Mutation

Mutation is a change outside that of normal evolution; be it forced or effected by outside sources to bring a change which would not normally have occurred in the time frame or along a particular path.

Examples
The Daleks were created by Davros mutating the Kaled race by subjecting them (or their cells) to a mutagenic process forcing them to evolve with the added effect of radiation. (DW: The Daleks, Genesis of the Daleks)

The Invisibility was a result of a mutation caused by solar flares on their planet Refusis. (DW: The Ark)

The spiders of Metebelis III mutated, becoming highly intelligent with great psycho-kinetic / telepathic powers as a result of mutation from the Metebelis or 'blue' crystal. (DW: Planet of the Spiders)

The Primords came into existence during an attempt to penetrate the Earth's crust when humans came into physical contact with Stahlman's Ooze mutating them into the animalistic Primords which favoured the heat like that of the the location of the Stahlman's Ooze. (DW: Inferno)

Reasons for Mutation
Some reasons for mutation are similar to that of evolution; the adaption to a situation, area or time however it is a forced and 'unnatural' change, outside that of what of would have occured during 'normal' evolution.