Devolution

Devolution was a process of development and change in organisms which occurred over long periods of time. As opposed to evolution, where the natural changes are thought to be positive, devolution in species was a kind of regression.

Bernice Summerfield discovered that the potfish spiders on Jaiwan were devolved from the planet's original inhabitants, and had lost their sentience. (PROSE: Genius Loci)

Inhabitants of the planet Klist were once taught how to reverse evolution by the Time Lords, with unfortunate results. (PROSE: The Quantum Archangel)