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 King Bowser, Daisy's father and the previous rule of Dinohattan.
De-evolution is usually a form of corporal punishment used by President Koopa in creation Goombas for use as a dumb, but strong and loyal police force. These goombas are set to a stage in physical evolution that is a bridge between their dinosaur-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 depending on particular ancestry.
This process was originally achieved through use a special, restraining machine, but Koopa later developed a portable gun for use as a weapon in his upcoming attack on the parallel dimension his world had split off from. Use on Humans results in small, monkey-like creatures resembling chimpanzees.
The process can also work in the opposite "direction", resulting in improved intelligence, though mental capabilities can only be extended to a point. Actual intelligence cannot move higher than a person is biologically-capable.