From the Super Mario Wiki, the Mario encyclopedia
President Koopa wielding a portable Devolution Gun.

De-evolution is a term used to describe the process of reverting a being to a lower stage in their evolution in the Super Mario Bros. film. De-evolution may affect level of intelligence as well, though not all de-evolution results in a loss of intelligence. The first known victim of this process was the King of Dinohattan.

De-evolution is usually a form of corporal punishment used by President Koopa in the creation of Goombas for use as a dumb, but strong and loyal police force. These Goombas are created by resetting them to a stage in their racial evolution that is somewhere between that of their saurian-ancestors and their current, humanoid form. This results in a tall and bulky creature with a small, underdeveloped reptilian head of either frog-like, raptor-like, or snake-like appearance, which depends on particular ancestry. Use of de-evolution on humans results in small, monkey-like creatures that resemble chimpanzees. De-evolution can be taken even further on both species, reducing them to primordial ooze (as was Koopa's ultimate fate in the film as well as the fate subjected to a technician due to sneezing in front of Koopa's presence) or fungus (as was how Koopa managed to overthrow the King).

This process was originally achieved through use of a special room-sized machine, but Koopa later developed the technology into portable-form to use as weapon in his upcoming attack on the dimension that his own had split off from.

The process can also work in the opposite "direction", which results in improved cognitive-thinking and comprehension. However, mental capabilities can only be extended to a certain point. Actual intelligence cannot move higher than a person is neurologically-capable.