Evolution is the natural process in the Tamagotchi Life Cycle when a Tamagotchi progresses through its life, 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. On some occasions, it can evolve into a Senior stage. 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 Marutchi, it will evolve into Tamatchi. If the user neglects Marutchi, it will evolve into Kuchitamatchi.
Babies generally evolve after one hour, but on some vintage Tamagotchis, it can last up to 24 hours. Child characters evolve after a period of time between 24 and 72 hours, and teen characters evolve after a period of time between 24 and 96 hours.
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 by age 10.
Conditions for evolution, and what a specific Tamagotchi character can become, vary between every release. Evolutions are not reversible, 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 may 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 sound effect and generally a special animation. The evolution animation may be portrayed with a fade-to-black effect (vintage releases), a flickering effect (modern releases), or a glowing burst effect (color-screen releases). However, on the TamaOtch, a sound effect resembling that of a vintage Tamagotchi being reset is heard, and the character switches directly to its next form with no animation. It can also evolve in its sleep, but no sounds or animation will occur. Also, the character will leave a poop on the screen during evolution.
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.