If the forces of evil didn't constantly mutate and get better at what they do (fight, steal, sabotage, blackmail, or whatever), the forces of good would've destroyed them eons ago. Simply put, evil, its shape, forms, powers and so on are dynamic, constantly changing and subject to survival of the fittest. This is what The Social Darwinist is talking about, and it's definitely what the Evilutionary Biologist is going to use against you. If you're lucky, they'll evolve according to a predictable pattern and you'll be able to anticipate any changes in tactics and equipment you'll need.
Most bad guys are content to evolve by normal Darwinian means, but some guys like to eat things and absorb their DNA. Sometimes, there is an entire race of beings that can do this. And they all do. If you ever find a race like this, add them to the list of species that have gone extinct because of man by any means necessary. Trust me, no one will miss them.
While good guys in general view this as a cosmic cruelty, the hero with a Superpowered Evil Side will be thankful for this, as it will also make him subject to this evolution and therefore he will also constantly get stronger. However, that's not good news if your evil side is the type that tries to take over your mind.....
Sure, good will also change, but in a more 'upgrade' like fashion, giving the bad guys periods between upgrades, during which they have the edge.
- Naraku, the main villain of InuYasha, is constantly adding new demons to his body, which is the bodies of hundreds of demons thrown together in a blender and solidified.
- The Hollows of Bleach. By eating other hollows, they gain power, and evolve through a distinct series of forms. Ichigo's inner hollow has also gotten more powerful between the instances we see him.
- Sosuke Aizen tried to weaponize this by fusing the Hogyoku to his chest and wishing it to evolve him into, as he put it, a "being that transcends Shinigami and Hollow". Fortunately, the Hogyoku didn't do what he thought it would.
- While the Kyuubi isn't getting stronger, the seal holding him inside Naruto was getting weaker, and more and more tails popped out everytime his power was used.
- Frieza from Dragon Ball goes through three transformation sequences before reaching his ultimate form. Even then, he gets a power upgrade later on by becoming a cyborg, though this does surprisingly little good as he gets prompty curb stomped almost immediately afterward. On the absorption side of things, Cell and Buu (especially Buu) also qualify as this trope. Interestingly, in Buu's case, the last form fought was also its original one, making it an aversion, at least at the end.
- The Akuma of D.Gray-Man have multiple evolutionary levels. The main character kills a level 1 Akuma in the first chapter. In an early arc two exorcists have trouble facing one level 2 Akuma. The heroes are now facing level 4 Akuma (albeit with difficulty)
- In the Manga this is the explanation for the demons- that were creatures who absorbed other creatures gaining more and more abilities while becoming more and more monstrous.
- Ordinary ghouls from Tokyo Ghoul are dangerous enough that only the most badass humans wielding the bleeding edge technology weapons are able to stand against them. However, would one start to eat other ghouls bodies, they'd develop kakuja, an ability to form armor and enhance their already-existing biological weapons. It also makes them unable to control themselves until they'll eat enough ghoul meat to stabilize kakuja. Needless to say, besides one particular organization there are not that much people who like them among both humans and ghouls.
- The Brood from the Marvel Universe are giant wasp-like aliens. Their Queen lays eggs inside their victims, which keep their hosts alive until they hatch. When this happens, the host and the symbiote merge into a fully grown Brood with the host's special genetic abilities. The process is almost flawless, as the Queen was very surprised when Wolverine's Healing Factor kills the symbiote she implanted in him.
- As a meta-example, many villains evolve throughout the decades to better reflect their adversaries and the time. Lex Luthor has gone from foreign dictator to insane scientist to politician to business man to reflect the changing views of society and many of Batman's villains have ceased to be gimmicky tricksters and evolved into dark and insane psychopaths. In the Dark Knight The Joker himself is described as a terrorist, playing on the recent fear of terrorism striking the west over the the last decade.
- Some time after The Death of Superman it's revealed that Doomsday, the monster from that story, has this as a superpower: he revives and evolves immune to anything that killed him before.
- Discussed in Batman Begins, where Jim Gordon mentions some of the advances Gotham's criminals have made to keep ahead of the police... and wonders what they'll become to counter Batman. He gets his answer one movie later.
- In Jumanji: Welcome to the Jungle, the game morphs itself into a game cartridge when its latest potential victim ignores its board game form in favor of his gaming console.
- This happens in Pacific Rim, where Kaijus become bigger, more intelligent and more dangerous to counteract the Jaegers killing them.
- The Vord in the Codex Alera, which have an annoying habit of coming up with new forms to counter anything they're faced with.
- In The Dresden Files, older people and beings change less. Older wizards are more set in their ways, and the big supernatural baddies tend to be thoroughly old-school and anti change. As Harry explains to Butters when the ME finds "a knife wound, but bigger", the White Council is "old school. Really, really old school."
- In the few aversions (Lara would be a monster/being aversion and Luccio is initially a partial aversion, but that turns out to be a big clue), it's always significant. Harry notes that he's often beat the monsters because he can change and they can't, and without that advantage...
- Harry kills Duchess Arianna this way, as she couldn't adapt to the terms of the duel that limited her (otherwise very effective) tactics.
- Not keeping up with things like GPS, gas stations (as combustible locations), kevlar, and other relatively mundane subjects have given Harry a vital edge on several occasions.
- In the Star Wars: Revenge of the Sith novelisation, Yoda's thoughts during his duel with Palpatine are shown to be his realisation that this is why the Sith have won; they had spent the last thousand years (starting with the institution of the Rule of Two) waiting and adapting to the changing Republic, until the moment was right to subtly install themselves in control, whereas the Jedi had not changed (and had probably become even more rigid in their conservatism and orthodoxy) and were essentially only suited for fighting the last war. This is why Yoda fled the battle and decided to do something more unconventional with regards to Luke.
- The quote from Harry Potter made by Snape to introduce himself as a Defense against the Dark Arts professor, refers to the Dark Arts in general terms. His words are wise for as long as there are witches and wizards and the desire to do harm, there will always be just as much creativity into doing harm by experimenting with the laws of natures, exploring the limits of magic (which are only bound by the limits of mind) and inventing new spells as before. There is even an example of this in recent wizarding history. Before Voldemort came along, Grindelwald was considered the most dangerous wizard in existence (helped but not determined by the fact that he had the Elder Wand). In a way, the Dark Arts were like an Evil Power Vacuum, with a greater dark force replacing the one that fell and who took dark magic even further.
- The aliens in Robert A. Heinlein's The Puppet Masters are scary good at this. Since they can occupy and invade human minds, whatever adaptations humans make to combat them are quickly discovered, and the aliens immediately counter-adapt. It soon becomes clear that the more adaptable species is going to win, and the other is going to be extinguished.
- In the Dreamers books by David Eddings, the Vlagh is an insectoid Mother of a Thousand Young who is endlessly improving her designs, creating minions for specialized roles like infiltration and resolving weaknesses as she becomes aware of them. "Evil" being relative, since she's currently stuck in a wasteland and wants her children to enjoy the resources that those pesky humans have been hogging for themselves.
- Sylar from Heroes.
- The Borg from Star Trek.
- The Krillitane from Doctor Who absorb traits from species they conquer via DNA absorption.
- According to the Expanded Universe, the Shadows from Babylon 5 have shades of this.
- The Replicators of Stargate SG-1 do this several times through the series. When first encountered, they're just a Hive Mind of robotic bugs seeking out technology- dangerous, but not particularly ambitious. Later, they construct several human-form Replicators, which are made of nanites, capable of interfacing with human minds, much tougher to destroy, and much more forward-thinking. One of the human-forms, Fifth, begins developing human emotions as well, and develops a grudge against Carter and Earth and a desire to conquer the Milky Way. When the good guys attempt to evolve in turn- by developing a Wave Motion Gun that destroys Replicators and leaves everything else intact- the Replicators reprogram themselves with an immunity.
- Warhammer 40,000:
- Evolution is fair part of the hat of the Tyranids. They evolve very rapidly, but there are special creatures called norn queens in the hive fleets, which (aside from controlling their reproductive processes) analyses the DNA of organisms from a recently consumed planet for useful adaptations to incorporate into the next generation of gruesome gribblies in the fleets production chambers.
- One notable incident is a minor fleet being halted over a Tau world because the Tau were doing exactly the same thing: studying the organisms brought to attack, and changing their loadouts to whatever worked best against it.
- A good part of the Mechanicus believes that all knowledge was once held by mankind, and seeking to discover things on one's own instead of recreating it from ST Cs is heresy. The Dark Mechanicus are those who've fallen to Chaos and started experimenting, resulting in things like giant demonic centaur tanks with guns for heads.
- Quite possibly the best example is this: In 40K, there is an embodiment of hope, who represents every sentient being's desire for things to change. That embodiment is Tzeentch, the ever-changing Chaos god of mutants, sorcerers, traitors and backstabbers and one of the greatest evils in a galaxy that has no shortage of evil. Ironically, he may also be a good part of the reason why Status Quo Is God, up until recently - He is fueled by the process of change as much as the result of it and for all his intricate schemes, all of them are doomed to fail because should one of them suceed, he may well go in a Puff of Logic.
- The Phyrexians of Magic: The Gathering are constantly improving upon their mechanical fighters, and they recycle the flesh of fallen enemies to create new soldiers (called newts).
- You wouldn't even know that the first Phyrexian was a vanilla 0/3.
- New Phyrexia has moved past its original mono-black affinity and now has factions aligned with each of the colors. New Phyrexia now has a wider variety of abilities pertaining to each faction.
- The Bugs in Body Harvest invade Earth four times over a hundred-year-period, facing the time-traveling human super soldier Adam Drake five times. In each new time period their forces have mutated to become more developed and stronger than in previous eras despite their failure to harvest enough human DNA.
- Psaro from Dragon Quest IV uses the secret of evolution changing his body to be like Estark and gain more power.
- A trademark of the Resident Evil games is the constantly evolving enemies, probably best reflected by the boss of Resident Evil 2, the G-Mutant, aka Birkin. Every time you defeat him he just evolves into his next form, so you have to "kill" him about a half dozen times in just as many forms.
- Fittingly for a race inspired by the Tyranids, the Zerg in Starcraft are constantly enhancing their abilities and expanding their ranks via the DNA of other races.
- The Grigori in Star Ocean: The Last Hope, they advance evolution in order to create a singularity. One of their targets, the Cardianon, evolved from lizard beasts to a space-faring race in only 200 years, at the cost of being obligatorily Always Chaotic Evil because they never developed a full conscience.
- This trope is central to the mythology of the Overlord series. As Gnarl tends to point out, "Evil always finds a way."
- Warning Forever has an enemy ship that constantly grows according to how you destroyed it last time. Attacked from behind? It grows more armor on its back. Attacked from the side? Same thing. Hit it from a blind spot? It grows a missile launcher. And it never ends.
- In the Legion of Super-Heroes cartoon after his apparent death Brainiac takes on a new, seemingly partially organic form and proclaims:
Evil does not die. It evolves.
- Too bad for him that the cartoon was cancelled before he could properly show off his new "evolution."
- This principle is stated at the end of the first season of Jackie Chan Adventures, when Uncle explains that destroying one evil will allow a stronger one to come into the world. This may be why most evil beings are sealed away instead of simply being killed off.
- In addition, nearly every MacGuffin collected by the good guys is lost to the forces of evil, once they are sealed off for good.
- Wolverine and the X-Men: The Sentinels in the Bad Future have apparently come down with a case of jealousy toward their Mutant enemies, and have begun trying to copy their powers. The only examples seen are one Sentinel using telekinesis, and a whole line of Sentinels based on Wolverine, healing factor included.
- Crackers. Every time companies come out with new ways to prevent theft, the crackers just try again and get sneakier.
- All serious criminals are like this. Gangs, conmen, forgers, embezzelers, launderers and so on. As the police learn new ways to track, stop and prevent their activities they learn a dozen new tricks.