Pokémon can undergo “Evolution”, a process in which they suffer a sudden, systemic and long-term transformation into a new shape often called evolutive Morph. This process and the shapes they can progress into are determined by species and largely codified in their genome. This is distinct to the capacity some Pokémon have of temporarily assuming a different form via a temporary, reversible transformation, more commonly called form or forme.

Evolution is one of the two paths a Pokémon can choose to release the tremendous amount of typeless energy they have stored for a long period of time during their lives, the other form being by executing an Overdrive attack. Of course, unlike an Overdrive attack, Evolution leads to a permanent change.

Pokémon are, for the most part, in control of when and how do they want to evolve. When they are not in control of this, it can be related to a heavy stress trigger (such as needing the energy release to heal a serious injury). Pokémon naturally choose to evolve at certain points when they reach a certain age (in general, their age of reproductive maturity).

Requisites for evolution are mostly uniform across species and involve having reached the previous evolutive stage (so no eg.: Digimon’s Warp Evolution) and having stored the required energy intake in the form of Typeless Energy.

The evolution process is codified into the genome and structural data of Pokémon as a whole, species-wide, and thus the effect Evolution can have is similar for most if not all of them. Because the genetic instruction is to modify or rebuild the body into a specific form, the process is capable of healing severe wounds and clearing non-genetic identifying marks (such as fur dye) if carried out correctly.

Evolution can change all physiological and psychological aspects of a Pokémon specimen. In drastic cases, Evolution could wipe out memories or skills learned by the individual; the psychological changes can result in the emergence of whole second personalities or consciousnesses (eg.: Dodrio), or the loss of separate / multiple identities (eg.: Beldum).

Evolutionary items, such as the Type Stones, basically serve the same purpose as Gems - they give a Pokémon a strong surge of typed energy and trigger the natural mechanism to filter this typed energy into Typeless Energy. Without a Type Stone, most Pokémon that evolve by stone would need a very long time or fulfilling very specific requisites to reach the next evolutive stage, sometimes even making it unfeasible to do so within their lifespan in the wild.

Most species regard Evolution as a sacred event -a remainder of the old eras and a means to become closer to their original beings, as Pokémon see their ability to unleash nature-modifying abilities as the display of their perfected stage.

Even in the wild, if a predator sees that their attack has triggered prey's Evolution, they will wait for it to finish or in certain cases just forgo the prey's life, under the understanding that a Pokémon who has evolved has earned the (semi-)divine right to experience the abilities their new body is capable of. In some regions such as La Plancha and Omixence, Pokémon guardians will actively hunt down those of their own kind who have used force against an evolving or recently evolved Pokémon.

There are outliers to this view of Evolution, of course, with most “Starter Pokémon” being included.

Scyther see evolution into Scizor as an aberration in the wild as they lose their speed, stealthiness, natural camouflage and elegance. Pokémon like Nidoran, Feebas and Larvitar see their evolution to the final stage as a risk, for their heightened power poses risk to their families and their unevolved peers, and they are usually forced to move to a different habitat upon evolution. Most “Starter” Pokémon seeing evolution into their final stages as unnecessary or thankless because they stop being small and cuddleable and in exchange they become less fit to perform activities with their humans other than competitive battling.

Certain Pokémon with the power and knowledge of how Evolution works can dispense the inverse process: De-volution, where a Pokémon’s power is released to the environment and their shape returned to the next earlier morph.

Because only the upwards reconstruction paths are actually encoded within a Pokémon's biological information, whereas the de-evolution path is not, the process is unnatural and extremely dangerous to undergo, and needs to be administered externally by a being of tremendous power and utmost control of it. Given those requirements, most Pokémon, including Legendaries, see this process as cheating or flat-out “evil”.

Unlike with normal evolution, undergoing de-evolution is tremendously likely (almost assured) to kill an injured or debilitated Pokémon.