Once upon a time, species #222 and species #333 moved to a new planet together.
This planet is horrible for us! There isn't enough air above ground for us to live on. There is NO other creatures existing above ground or in the sky. What are we supposed to eat and how are we supposed to survive in this nature?
This planet is literally designed for us! We are thriving and having so much fun under the water and we all feel super comfortable!
Gradually, because of the lack of food and the difficulties with their situations, species #333 were dying out.
On the other hand, species #222 are reproducing and getting stronger in this planet!
Year after year, due to directional selection, nature selected the trait of being able to swim and breathe under water to survive, which made species #222 flourish on the planet.