A creature exhibits power to change shape, unpredictably and often uncontrollably. When shapeshifting isn't exotic enough to explain this, Hollywood Evolution or Magic Genetics might do just fine. Suggest that the creature has an Unstable Genetic Code, and it can "evolve" without regard to Evolutionary Levels.
(Note that in these stories, a change on the genetic level always means an [almost] immediate change on the physical level. The writer's understanding apparently is that the genes tell the body what shape and it form it has to be, rather than merely providing a set of patterns for a lifetime of development which also require a variety of external conditions at each stage to work.)
Fridge Logic may come into play about the specific wording used, which would imply random and uncontrolled shapeshifting. As this is not usually the case, something like "malleable genetic code" might be more appropriate.
Another instance of fridge logic is that we never see this unstable and constantly shifting genetic code accidentally erase the coding that allows it to change in the first place (yes, some of your DNA tells the rest of your DNA how to work).
- In Bio-Meat: Nectar, the USBM's reproductive and ambulatory abilities (which it was never designed to have) are all explained with this trope.
- In Dragon Ball Super, this is what leads to Zamasu's defeat, due to fusing with someone who was mortal, while himself was immortal, his body becomes very unstable. Combined with his now imperfect immortality, the results aren't pretty.
- Vee the Eevee of Pokémon Adventures was subjected to experiments by the resident bad guy organization, Team Rocket, and as such can now freely evolve and devolve between any of Eevee's first-generation evolved forms — Vaporeon, Jolteon, and Flareon — at will. However, it caused him pain every time he used the ability, and he was much happier when he lost it by permanently evolving into an Espeon.
- The Deviants, and therefore, the Skrulls, in the Marvel Universe were originally this.
- X-Men: This was the explanation used for Toad's initial pathetic Quasimodo-like appearance and "abilities". When it was corrected, he suddenly Took a Level in Badass and turned into a sleek, cocky and dangerous acrobat molded in the likeness of Ray Park.
- In Safe Havens, Samantha has messed with her DNA so many times it's now unstable, causing her to randomly change at inopportune times.
- In Resident Evil (2002), the protagonists see a horrifying monster eating a dead body:
Red Queen: One of the Hive's early experiments, produced by injecting the T-virus directly into living tissue. The results were unstable. Now that it has fed on fresh DNA, it will mutate — becoming a stronger, faster hunter.
- In X-Men: Days of Future Past, this seems to be the case with Mystique, as her blood cells constantly change when under a microscope. Trask says that her DNA might be the "key to mutation itself."
- In Doom: Endgame, this describes the Newbies in a nutshell. All sentient life aside from humans and Newbies take thousands of years to evolve, languishing with Medieval Stasis. Humans develop blindingly fast, able to progress from sailing ships to inter-system star ships in a mere five centuries. The Newbies managed to advance from hunter-gatherers to a force able to destroy the Fredworld in half that time. They evolve so fast that the dead one found on Fredworld fears he's now a different species after a forty year separation from his kind. When Fly finally tries to kill him, the Newbie starts evolving new organs to reroute around the damage caused by the bullets. When Fly and Arlene catch up to them on Skinwalker, the Newbies have become microscopic and have infected a human fleet. This becomes the Newbie's downfall, they drag the soul of a Newbie into the computer simulation and the processing speed accelerates the evolution until it disappears from the dimension.
- Doctor Who:
- "Planet of Evil" has a scientist get unstable DNA by being infected with anti-matter. Which is even sillier.
- "The Lazarus Experiment": A scientist alters his own DNA to become young again but it becomes unstable, causing him to start changing from a human into a monster and back again. Lampshaded by medical student Martha.
- The Outer Limits (1995): In the episode "Double Helix", a high-school teacher activated the introns in his DNA. This resulted in a map growing on his back, which he is intended to follow.
- In The Machine: Bride of Pin*Bot, this is what presumably causes The Bride to revert to her robot form.
- The Hitchhiker's Guide to the Galaxy radio show described the Haggunenons this way. (They didn't appear in the books, which replaced the Haggunenon subplot with one involving Disaster Area.) To elaborate, it is stated that when reaching for sugar for their coffee, they may spontaneously evolve into something with far longer arms, but which is incapable of drinking coffee.
- Dungeons & Dragons 3E has both Chaos Beasts from the "Monster Manual", and the Hagunemnons (yes, a HG2G reference) from the "Epic Level Handbook". Both of them have no real form, but constantly morph between countless shapes.
- In Warhammer 40,000, various planets - especially Death Worlds - tend to have monstrous creatures as one-offs and adaptive mutations. Subverted at times when it's actually Chaos magic mutating them; played straight at other times, ESPECIALLY with the Tyranids.
- In Warhammer Fantasy Roleplay, Chaos Marauders are blessed by Tzeentch, The Lord of Change, with the power to transmutate their arms into large weapons. Their effects and purpose can vary depending on the shape which are Spike, Blade, Claw, and Club.
- "Hazmat" in Marvel Nemesis: Rise of the Imperfects. He has unstable DNA (a "living mutation") due to injections of untested immunization vaccines.
- After being fatally shot, William Birkin in Resident Evil 2 injects himself with a syringe that contains the G-virus, which at first causes his wound to heal almost instantly. But it doesn't stop there.
- In Resident Evil 6, this is a seemingly unintended side-effect of the C-virus. While it can regenerate tissue, considerable regeneration of tissue in the same area can cause genes to break-down and result in spontaneous mutation. Usually in some manner to counteract the threat causing the damage; the instability is that it's impossible to predict how a J'avo will mutate. Even the Complete Mutation process, which transforms a J'avo into an entirely new species of B.O.W, is a random pick procedure.
- The point of the Ultimate Being in the first Parasite Eve game. It would be able to take over the world as it could change its DNA at will to overcome any resistance.
- Eevee is said to possess this in its Pokedex entry. It has eight different evolved forms, each of different types... and counting (it started with only three)!
- Ditto's DNA is also said to be unstable, but regularly malleable. This is apparently what lets it shape itself into any other Pokemon, and breed with any Pokemon capable of it.
- The Zerg of Starcraft have this trait, along with a hyperactive metabolism that can rapidly replace most of a particular creature's body. Background details in Starcraft II explain that a Zerg organism is essentially composed entirely of highly mutable stem cells of two types: "Type A" that experience rapid mutations, and "Type B" that hunt them down and destroy them. The surviving Type A cells go on to create the next generation of Type B, leading to evolution at a cellular level that quickly spreads through the entire organism. Combined with their psychic Hive Mind, this leads to the rapid "mutations" that serve zerg forces as in-game upgrades and unit production. Then there's the Primal Zerg introduced in Heart of the Swarm, who are the Zerg who never left their homeworld of Zerus and are not part of the Hive Mind; their cells are just as mutable, but they mutate by fighting, killing, and devouring each other, with the winners absorbing desirable traits from their prey.
- At one point, you ingest a chemical mixture that causes you to randomly cycle through all the plasmids in the game, including plasmids you don't have. This is justified, since it's the chemical that's meant to undo the biological conditioning and genetic manipulation Frank Fontaine forced upon your character when he was a child. Ingesting the chemical is the only way to be free of Fontaine's control. You need to drink the chemical again to end the random cycling.
- More generally, an unstable genetic code is one of the long-term side effects of splicing with ADAM, which is the reason it's so physically addictive (and why most of its users eventually go insane).
- Evolva: The Genohunters are able to change their DNA (and thus their shape) in mere seconds.
- Mass Effect: The Vorcha possess clusters of non-differentiated cells, which allow them to easily adapt to environments, along with allowing them to heal faster. Due to their short lifespans, however, they usually only have enough time to adapt to a single environment.
- Danielle "Dani" Phantom, a female clone of Danny Phantom, has unstable DNA that causes her to melt into ectoplasm whenever she turns into a ghost and uses her powers. But she got better.
- Beast Boy from Teen Titans (2003) is stated to have this. Apparently, this came from him getting sick as a kid in Africa and the (experimental) cure for which threw his genetic code out of whack, turning him green and granting him the ability to turn into animals.
- Agent 57 from Danger Mouse explains how he can change shape in "The Spy Who Stayed In With A Cold." He ran into a molecular fragmenter with his tea trolley which left his molecular structure unstable allowing him to change into different things, but when he gets a bad cold it upsets his electrolyte balance, thus causing him to change into things he did not intend.
- One Monster of the Week in Godzilla: The Series exhibits a variation on this theme, being able to change shape due to being a blob made of "raw" or "blank" DNA that can rewrite itself with bits of the DNA of things it's touched.
- Street Sharks: Dr. Paradigm was injected with "unstable DNA" early on, resulting in his periodic transformations into a humanoid piranha.
- While not quite as extreme as some of the examples above, in-generation changes in an organism's genetic code through horizontal gene transfer is commonplace among bacteria and archaea, and not unheard of in eukaryotes.
- Plants sometimes express mutations in one portion of their anatomy, such as a single root or branch, that make that part visibly different from its parent stalk. Such mutations can actually prove valuable, as with the 'golden delicious' strain of apple, which was derived from a single mutant branch of another strain of apple tree.
- The genes of currently living organisms have been shown to be less stable than science used to think. Epigenetics studies genetic changes caused by environmental factors, which can be passed on to the next generation like 'regular' genetic traits. There are also many substances that can directly alter genes, usually by damaging them.
- Randomly changing a gene is far more likely to result in an adverse effect than a good one. It's like randomly changing a word in a story. You might get lucky and actually improve the story, but it's much more likely that the sentence it's in will no longer make potato.
- In a normal process of human development, progenitors of one of subpopulations of white blood cells, specifically one that produces antibodies, undergo heavy mutagenesis in the part of genes encoding antigen-binding part of antibodies.
- Actually, each DNA duplication results in a significant amount of mutations, though for humans the rate per nucleotide (a 'letter' of the code) is much smaller than for bacteria... But since humans have so much DNA, the actual rate of mutation puts bacteria to shame.
- Technically speaking, it's what sexual reproduction is about. The point is to mix-and-match genes from the parents in hope that result would be something better. It works (usually), and it's why animals more complex than amoebas reproduce this way, not asexually.
- There actually are quite a variety of substances or conditions that can cause (relatively) rapid changes in the shape and growth of human beings, often resulting in distorted or even dangerous transformations. They just happen to take effect inside the body at the cellular level, and the deformed mutants that result are often better known as "cancer".