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Mutation was a biological 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 same time frame or along a particular path of evolution. Something was said to be mutagenic if it caused mutation.
Some reasons for mutation were similar to that of evolution — the adaption to a situation, area, or time; however it was a forced and "unnatural" change, outside of what of would have occurred during "normal" evolution. Other mutations were caused by accident or by deliberate design of another individual or species.
Mutations could be caused by deliberate subjection to a mutagenic process, (TV: Genesis of the Daleks) by exposure to radiation, (TV: "The Ambush", The Ark, PROSE: The Sons of the Crab) by exposure to DNA or toxins from another species, (PROSE: The Weeping, TV: Cold Blood) or by other means.
According to one account, the Daleks were created by Davros mutating the Kaled race by subjecting them (or their cells) to a mutagenic process, forcing them to evolve from the effect of radiation. (TV: Genesis of the Daleks) According to another account, the surviving Dals and Thals of a neutronic war involving atomic weapons mutated in the war, the Dals being the "forebear" to the creature inside the Daleks' armour. (TV: "The Ambush") Many mutants also lived in the Lake of Mutations on Skaro. According to Ganatus, the creatures in the lake were bred and crossbred until the original species had "long been replaced". (TV: "The Expedition") According to a definition by the Movellans, Davros himself was a humanoid mutant. (TV: Destiny of the Daleks)
The spiders of Metebelis III mutated, becoming highly intelligent and having great psycho-kinetic / telepathic powers as a result of mutation from the Metebelis or "blue" crystal. (TV: 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. This mutated them into the animalistic Primords which favoured heat like that provided by the Stahlman's ooze. (TV: Inferno)
Although the physical changes of the Solonians were believed by the humans of the Earth Empire to be harmful mutations, they were actually part of the species' natural life cycle, where seasonal metamorphosis occurred. (TV: The Mutants)
Mawdryn and his followers stole a metamorphic symbiosis regenerator from the Time Lords and used it on themselves, only to find that their immortality was one of unending mutation and regeneration. (TV: Mawdryn Undead)
The Time Lords became what they were through billions of years of exposure to the Untempered Schism. Similarly, Melody Pond was mutated in utero by being conceived in the TARDIS as it was flying through the time vortex. (TV: A Good Man Goes to War)
Saibra, a woman who identified as "mutant human", had a condition which enabled her to change form by replicating living cells upon contact. She was given a gene suppressant to control her condition by the Twelfth Doctor. (TV: Time Heist) Other mutant humans included Xais, as well as Eddie and Charlie Nisbett. (AUDIO: The Romance of Crime)