Evolution

Evolution is a sudden process in which a Pokémon changes into a new form. Despite its name, Pokémon evolution shares more similarities with than the theory of. In the video games, evolution usually results in the evolved Pokémon's stat values changing. Single-form Pokémon do not have any evolutions. Few Pokémon do not want to evolve, with Ash's Pikachu being one of them.

Most Pokémon evolve after reaching a certain level once they have acquired enough experience points from Pokémon battles. Some other ways in which a Pokémon can evolve include the use of an evolutionary stone, trading the Pokémon to another player (and sometimes for a specific Pokémon), or having the Pokémon's friendship reach a certain level. Some evolutions occur based on a circumstance or condition, such as the time of day, carrying a unique item while being traded, leveling up in a certain area, having a certain Pokémon in the party, knowing a certain move, or in the case of Inkay, holding the game console upside down. Starting with Generation VII, several Pokémon from earlier generations have an evolutionary variant unique to the Alola region.

The player can choose to halt evolution at any time before the animation is finished by pressing the B button, unless the evolution was initiated by using an evolutionary stone on the Pokémon. Pokémon whose evolutions are canceled will attempt to evolve again after the condition for evolution is met again. The main purpose for preventing an evolution is because many Pokémon can learn moves sooner than their evolved form.

Evolutionary stages
All Pokémon can be placed at one of four evolutionary stages (though no existing Pokémon line includes more than three stages out of the four stages) – Baby, Basic, Stage 1, and Stage 2. A Basic Pokémon is usually the first one in the evolutionary line, but some do evolve from Baby Pokémon. Many of these Basic Pokémon can evolve, and the forms just above the basic level are known as Stage 1 Pokémon. When these Stage 1 Pokémon evolve, the new form is known as a Stage 2 Pokémon. When bred under certain conditions, Basic or Stage 1 Pokémon may produce an egg which hatches into a Baby Pokémon that then evolves into the Basic form.

A Pokémon higher up on this evolutionary scale is called an evolved form of the previous stages; a Pokémon lower down on the scale is often said to be a pre-evolved form of later Pokémon in the chain. Some Pokémon, such as Eevee, Slowpoke, Tyrogue, Clamperl, and Minomutchi have branching evolutionary chains. All of these branches occur at the evolution of Baby, Basic or Stage 1 Pokémon; there may be multiple Basic, Stage 1, and/or Stage 2 Pokémon in an evolutionary chain, but only the Wurmple line contains a divergence in evolution prior to the final stage.

Methods
There are several ways in which a Pokémon can evolve, some of which involve two simultaneous methods.

Leveling up
As in many RPGs, a Pokémon’s overall strength is measured by its level, which ranges from 1 to 100. The most common cause of evolution is leveling up either by gaining experience points or by being fed Rare Candy, which instantly elevates Pokémon by one level. A Pokémon that evolves in this manner will evolve once it reaches a certain level. For example, Bulbasaur evolves into Ivysaur at level 16, which evolves again into Venusaur at level 32. Level-based evolution can be halted by the trainer at any time before the process is complete, or prevented outright by having the Pokémon hold a special item, the Everstone. This is useful because, with this method of evolution, higher-evolved Pokémon learn new moves at a slower rate. For example, in some games, Bulbasaur learns its final and most powerful move, SolarBeam, at level 48, but Ivysaur learns it at level 54, and Venusaur doesn't learn it until level 65.

Certain items
Some Pokémon evolve through the use of a certain item. In most cases it is an evolutionary stone, which have elemental properties that affect specific Pokémon. Some Pokémon that evolve through a certain item are part of a branched evolutionary chain, and their evolution depends on which item used on them. The most notable example is Eevee, who has eight eeveelutions, five of which are each the result of exposing Eevee to a certain evolutionary stone.

Starting with Pokémon Diamond and Pearl Versions, certain Pokémon do not immediately evolve by coming into contact with an evolutionary stone. In most situations, instantaneous evolution may occur, as with Growlithe to Arcanine. Holding the evolutionary stone is just another way of making it evolve.

Trading
Some Pokémon evolve if they are traded among Pokémon Trainers. This generally involves the connection of two handheld systems and the trade of a Pokémon from one game to another. Starting with Generation II, most Pokémon only evolve if they hold a certain item while being traded. Like evolutionary stones, the Pokémon consumes their held item once they evolve. There are two Generation V Pokémon that only evolve if traded for a certain Pokémon.

Friendship
Since Generation II, the games track a hidden statistic measuring a Pokémon's friendship with their trainer, the player character. It changes based on several factors, such as how often a Pokémon is defeated in battle or if it has been fed stat-enhancing items. The player character can gauge a Pokémon's friendship level by taking it to certain NPCs, or using Pokémon Stadium 2 or Pokémon Box to monitor the strength of its friendship with the trainer. If certain Pokémon have a high friendship, they evolve upon leveling up. A few Pokémon from the Generation I games received an evolution through this method in the Generation II games. Similarly, a few Generation I Pokémon received a pre-evolution that evolves into them through friendship. There are a few Pokémon that only evolve at a certain time of day or night.

Gender-specific
Starting with Generation IV, there are a few Pokémon who can only evolve if they are a certain gender. A majority of them evolve through leveling up, although two Pokémon evolve by using a Dawn Stone.

Attack-based
Starting with the Generation IV games, certain Pokémon evolve when they learn a certain attack.

Time-based
Starting with the Generation VI games, some Pokémon evolve if leveled up at a certain time of day.

Similarly, in as early as Generation IV, there are three Pokémon that evolve through similar conditions but must be holding an item. Most Pokémon that evolve by holding an item do so if traded.

Special evolutions
Several Pokémon evolve under specific conditions, many of which is still based on the Pokémon's level.

Mega Evolution
Mega Evolution is a mechanic introduced in Pokémon X and Y, and it further increases the abilities of specific Pokémon. If the player character possesses an item called the Key Stone and a Pokémon is holding a Mega Stone that corresponds to its species, that Pokémon will be able to Mega Evolve during battle. The player cannot perform more than one Mega Evolution per battle. Unlike normal evolution, this transformation is temporary and the Pokémon will revert to its normal form after the battle. Mega Evolved Pokémon have higher stats than their normal forms and may also have a different type or ability.

Primal Reversion
Primal Reversion is a mechanic introduced in Pokémon Omega Ruby and Alpha Sapphire. Only two Pokémon, Groudon and Kyogre, are capable of Primal Reversion. These Pokémon will undergo Primal Reversion automatically if they hold the Red Orb or Blue Orb (respectively) when sent into battle. Like Mega Evolution, Primal Reversion increases the Pokémon's stats and changes their Abilities; unlike Mega Evolution, Primal Reversion can be performed more than once per battle.

Bond Phenomenon
Bond Phenomenon is a mechanic that debuted in the anime's XYZ series and was introduced in Pokémon Sun and Moon. The only Pokémon capable of this transformation is Greninja with the Battle Bond Ability. Greninja will transform into Ash-Greninja if it knocks out a Pokémon during battle, giving it an appearance similar to Ash Ketchum, increased stats and a stronger Water Shuriken. Unlike Mega Evolution, Primal Reversion and Ultra Burst, Bond Phenomenon does not require an item to activate.

Ultra Burst
Ultra Burst is a mechanic introduced in Pokémon Ultra Sun and Ultra Moon, exclusive to the Legendary Pokémon Necrozma. Necrozma may undergo Ultra Burst if it is in its Dusk Mane or Dawn Wings form and holding the Ultranecrozium Z Z-Crystal, which will allow it to transform into Ultra Necrozma. Like Mega Evolution, Bond Phenomenon and Primal Reversion, Ultra Burst increases Necrozma's stats and is a temporary transformation.

Dynamax & Gigantamax
Dynamax and Gigantamax are mechanics introduced in Pokémon Sword and Shield. If the player character has an item called the Dynamax Band, it drastically increases the Pokémon's size, its HP by 50% and can use Max Moves. The Pokémon can further increase its HP by 5% from using Dynamax Candy to increase its Dynamax Level. Gigantamax is similar to Dynamax, but it changes the Pokémon's form, similar to Mega Evolution. Gigantamax Pokémon use G-Max Moves, exclusive to the 26 Pokémon capable of Gigantamaxing. Each Gigantamax Pokémon must have their respective move type in order to use their respective G-Max Move. Dynamaxing and Gigantamaxing can only be used in Gym Stadiums, Max Raid Battles and other areas except for wild or trainer battles.

Zacian, Zamazenta and Eternatus are the only Pokémon that cannot Dynamax.

Eternamax
Eternamax is a mechanic introduced in Pokémon Sword and Shield, and is exclusive to the Legendary Pokémon Eternatus. Eternatus changes into its Eternamax form after defeating it for the first time. Its power prevents the player's and Hop's Pokémon from attacking. Eternatus is attackable after the player summons the Legendary Pokémon Zacian and Zamazenta, resulting in a Max Raid Battle where the player can capture Eternatus and end its transformation.

Eternamax Eternatus can be used in battle (if Eternatus uses the move Eternabeam), but the player cannot currently obtain this form of Eternatus.

Pokémon Yellow Version
In Pokémon Yellow Version, the player character's Pikachu, who is based on Ash's Pikachu, refuses to evolve into Raichu if the player character tries to use a Thunderstone on it.

Pokémon Pinball series
In Pokémon Pinball and its sequel, Pokémon Pinball: Ruby & Sapphire, the player must accomplish certain tasks to enter EVO mode, and may then evolve any Pokémon caught in that round of play by using the ball to collect a set of three icons representing experience, appropriate evolution stones, hearts for friendship, link cables for trade, and similar attributes, scattered around the board, representing the method by which it evolves, before time runs out.

Pokémon Mystery Dungeon: Red Rescue Team / Blue Rescue Team
In Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team, the world of Pokémon is plagued by natural disasters, including the inability of all native Pokémon to evolve. Once the player's team has defeated Rayquaza, the Luminous Cave, where evolution can occur, appears at Whiscash Pond. Evolution requires the same conditions as in the main series, except that happiness evolution instead occurs only if the Pokémon's IQ is high enough, and Pokémon which evolve through trading must use a Link Cable item in addition to any other required item. Eevee will evolve into Espeon or Umbreon once its IQ is high enough by using the Solar or Lunar Ribbon respectively, and Feebas evolves with the Beauty Scarf. Shedinja appears automatically after Nincada evolves. Wurmple will evolve into either Silcoon or Cascoon depending on its personality value at level 7.

Pokémon anime
In the anime, some Pokémon evolve under different conditions. One notable change is that Pokémon often evolve in the middle of a battle, which cannot happen in the games. In "March of the Exeggutor Squad", an Exeggcute evolved without the use of a Leaf Stone.

Usually in the anime, a Pokémon would evolve by glowing as a white silhouette, changing shape, and then the glow would fade to reveal its new form. However, there were a few exceptions; Ash's Caterpie evolved into Metapod by shooting string on itself and shedding its skin. Then Metapod would evolve into Butterfree by flashing, and the butterfly Pokémon would emerge from the shell, like a real butterfly coming out of pupation. Beedrill and its pre-evolved forms would also follow this practice. Eventually later in the series, these Pokémon types would evolve the normal way how other Pokémon do: by glowing and changing shape. Also, when Poliwag would evolve into Poliwhirl, it would stop changing shape halfway and appear as a spinning circle of light before resuming its evolution to its next form. This is not present in its evolution into Poliwrath, but it does appear as a rotating circle of light when evolving into Politoed. In "The Flame Pokémon-athon", while Ponyta was evolving to Rapidash to win the race, its white silhouette while evolving was also in a rainbow-ish color.

Pokémon Trading Card Game
In the Pokémon Trading Card Game, some Pokémon can evolve if the player places the correct card on top of the original Pokémon. Evolution is usually a strategic necessity, as the evolved Pokémon are more powerful and may be able to take advantage of tools and circumstances that Basic Pokémon cannot.