Evil Evolves

Kill 'em all and they come back for more.

"The Dark Arts are many, varied, ever-changing and eternal. Fighting them is like fighting a many-headed monster, which, each time a neck is severed, sprouts a head even fiercer and cleverer than before. You are fighting that which is unfixed, mutating, indestructible."

—Professor Severus Snape, Harry Potter

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 Super-Powered 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.

Compare Adaptive Ability, see also As Long as There Is Evil.

Examples of Evil Evolves include:

Anime and Manga

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.
- While not technically evil, Inuyasha's sword is also capable of absorbing powers from the demons he slays with it.
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.
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 Z 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. Note that Buu had to technically DE-evolve to reach his 'ultimate' form.
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.

Comic Books

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 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.

Literature

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 refers to the Dark Arts in general terms. Before Voldemort came along, Grindelwald was considered the most dangerous wizard in existence (being that he had the Elder Wand). In a way, the Dark Arts are like an Evil Power Vacuum, with a greater dark force replacing the one that fell. Of course, based on that logic, there's a chance a more terrible wizard than Voldemort will rise.

Live-Action TV

Sylar from Heroes.
The Borg from Star Trek.
The Krillatane from Doctor Who absorb traits from species they conquer via DNA absorption.
According to the EU, the Shadows from Babylon 5 have shades of this.

Tabletop Games

Evolution is fair part of the hat of the Tyranids of Warhammer 40,000. 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.
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.

Video Games

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.
Warning Forever has enemy ship that constantly grows according to how you destroyed it last time. Attacked from behind? It grows more armor on back. Attacked from side? Same thing. Hit it from the blind spot? It grows missile launcher. And it never ends.

Western Animation

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.

Real Life

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.
In general, all law enforcement today. Internet and technology march so fast ahead that legislation has hard time keeping up, therefore applying old, outdated laws.