokemon (unless you spend some PokeCoins to expand your Storage), and it can fill up fast when you don't transfer your extras of given species.
You can Transfer any Pokemon by accessing their Pokemon Profile and scrolling to the very bottom of their page. You can even transfer multiple Pokemon at once to save a little time by holding down on one of your Pokemon after just entering your Storage, and then clicking on any Pokemon you'd liked transferred with it. Transferred Pokemon go to Professor Willow - who will give you a Pokemon Candy for their species of such - but you actually can never get them back after doing so. And that's why a trainer must choose diligently and wisely on when and which Pokemon to transfer, and which ones to ultimately keep in their Storage for battling Gyms and such.
Most players use to always keep Pokemon of the same species that had the highest Combat Power Level and transfer the rest; however, that all changed after the Individual Values Apprasial feature was released. Now, players usually have their Pokemon examined, keeping the ones with better overall stats. Sometimes this means giving up one of the species that has a higher current CP, but in the grand scheme of things, the Maximum Possible Combat Power a Pokemon has is what matters most (what IVs play a key role in).