When a Pokémon evolves, it immediately attempts to learn all moves that the evolved form can learn at its current level. Additionally, starting in Generation VII, some Pokémon have Evolution moves, which are moves that Pokémon will always attempt to learn when evolving into that species, regardless of level.
Professor Elm and Professor Rowan are the leading experts in Pokémon evolution. According to Rowan's research, over 90% of all Pokémon are connected to at least one other through evolution. Currently, the actual percentage is 80% of all known Pokémon. Rowan's research focuses on whether evolution is a form of maturity in Pokémon, and looking at the implications of the process on Legendary Pokémon (which could not evolve in Generation IV, when Rowan debuted).
In the anime, Evolution happens in much the same way as it does in the games; though level-based evolutions and trade-based evolutions do not occur using those methods, there are similarities in the way they come about. For example, Misty's Poliwhirl evolved into Politoed because it found Ash's King's Rock and was holding it when Misty sent it out, while in the games it is required that Poliwhirl be traded while holding the King's Rock for the evolution to take place (it should be noted that Poliwhirl had been through a machine in connection with it being healed at the Pokémon Center while holding the item). When a Beedrill attacked Ash's Metapod, it caused a crack to appear on its shell, which Butterfree came out of (although later examples of Metapod evolving into Butterfree were treated as the more familiar form of evolution used in the anime).
Uniquely, if Tandemaus levels up to level 25 or higher in a battle it was not sent out in, it will evolve into Maushold immediately after the battle ends with no animation.
However, some Pokémon originally from the Generation III games can change Ability slot upon evolution. When Abilities were introduced in Generation III, some Pokémon only had one possible Ability but were given a second Ability in Generation IV. If a Pokémon whose species gained an Ability after Generation III is transferred to a Generation IV or V game, it will initially retain its original Ability; upon evolving, however, its Ability slot will be recalculated and its Ability may change. When a Pokémon is transferred to Generation VI or Generation VII, however, it will be locked into the appropriate slot for its current Ability.
For some evolutions into Pokémon with multiple forms, the form of the evolved Pokémon depends on how it evolved. For example, Wormadam's cloak depends on the cloak Burmy had when it evolved. Similarly, the form of Toxtricity which Toxel evolves into depends on its Nature.
Certain Pokémon, especially baby Pokémon, requires friendship with its trainer and if they are fond enough, they are willing to evolve. Some Pokémon only evolves in a certain time of the day with friendship.
This is a list of Pokémon by their evolution family. That is, it is a list of Pokémon in their National Pokédex order, but it also includes later-released evolutions and pre-evolutions closer to the first Pokémon in their family to be released. This means that Pichu will appear before Pikachu, instead of at its National Pokédex position of 172, and Magby and Magmortar will appear next to Magmar. Evolution families are named by their basic evolutionary form. For example, although Azurill is the lowest Pokémon in its evolutionary line, it is still known as the Marill family, because Marill was released first.
Pokémon gain experience after battling wild Pokémon and Pokémon Trainers. After gathering enough experience points, the Pokémon levels up. When a Pokémon reaches a particular level, it will evolve into its next form (if it has one). There are ways to share experience, such as allow a Pokémon to hold Exp. Share: a hold item that allows the user to earn experience even if it did not participate in the battle.
These are evolutionary families in which a Pokémon can only ever evolve once. These are the most common type of evolutionary families. An example of this type of evolution family is below.
These are evolutionary families with a single member, which cannot evolve. Many of the Pokémon that have no evolutionary relatives are Legendary or Mythical Pokémon, although there are some Legendary and Mythical Pokémon that can evolve.
Certain Pokémon will be able to Mega Evolve with the use of a Mega Stone, a held item, if its Trainer has a Key Stone. However, the Pokémon will revert into their non-Mega Evolved form at the end of the battle, or after the Pokémon faints.
Additionally, a difference can be seen in the fact that Pokémon evolve during a battle, as opposed to after it. Pokémon may also evolve when they are needed to, for an extra boost of power or gaining new abilities, instead of after a set amount of training, such as when Ash's Charmeleon evolved into Charizard to battle an Aerodactyl just three episodes after it evolved from Charmander, where the game requires Charmeleon to grow twenty levels to reach that stage. In addition, Pokémon can sometimes choose not to evolve, even if they evolve by a 'natural' method such as leveling up. This was shown when Ash's Bulbasaur refused to evolve during an evolution festival for all Bulbasaur to evolve in Bulbasaur's Mysterious Garden. It appears that Evolution has emotional implications for Pokémon - some Pokémon, such as Team Rocket's Meowth, dislike their evolved forms, while others such as Ash's Pikachu simply want to prove they can be powerful without evolving. Conversely, when Pokémon do evolve, this can often be linked with an experience that causes them to mature emotionally or deal with an emotional issue, such as when the Poochyena in A Bite to Remember evolved, or the Paras in The Problem With Paras. Poochyena, for some reason, had an aversion to using the move Bite, while Paras was extremely timid and weak in battle. Both of them evolved shortly after overcoming these issues. In Enter Pikachu!, it was revealed that Ash's Pikachu evolved from a Pichu in the wild during a silent, tearful parting with a pack of Kangaskhan that had acted as his surrogate family. On the other hand, evolution is not always linked with maturity or the age of a Pokémon where the episode Tree's a Crowd features an elderly Treecko. Another example is in Odd Pokémon Out! where even after evolution, Ash's Donphan still acted quite playful as it did when it was a Phanpy.
A Pokémon card in a player's hand must specifically state that it evolves from a card in play on the player's bench. For example, Dark Blastoise states on the card "Evolves from Dark Wartortle". This means that any card named Dark Wartortle may be evolved into Dark Blastoise. However, a card simply named Wartortle cannot. Likewise, Pokémon such as Rhyhorn cannot be evolved into a Pokémon that says on it "Evolves from Team Magma's Rhyhorn".
Devolution is a form of reverse evolution when a Pokémon reverts to a previous state. The only instance devolution happens in the games is in Pokémon Super Mystery Dungeon, in which the player Pokémon and partner Pokémon evolve and soon devolve three times during the main story, and Pokémon Mystery Dungeon: Gates to Infinity, which includes the Devolve Orb that devolves all enemies in the room.
Evolution can be prevented by simply press the 'B' button on the Game when the Pokémon is attempting to evolve. This is useful as some Pokémon will learn moves that cannot be learnt in a later form or if you want a Pokémon to learn a move earlier than usual. For example, Gabite. If you let Gible evolve into Gabite at level 24 it will learn Dragon Rush at level 49, but if you keep Gible it will learn Dragon Rush at level 37. Sometimes it doesn't matter if you let it evolve or not. The Pokémon can hold an Everstone, which keeps them from evolving, so that you don't have to keep cancelling the evolution every time the Pokémon levels up. Although some Pokémon can evolve more than once (Poliwhirl, Cascoon, etc.), some basic Pokémon just can't evolve, due to undiscovered forms, or if they are really rare (Chatot, Groudon, etc.). There are also Pokémon that can evolve into different things like love, choice, etc. (e.g. Eevee, Poliwhirl, Wurmple, or Kirlia)
In Pokémon Masters EX, Pokémon from certain sync pairs are allowed to evolve through sync pair stories, where they must win a 1-on-1 battle. These sync pair stories become available once the sync pair reaches a specific level, and can be played at the cost of evolutionary items. Evolution requires Evolution Shard ×5 and the sync pair to be at least level 30; if the Pokémon can evolve again, another evolution requires Evolution Crystal ×3 and the sync pair to be at least level 45. Evolved Pokémon will have increased stats, and in some cases their moves will change. If a Pokémon reaches its final evolutionary stage, then it also receives a new sync move.