Chao can have a gradual evolution in both in the child and adult stages of its life. In the adult stage, this is called second evolution. A chao's "first evolution", where it enters a cocoon, is decided only by its current gradual evolution; stats have absolutely zero effect on first evolution. This is why people sometimes have their chao evolve into an unexpected type.
Gradual evolution works by storing three values in the chao's data: an Alignment slider, a Run ↔ Power slider, and a Swim ↔ Fly slider. These three sliders each run from -1.0 to 1.0, with 0 representing neutral.
There is also an Evolution Strength or Magnitude slider, which runs from 0 to 1.2. This increases slowly over time as the chao ages, and it is what determines how visible the chao's current gradual evolution is. However, I won't talk about this much here. For more information, see my thread: [SADX & SA2B] Chao age and lifespan.
For a child chao:
- Each Swim animal subtracts 0.1 from the Swim ↔ Fly slider.
- Each Swim Chaos Drive subtracts 0.05 from the Swim ↔ Fly slider.
- Each Fly animal adds 0.1 to the Swim ↔ Fly slider.
- Each Fly Chaos Drive adds 0.05 to the Swim ↔ Fly slider.
- Each Run animal subtracts 0.1 from the Run ↔ Power slider.
- Each Run Chaos Drive subtracts 0.05 from the Run ↔ Power slider.
- Each Power animal adds 0.1 to the Run ↔ Power slider.
- Each Power Chaos Drive adds 0.05 to the Run ↔ Power slider.
- Each Blue, Gold, and Black animal randomly adds or subtracts 0.1 from one of the type sliders.
- The last bite of a Dark Fruit subtracts 0.15 from the Alignment slider.
- The last bite of a Hero Fruit adds 0.15 to the Alignment slider.
- Each Swim animal subtracts 0.05 from the Swim ↔ Fly slider.
- Each Swim Chaos Drive subtracts 0.025 from the Swim ↔ Fly slider.
- Each Fly animal adds 0.05 to the Swim ↔ Fly slider.
- Each Fly Chaos Drive adds 0.025 to the Swim ↔ Fly slider.
- Each Run animal subtracts 0.05 from the Run ↔ Power slider.
- Each Run Chaos Drive subtracts 0.025 from the Run ↔ Power slider.
- Each Power animal adds 0.05 to the Run ↔ Power slider.
- Each Power Chaos Drive adds 0.025 to the Run ↔ Power slider.
- Each Blue, Gold, and Black animal randomly adds or subtracts 0.05 from one of the type sliders.
When it comes time for the chao to enter a cocoon and have its first evolution, its new type is decided only by its current gradual evolution. The check works as follows:
- Alignment:
- If the chao's Alignment value is equal to or greater than 0.5, it will evolve into a Hero type.
- If the chao's Alignment value is equal to or less than -0.5, it will evolve into a Dark type.
- Swim type:
- If the chao's Swim ↔ Fly value is equal to or less than -0.5, and the chao's Run ↔ Power value is equal or closer to zero, it will evolve into a Swim type chao.
- Run and Power types:
- If the chao's Run ↔ Power value is equal to or less than -0.5, and the chao's Swim ↔ Fly value is closer to zero or equal on the positive side, it will evolve into a Run type chao.
- If the chao's Run ↔ Power value is equal to or greater than 0.5, and the chao's Swim ↔ Fly value is closer to zero or equal on the positive side, it will evolve into a Power type chao.
- Fly type:
- If the chao's Swim ↔ Fly value is equal to or greater than 0.5, and the chao's Run ↔ Power value is closer to zero, it will evolve into a Fly type chao.
- Normal and Chaos Chao types:
- If the chao's Swim ↔ Fly and Run ↔ Power values are both greater than -0.5 and less than 0.5, it will become either a Normal or Chaos Chao type.
- If the chao's number of reincarnations is equal to or greater than 2, its happiness is equal to or greater than 51, and it has been given at least one of every animal in its current life, then it will evolve into a Chaos Chao type chao. Otherwise, it will evolve into a Normal type chao.
- The Swim type takes precedence over all other types.
- Both the Run and Power types take precedence over the Fly, Chaos Chao, and Normal types.
- The Fly type takes precedence over the Chaos Chao and Normal types.
- The Chaos Chao type takes precedence over the Normal type.
Some other things worth noting from this are:
- Chaos Drives influence chao half as much as animals do.
- It takes five animals (or ten Chaos Drives) to make a fresh child chao set to evolve into a certain type.
- It takes ten animals (or twenty Chaos Drives) to make a fresh child chao fully lean towards a certain type.
- It takes at most fifteen animals (or thirty Chaos Drives) to make a child chao that you are unsure about set to evolve into a certain type.
- It takes at most twenty animals (or forty Chaos Drives) to make a child chao that you are unsure about fully lean towards a certain type.
- It takes four last bites (or complete fruit for simplicity) of a Hero Fruit or Dark Fruit to make a fully neutral chao set to evolve into a certain alignment.
- It takes seven last bites (or complete fruit for simplicity) of a Hero Fruit or Dark Fruit to make a fully neutral chao fully lean towards a certain alignment.
- To be as safe as possible when trying to get a Chaos Chao, avoid giving it any additional Blue, Gold, or Black animals (beyond the required ones), and give it an equal number of Swim, Fly, Run, and Power animals or Chaos Drives.