Mutations and their causes
Mutations result from organic cell DNA coding changing in some way, whether it be helpful, hindering or neutral. Their causes include a large range of conditions, like radiation, viruses, mutagenic chemicals or simple DNA replication errors. These mutants are spread throughout the wasteland and characterize one of the many effects of the Great War.
The various types of mutant creatures that inhabit the Wastelands were mostly created by radiation. This is where mantises, geckos, radscorpions, brahmin and other various mutant species originated from. Radiation is also how ghouls - decrepit, ragged, almost rotting, zombie-like victims of massive radiation poisoning - are created. Many ghouls were once Vault dwellers living in Vault 12 under the city of Bakersfield (better known as the Necropolis after the War). As part of the vast Vault Experiment Program, the Vault 12 door was designed not to close properly, thus those within the Vault were exposed to massive amounts of radiation, and the majority of them became ghouls.
Ionizing radiation causes mutations by disrupting and breaking molecular bonds in the body, resulting in the molecules no longer being able to perform its required tasks. Providing it does not kill the cell outright, the radiation changes the cells' genetic code, whereby it develops a mutation which spreads through the organism via cell replication and reproduction.
 Forced Evolutionary Virus
The other major source of mutations in the Fallout world is the Forced Evolutionary Virus (F.E.V.). The super mutants, floaters, centaurs and possibly wanamingos were products of FEV infection. Harold was also mutated by FEV, even though he looks like a ghoul.
It was initially called the Pan-Immunity Virion Project and was created to fight a disease called the New Plague. However, abnormal side effects were observed in test subjects. The test animals began to grow dramatically and their brain activity increased. Seeing the potential there, the United States Army sent its own people to secure the project, renamed it FEV: Forced Evolutionary Virus, and began performing experiments on humans within the Mariposa Military Base, which would turn soldiers into abnormally strong and intelligent supermen.
After the Great War, and the exodus of Roger Maxson and his troops to the Lost Hills Bunker, the base sat quiet until it was discovered by a group of explorers including Harold and Doctor Richard Grey, most of whom died when they encountered the automated defense systems. Soon, only Grey and Harold remained. Harold was knocked out (he never did find out how) and woke up out in the desert looking like a ghoul. Grey was knocked into a vat full of FEV by a crane and managed to crawl out. He began to mutate in horrible ways, turning into a gooey mess of skin, bone and organs. He developed psychic powers, which were enhanced by consuming living minds to expand his own brainpower. Slowly, he started doing his own experiments with FEV.
Grey found that many of the test subjects were metamorphosing into giant, mindless brutes with the brains of a child, if they survived at all. However, a rare few test subjects yielded super mutants. Immune to disease and radiation and blessed with exceptional strength, intelligence and endurance, super mutants were superior to humans in every way. They also possessed the exceptional lifespan of the mutated, but were total sterile. Grey—who now called himself the Master-- lamented the needless destruction of the war. He decided that he would have to force humanity to evolve. Those who could not evolve would die.
Creating super mutants was a very hit or miss process. The great majority of super mutants produced by the Master and later his Lieutenant in Mariposa's vats were big, dumb brutes. Physically, there were vastly superior to humans, but they had the intelligence of children. What exactly causes some mutants to be brilliant and others to be stupid is unknown. As a rule of thumb, the less damaged or altered by radiation the subject's DNA, the better.
Aside from super mutants, the Master also created a variety of other creatures. No one is sure where the floaters came from, but then again most would rather not know. Centaurs were created by tossing a varied mix of dogs, cats, brahmin and other animals into a vat and seeing what came out the other end.
 Military and scientific intervention
Although methods like radiation and FEV seem (for the most part) unregulated, there have been many instances were mutation was the goal in mind.
Wanamingos were a pre-War experiment conducted by the U.S. military to make killing machines to aid in the conflict with China. However, after the bombs fell, wanamingos escaped into the Wasteland. Deathclaws were of similar origin. Created by the United States military, this FEV dipped Jackson's chameleons were designed to replace humans in high risk, close combat and seek-and-destroy operations.
Frank Horrigan, the Enclave's most powerful enforcer, was produced by a series of treatments involving a carefully modified version of the FEV, surgery, a variety of other drugs and the addition of a suit of power armor. Lastly, there is Harold, who looks a lot like a ghoul, but actually is a different kind of mutant than the standard necrotic meta-humans. Ghouls are the result of massive radiation exposure, while Harold was mutated by brief FEV exposure without being dipped into one of the nutrient vats containing the virus. FEV's mutational results are at best unpredictable, depending on the subject's genetic quirks and amounts of exposure to the post-nuclear wasteland's ambient radioactivity, so one never can tell what the mutational result of an FEV infection will be (just look at the Master). Harold's existence was an oddity, the result of a unique combination of a specific degree of exposure to the virus, a specific degree of prior radiation exposure within his own body and any number of other factors. While FEV does not produce ghoul-like mutants like Harold as reliably as it does super mutants when humans are infected by it, it does occasionally spit out something that looks like one.
Another important exception is Talius, the ghoul-like creature that was hiding with the Followers of the Apocalypse and who was turned into his current state by FEV exposure, much like Harold, though under different circumstances.
In the Big MT research facilities, many mutations were deliberate, but without clear intention. Cazadores for one - large mutant wasps - were created by Dr. Borous. He was convinced they couldn't escape the research crater and were sterile, but this has been proven false by sightings of cazadors in large colonies in the areas surrounding the research facility. Also the handiwork of Dr. Borous, nightstalkers were the result of the genetic hybridization of coyotes and rattlesnakes. Borous admits nightstalkers were created after Dr. Calis dared him to splice mountain lion, eagle and condor genes together, But instead used the combination of coyotes and rattlesnakes.
 Other mutants
There are a few other types of mutant creatures in the Wastelands that don't directly stem from FEV or radiation. These are mostly the intelligent animals and one spore plant encountered in Fallout 2. The intelligent spore plant and radscorpion in Broken Hills were another, separate experiment conducted by a scientist there. Occasionally, other intelligent animals have appeared here and there in the Fallout games, but all have been flukes. A race of intelligent raccoons living in an colony known as the Burrows was originally intended to appear in Fallout, but was cut due to style issues.
There are human societies that have undergone gradual mutations over several generations due to either environmental conditions like exposure to the modified airborne strain of FEV that permeates the Wastelands of the Core Region, radiation exposure, or other reasons. Two examples are the Slags (mutated by genetic adaption to life underground) and the Beastlords (mutated by a unique form of radiation). Additional forms of these such mutants include the trogs of the Pitt, the swampfolk of Point Lookout, and the tunnelers of the Divide.
Over the years, different people have been in charge of Fallout canon and have come up with different interpretations on this subject. Mutant creatures and where they come from is one of the most controversial topics in Fallout canon and it's very easy to annoy veteran scholars of the setting by getting things wrong or even just partially right.
First, Chris Avellone seemed to prefer the "FEV explains everything" theory of mutation. He suggests that when the Glow was hit by the Chinese warheads, the tanks holding the virus burst and the FEV was vaporized and shot into the air. It was then mutated by the radiation from the blast, and went on to help radiation create the various mutant animals we know and love as well as ghouls. The problem with this theory is that first off it's a bit redundant since Fallout radiation is already fully capable of producing the mutations seen in the gameworld. Second, we know from the Lieutenant's mention of inoculation from mutant FEV that any airborne strain of FEV could not cause these mutations. If it did, why didn't any humans mutate from exposure to the airborne FEV? Furthermore, if radscorpions, geckos, molerats, deathclaws and other creepie-crawlies are all products of FEV, how can they breed as much as they do? Remember, FEV causes sterility. Avellone eventually admitted that he was wrong, and that most mutations in the Fallout world were caused by the ambient radiation.
Next, Chris Taylor offers a different point of view on how ghouls are made. He suggests that when people with too much radiation damage are dipped, you get ghouls. When people with mild radiation damage are dipped, you get stupid super mutants. When people with very little radiation damage are dipped, you get intelligent super mutants. This would mean Harold is a normal ghoul. However, since the population of Vault 12 was never dipped, not all ghouls could be made this way, only that FEV would speed up the process slightly.
 Enclave Mutation Policy
The Enclave holds the view that any "human" who has been "exposed" to the post-nuclear world's ambient environmental radiation is a mutant and that all mutants should be destroyed. The Enclave's definition of "exposure" would seem to mean that only Enclave members and those people still residing in Vaults count as "pure" human beings.
 Related holodisks
- Alpha Experiment Disk
- Delta Experiment Disk
- FEV Experiment Disk
- FEV Research
- Richard Grey Audio Diary
- Vree's autopsy report
- Captain Maxson's Diary
- The Project