Evolution is the natural process in the Tamagotchi Life Cycle when a Tamagotchi progresses through one form to the next, through aging or other conditions.
All Tamagotchis naturally evolve over time as they age, starting from the egg to the baby, child, teen, and adult stages. The majority of Tamagotchi characters have a variety of different forms they can evolve into, but what they become depends predominantly on how they were cared for in their current stage. For example, on the original Tamagotchi, if the user provides good care to a Marutchi, it will evolve into Tamatchi. If the user neglects the Marutchi, it will evolve into Kuchitamatchi.
Other conditions may be involved in order to raise specific characters, including Skill Points and Bonding. Evolution can also be deliberately invoked by the user, provided specific conditions are met. Overfeeding, underfeeding, not responding to calls for attention, marriage, and specific items are all ways of trigger a Tamagotchi to evolve into another form. On some releases, it can evolve further into a Senior if it hasn't married after a while.
Conditions for evolution, and what a specific Tamagotchi character can become, vary between every release. There is no way to reverse an evolution, though some characters that evolve through severe neglect (such as Debutchi) can be reverted to their previous form. On the Genjintch Tamagotchi, if the user gives the Tamagotchi severe neglect, it may regress to its previous form.
A Tamagotchi close to its time to evolve will fall ill, typically within 8-24 hours of its evolution (about 20 minutes for the baby stage). The evolution itself is indicated with a loud, unique sound effect and a special animation. The evolution animation may be portrayed with a fade-to-black effect (vintage releases, though some may differ), a flickering effect (modern releases), or a glowing burst effect (color-screen releases).
Evolving into its next form may trigger other events in the Tamagotchi's life, such as getting extra money or being accepted into a new school.
Evolution in the Anime
Evolution also exists in the anime series, but is rarely seen.