How does nincada evelolve in HeartGold?

Nincada doesn't evolve in HeartGold (or SoulSilver) in the traditional sense. It evolves through a special method:

1. Level up Nincada to level 20 during the day.

2. During the night, Nincada will evolve into Ninjask and Shedinja.

Important Notes:

* Only one Shedinja can be obtained per game.

* Shedinja's stats are determined by Nincada's stats at the time of evolution.

* Shedinja will inherit the same held item as Nincada.

* You need an empty space in your party to obtain Shedinja.

So, while Nincada technically "evolves" into Ninjask, the Shedinja is a unique side effect of this evolution.