Constructive neutral evolution

Constructive neutral evolution (CNE) is a theory that seeks to explain how complex systems can evolve through neutral transitions and spread through a population by chance fixation via genetic drift. Constructive neutral evolution is a competitor for both adaptationist explanations for the emergence of complex traits and hypotheses positing that a complex trait emerged as a response to a deleterious development in an organism.

Source: Wikipedia — Constructive neutral evolution (CC BY-SA 4.0)