Evolution
During the course of a Pokémon's development, under certain circumstances specific to that Pokémon's species, it may evolve (Japanese: 進化 shinka) into a different Pokémon. This change is not merely physical, however, as Pokémon of a higher evolutionary stage have different (and usually more powerful) base stats than their predecessors, may have different moves that can be learned, and sometimes change their types, though usually at least one of the types of the previous form is preserved. Other statistics, such as nature and EVs, as well as alternate coloration, are preserved.
Evolution families
An evolution family is a group of Pokémon who will all, if bred with Ditto, make a Pokémon egg that will hatch into the same Pokémon, excluding baby Pokémon. This also means that the most basic form has the potential to become any of the rest of the family, although it will only be able to follow one evolutionary path, ultimately.
Stages of evolution
Pokémon can be classed into different evolutionary stages, based on their appearance in their evolution family. All Pokémon fall into one of four groups: baby Pokémon, unevolved Pokémon, first-evolution Pokémon, and second-evolution Pokémon. These groups are also the basis for the TCG's grouping of Baby Pokémon, Basic Pokémon, Stage 1 Pokémon, and Stage 2 Pokémon, respectively.
Due to the fact that no evolution family contains both a baby Pokémon and a second-evolution Pokémon, many regard baby Pokémon as the most basic form, while moving their evolved counterparts one level higher. For example, originally, Pikachu was regarded as an unevolved Pokémon, however, with the release of Pichu in Generation II, many now considered it more on par with Pokémon like Charmeleon, though its TCG classification remained the same.
Two-evolution families
- Main article: Pokémon that are part of a three-stage evolutionary line
Perhaps the most well-known types of evolution families are those that feature two separate evolutionary events in the Pokémon's development. Indeed, this type of evolution family is what all of the starter Pokémon in the main series are a part of, including Pikachu. An example of this type of evolution family is below.
| Lowest | Trigger | Middle | Trigger | Highest |
|---|---|---|---|---|
 Dratini |
Level 30 |  Dragonair |
Level 55 |  Dragonite |
One-evolution families
- Main article: Pokémon that are part of a two-stage evolutionary line
By far the most common type of evolution family, these families are based in a Pokémon that will only ever evolve once in its development. About one third of all Pokémon that would later get a baby form were part of this kind of evolution family before their baby form was revealed. An example of this type of evolution family is below.
| Lowest | Trigger | Highest |
|---|---|---|
 Rattata |
Level 20 |  Raticate |
Pokémon that do not evolve
- Main article: List of Pokémon that do not evolve
The least common type of evolution family, of course, is that in which no evolutionary event takes place, meaning that it is made up of only one member. Many of the Pokémon that have no evolutionary event are, of course, legendary Pokémon. However, there are still 42 other Pokémon that do not evolve. Below is a list of all non-legendary Pokémon that do not evolve.
It must be noted that not belonging to an evolutionary family is not an indication of strength overall. Some Pokémon, like Pinsir and Skarmory are comparable to fully evolved Pokémon while others, like Luvdisc and Pachirisu, are more comparable to unevolved Pokémon. Often this indicates a Pokémon's possibility to be eligible for future new evolutions or pre-evolutions.
Branch evolution families
- Main article: List of Pokémon with branched evolutions
Several families, while also one- and two-evolution families, are also branch evolution families. What this means is that there is a split in the evolutionary line at some point so that even though two Pokémon of the same species evolve the same amount of times, they can become one of two or more entirely different creatures. Eevee is the best-known example of this, evolving seven different ways depending on the method used. An example of this type of evolution family is below.
| Lowest | Trigger | Middle | Trigger | Highest |
|---|---|---|---|---|
 Poliwag |
Level 25 |  Poliwhirl |
Water Stone |  Poliwrath |
| Trade holding King's Rock |
 Politoed |
Advantages
A major difference between the final forms of an evolution family with a branch in evolution is in the way that their base stats line up. For example, Kirlia evolves into both Gardevoir and Gallade, which both have 518 total base stats. However, Gallade's base stat in Attack is 125 and its base stat in Special Attack is 65. The reverse is true for Gardevoir, whose Special Attack is 125 and whose Attack is 65. This is true of many opposing evolutions, with one focusing in one specific stat, the other focusing in a separate stat, and both having the same total stats. This is especially obvious in the Eeveelutions, who each have exactly the same base stats, though organized differently.
A listing of the stat focuses is below.
| Basic form | Evolutions | Types | Difference | |||
|---|---|---|---|---|---|---|
|
Oddish | 
|
Vileplume | Grass | Poison | Special Attack is 100, Special Defense is 90 |
|
Bellossom | Grass | Special Defense is 100, Special Attack is 90 | |||
|
Poliwag | 
|
Poliwrath | Water | Fighting | Attack is 20 higher, Defense is 10 higher |
|
Politoed | Water | Special Defense is 20 higher, Special Attack is 10 higher | |||
|
Slowpoke | 
|
Slowbro | Water | Psychic | Defense is 110, Special Defense is 80 |
|
Slowking | Water | Psychic | Special Defense is 110, Defense is 80 | ||
|
Eevee | 
|
Vaporeon | Water | Highest stat is HP | |
|
Jolteon | Electric | Highest stat is Speed | |||
|
Flareon | Fire | Highest stat is Attack | |||
|
Espeon | Psychic | Highest stat is Special Attack | |||
|
Umbreon | Dark | Highest stat is Special Defense | |||
|
Leafeon | Grass | Highest stat is Defense | |||
|
Glaceon | Ice | Highest stat is Special Attack | |||
|
Tyrogue | 
|
Hitmonlee | Fighting | Large difference between Attack and Defense | |
|
Hitmonchan | Fighting | Speed lower than Defense, Attack and Defense more equal | |||
|
Hitmontop | Fighting | Attack and Defense equal, Speed at minimum | |||
|
Wurmple | 
|
Beautifly | Bug | Flying | Attack and Special Attack higher than Defense and Special Defense |
|
Dustox | Bug | Poison | Defense and Special Defense higher than Attack and Special Attack | ||
|
Ralts | 
|
Gardevoir | Psychic | Special Attack is 125, Attack is 65 | |
|
Gallade | Psychic | Fighting | Attack is 125, Special Attack is 65 | ||
|
Snorunt | 
|
Glalie | Ice | All stats are 80 | |
|
Froslass | Ice | Ghost | HP, Defense, Special Defense each 10 lower, Speed 30 higher | ||
|
Clamperl | 
|
Huntail | Water | Attack is 104, Special Attack is 94 | |
|
Gorebyss | Water | Attack is 84, Special Attack is 114 | |||
|
Burmy | 
|
Wormadam | Bug | Grass | Special Attack and Special Defense higher by 10 |
|
Wormadam | Bug | Ground | Attack and Defense higher by 10 | ||
|
Wormadam | Bug | Steel | Equal Special & Normal stats | ||
|
Mothim | Bug | Flying | Lower Defenses but higher HP, Attacks, and Speed | ||
Methods of evolution
- Main article: Methods of evolution
The various triggers for a Pokémon's evolution are almost as varied as the Pokémon themselves. The most common of them is evolution by leveling up at or above a certain level. Other methods include leveling up when happiness has reached a high level, trading the Pokémon, trading the Pokémon holding an item, leveling up holding an item, or even using an evolutionary stone on it. Additionally, holding an Everstone prevents a Pokémon from evolving.
Most commonly, Pokémon that can evolve into more than one Pokémon will have the ways in which the evolution is activated being slightly similar, such as having both be by evolutionary stone or by holding an item and trading. Closely-related Pokémon, such as Nidoran♀ and Nidoran♂, will also have very similar, if not identical, evolution methods.
In the anime
In the anime, evolution happens in much the same way as it does in the games; though level-based evolution nor 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. When a Beedrill attacked Ash's Metapod, it caused a crack to appear on its shell, which Butterfree came out of it. The other aspect about the anime is the evolving is very different. For example, when Metapod evolve they leave their shells behind. However, starting from the Advanced Generation series it's changed (eg. Cascoon just turn into Dustox not leaving anything behind).
Additionally, a difference can be noted in the fact that Pokémon evolve during a battle, as opposed to after it. There are also several instances of an evolutionary trigger being incorrect, such as March of the Exeggutor Squad where several Exeggcute evolve into Exeggutor without the aid of a Leaf Stone. Pokémon may also evolve when they are needed to, for an extra boost of power, instead of after a set amount of training, such as when Ash's Charmeleon evolved into Charizard. In addition, Pokémon can sometimes choose not to evolve, even if they evolve by a 'natural' method such as leveling up. It appears that evolution has emotional implications for Pokémon - some Pokémon, such as Team Rocket's TRMeowth ([[{{{2}}} (move)|{{{2}}}]]), dislike their evolved forms, while others such as Ash's Pikachu simply want to prove they can be powerful without evolving.
For a list of all evolutions that Pokémon belonging to the main cast have undergone, see List of anime Pokémon by evolution.
In the TCG
- Main article: Evolution (TCG)
Evolution in the TCG functions similarly in many aspects to that of the games, however, there is no different requirement that needs to be met depending on the Pokémon species to be evolved to move on to the next stage.
There are four different stages of evolution in the TCG, Baby Pokémon, Basic Pokémon, Stage 1 Pokémon, and Stage 2 Pokémon. Of these, only Baby and Basic Pokémon may be placed onto the Bench during the setup phase and during play; Stage 1 and Stage 2 Pokémon are considered to be evolution cards and therefore unable to be played except on top of their corresponding pre-evolved forms. The stage of evolution is indicated in a conspicuous place on each and every Pokémon card, though the placement differs among the four generations of cards.
Within the deck and discard pile, only Stage 1 and Stage 2 cards are considered to be "evolution cards" for the purpose of a Trainer card or Pokémon Power which allows them to be searched for. In play, a Basic Pokémon card can be considered an evolution card if it is evolved from its Baby stage.
In relation to the real world
Evolution in Pokémon, for most species, is more akin to metamorphosis than to actual evolution. The Pokémon whose evolution is closest to the real-world definition of evolution is Eevee, whose cellular structure changes due to its environment.