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 and Sentret 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, Swablu and Wailmer needs 400 candies each to evolve into Gyarados, Altaria and Wailord respectively.
 * 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 six evolution items: Dragon Scale, King's Rock, Metal Coat, Sun Stone, Up-Grade and Sinnoh Stone. It is always just one evolution item required to perform singular evolution.

Distance walked as Buddy
There are currently in game five Pokémon that need certain distance walked as Buddy Pokémon to be evolved.
 * Eevee requires 10 km walked as Buddy Pokémon to evolve into either Umbreon or Espeon.
 * Espeon evolves from Eevee only when evolution is performed at day.
 * Umbreon evolves from Eevee only when evolution is performed at night.
 * Feebas requires 20 km walked as Buddy Pokémon to evolve into Milotic.
 * Mime Jr., Bonsly, and Happiny all require 15 km to be walked as a Buddy Pokemon to evolve into Mr. Mime, Sudowoodo, and Chansey respectively

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 another.

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.
 * This concept was scrapped. The evolution of Clamperl is finally decided to be random.
 * It was also stated there that Feebas would be required to be walked a minimum of 20 km as Buddy Pokémon, before it had been released to the gameplay.
 * Searching the phrase "evolve" in Pokémon collection search return the list of all Pokémon with enough amount of candies to be evolved.
 * Every Pokémon gives back 1 of its candy type when evolved. So for example, if a player has 23 Pidgey candies and 2 Pidgeys (which need 12 candies to be evolved) they can evolve both of them, as the first one gives back 1 candy and then the player still has 12 remaining. After evolving both Pidgeys, there will still be 1 candy remaining.
 * Pokémon can be evolved at any health, even when fainted. After evolving, the new Pokémon would be at full health.