Evolution

During the course of a Pokémon's development, under certain circumstances specific to that Pokémon's species, it may evolve (Japanese:  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 s, as well as, are preserved. It is similar to.

Professor Elm and Professor Rowan are the leading experts in Pokémon evolution. According to the latter's research, over 90% of all Pokémon are connected to at least one other through evolution. Rowan is currently investigating whether evolution is a form of maturity in Pokémon, and looking at the implications this process has on legendary Pokémon, which don't evolve.

Evolution families
An evolution family is a group of Pokémon who will all, if bred with or a Pokémon in the same Egg Group, 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 ultimately be able to follow only one evolutionary path.

Stages of evolution
Pokémon can be divided into different evolutionary stages, based on where they appear in their evolution family. All Pokémon fall into one of four groups: baby Pokémon, unevolved Pokémon, first-evolution Pokémon, and second-evolution Pokémon.

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, was regarded as an unevolved Pokémon, however, with the release of  in Generation II, many now consider it to be more on par with Pokémon like, though its TCG classification remains the same.

Two-evolution families
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 (excluding the starter in, as  did not yet exist and it could not be evolved into ), as well as all pseudo-legendary Pokémon. An example of this type of evolution family is below.

One-evolution families
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.

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 relatives are, of course, legendary Pokémon. Many Pokémon that cannot evolve are also novelty Pokémon such as Smeargle and Ditto for example. However, there are still 56 other Pokémon that do not evolve. Below is a list of all non-legendary Pokémon that do not evolve.

Not belonging to an evolutionary family is not indicative of strength, or a lack thereof. Some Pokémon, such as and, are comparable to fully evolved Pokémon while others, like  and , 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
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. 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.

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. There are 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.

Methods of evolution
The various triggers for a Pokémon's evolution are almost as varied as the Pokémon themselves, and some Pokémon have a unique evolution method. The most common of them is evolution by leveling up at or above a certain level. Other methods include the following:
 * leveling up when friendship has reached
 * leveling up while holding an item
 * leveling up while knowing a certain move
 * leveling up in a certain location
 * trading the Pokémon
 * trading the Pokémon while holding an item
 * using an evolutionary stone on it.

Additionally, holding an prevents a Pokémon from evolving.

Pokémon that can evolve into more than one Pokémon will usually 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 and, will also have very similar, if not identical, evolution methods.

Some Pokémon have different evolutions depending on their gender. Also, there have been situations in which the current party must be configured in a specific manner for some Pokémon to evolve. So far, only two Pokémon need to have these special requirements.