APA Dictionary of Psychology

a subsystem of the brain (or mind) that is a product of natural selection and is generally seen as having evolved as a result of its success in solving a problem related to survival or reproduction during the evolution of a species. For example, the elements of the brain’s visual system that enable organisms to perceive objects in three-dimensional space (despite the fact that vision involves the projection of light onto a two-dimensional surface, the retina) would be seen as an evolved mechanism that solved the problem of determining the distance between oneself and objects in the environment.