Evolution

Evolution is game mechanic that allows Trainer to make Pokémon stronger and more advanced by feeding it with certain amounts of Candy. Evolved Pokémon generally have higher CP and HP, making them more valuable addition to Trainer's collection.

Evolution costs
Every Pokémon that can evolve requires certain amounts of candy to be evolved. There are some other Pokémon that require Evolution Items or distance walked as Buddy Pokémon.

Candy
There are several patterns for candy costs regarding the number of members of each Pokémon family.
 * The most common patterns for evolution families:
 * When Pokémon have only two stages of evolution in its family, a second form costs 50 candies to evolve from a first one.
 * When Pokémon have three stages of evolution in its family, a second form costs 25 candies to evolve from a first one, and a third form costs 100 candies to evolve from second one.
 * Patterns for evolution families which Baby Pokémon were added to:
 * When Pokémon has Baby Pokémon introduced that makes its one-stage Pokémon family a two-stage one. A second form costs 50 candies to evolve from a first one (Baby Pokémon).
 * When Pokémon has Baby Pokémon introduced that makes its two-stage Pokémon family a three-stage one. A second form costs 25 candies to evolve from a first one (Baby Pokémon) and third form costs 50 candies to evolve from a second one.
 * Only exception here is Marill family, as the costs of evolution into Azumarill from Marill and Marill from Azurill are 25 candies each.
 * Patterns for evolution family of the most common Pokémon:
 * When Pokémon is part of two-stage evolution family, a second form costs 25 candies to evolve from a first one.
 * ​ It applies to common Pokémon such as Rattata, Sentret and Ledyba and Pokémon that have branched evolution families such as Eevee and Tyrogue.
 * When Pokémon is part of three-stage evolution family, a second form costs 12 candies to evolve from a first one, and a third form costs 50 candies to evolve from a second one.
 * It applies to common Pokémon such as Weedle, Caterpie, Pidgey and Wurmple.
 * Patterns for special evolution family:
 * Being a part of two stage Pokémon families, Magikarp and Wailmer needs 400 candies each to evolve into Gyarados and Wailord respectivelly.
 * Being a part of two stage Pokémon family, Feebas requires 100 candies to evolve into Milotic.
 * It also requires a 20km walked as Buddy Pokémon for its evolution.

Evolution Items
There are several Pokémon that requires special evolution items to evolve. There currently only 5 evolution items and 8 Pokémon families in which such item is needed to further evolution. It is always just one evolution item required to perform singular evolution.

Distance walked as Buddy
There are currently in game three Pokémon that need certain distance walked as Buddy Pokémon to be evolved. Two of them currently available in game - Umbreon and Espeon, and third one - Feebas, is yet to be released.
 * Eevee requires 10km walked as Buddy Pokémon to evolve into either Umbreon or Espeon.
 * Espeon evolve from Eevee only when evolution is performed at day.
 * Umbreon evolve from Eevee only when evolution is performed at night.
 * Feebas requires 20km walked as Buddy Pokémon to evolve into Milotic.

Pokémon families
Each Pokémon is a member of a Pokémon family (also known as Evolutionary line). It is basically a group of Pokémon that are related with each other as one of such Pokémon evolve into or from other one.

Pokémon evolution families can range between a single to three-stage evolutions, but there can also be more than three members within a family, as some of Pokémon families might be branched.

Trivia

 * On the official support page for the Pokémon GO, Deepsea Scale and Deepsea Tooth are mentioned.
 * In the main series games, these items required to evolve Clamperl into either Huntail or Gorebyss respectively.
 * It's also stated that Feebas is required to be walked a minimum of 20 km as Buddy Pokémon.