What is irreducible complexity.

Irreducible complexity (IC) is the idea that certain biological systems cannot evolve by successive small modifications to pre-existing functional systems through natural selection.

Translated to regular english: Irreducible complexity is the concept that we will, as of being a prosperous species, never need to evolve the way the systems of our body, such as the respiratory system and digestive system, work. You're gonna be breathing the same way as your ancestors!