Genetic Adaptation

This is a counterpart to the Fisher Kingdom, except it is the outsiders that willingly impose the transformations upon themselves, rather than the environment.

For example, humans are attempting to colonize an alien planet, but unfortunately, it's quite unpleasant to live on: to actually survive on the planet, you'd either have to wear space suits and build sealed cities to shield you from its atmosphere and hazards, or go native. But if you have the technology, you can do exactly the latter: Change and adapt your species to enable you to survive on the planet and become one of its native inhabitants.

There are two ways of doing this: The short-term version affects specific individual persons only, while the long-term solution affects the entire colony, becoming a part of the planet's evolutionary chain, possibly becoming a dominant species with the original humans remembered only as ancient Precursors. Over time, the transformed may also become mentally alien as well as they adapt to the planet's nature, or the mental changes are immediate and directly connected to the process of transformation.

Expect lots of Hollywood Evolution and Lego Genetics.

If the transformed species ever meets its original version, expect to find An Aesop about What Measure Is a Non-Human? (if from the humans' point of view) and/or Humans Are the Real Monsters (if from the natives' point of view).

Not to be confused with Empathic Shapeshifter.

Examples of Genetic Adaptation include:

Anime and Manga

A Doraemon Tool allows an individual to adapt to any environmental condition, no matter how extreme (high temperature, high gravity, non-breathable atmosphere, etc). The drawback is that it must be reapplied to each individual every 24-hours. Giant and Tsuneo was nearly crushed by the oceanic pressure when they wandered away from the underwater summer camp without Doraemon's permission.

Comic Books

Marvel's 1970's Guardian of the Galaxy series. Lead by Vance Astro, the secondary members of the team were former humans bio-engineered to survive the harsh conditions of the other planets of the Solar System.

Film

This is a major part of Avatar‍'‍s plot, though humans don't change their bodies, instead remotely controlling eponymous avatars.

Literature

In Clifford Simak's City series of novels, the only way to survive on Jupiter is to become a jupiterian. When such a technology is discovered, people find out that jupiterians are absolutely superior to humans in every single way: they are immortal, superintelligent, telepathic, et cetera. Thus everybody just move son Jupiter, and humanity as a species ceases to be.
Way Station, another Clifford Simak novel, briefly mentions a race of aliens who colonized numerous planets regardless of the conditions by physically and mentally changing themselves every time to perfectly fit them. The main character wonders whether can they be counted as the same race.
In one Sergey Lukyanenko short story the protagonist, who possesses the ability to automatically adapt to anything pefrectly, is hired to deal with a dangerous predator on a distant planet. Gradually adapting to the Death World in question he understands that the dangerous predator in question could not possibly hunt humans for fun as described, as he actually became exactly like him, physically and mentally. When the two finally meet, they start playing as cats do... which gives the colonists a perfect opportunity to kill both with a single rocket, since both aren't technically human and they don't have to pay him this way. He survives, and briefly mentions before leaving that they are the ones who are not human.
In Hyperion cantos, we eventually learn that this is how the Ousters came to be - pioneers of interstellar travel, they had to radically alter their biology to survive. When rest of the humanity finally caught up with Hawking drives and means for terraforming, Ousters became persecuted.
In Surface Tension by James Blish, humans colonize a mostly water-covered planet by creating a race of humanoids out of their own genes hand-crafted to best suit this planet, and leaving all their knowledge, up to and including how to build spaceships, in form of tablets to be read when they develop enough to manage to do so. The driving point of the story is that being made to perfectly suit the world in question includes being microscopic.
In Jack Chalker's "Downtiming the Night Side," it's said that most of the humans who moved to other planets had to radically alter their bodies to survive; most don't even look human now, though they still think of themselves as such.
In All Tomorrows, humans intentionally modified themselves to live on Mars. Then they went on to colonize other worlds, but kept their new form - large chests, bigger heads, slender hands and so on. Then the Abusive Precursors stepped in and modified the humans on each planet into something that can perfectly live on the world they colonized, but is rarely even humanoid, just to have fun.
The Sci-fi novel I, Weapon deals with a universe that is essentially the aftermath of this with hundreds of variant human species who have adpated to the various planets of the universe, albeit in some cases with extensive genetic manipulation.

Tabletop Games

In GURPS Transhuman Space, Mars is terraformed, but not to the extent that normal humans can cope with the conditions. Most of the population still live in pressure-domes, but a sizable minority have been "bio-modded" to fit the environment. In addition a large minority of the Rust China population are bioroids genetically engineered for Mars.

Video Games

In Killzone, the Hellghast are the result of colonists adapting to a Death World.They aren't happy about it.

Web Comics

The 'Amphibians' of Gene Catlow colonize other planets using this method - but on Earth, they screwed up somehow. Seeing as Earth is mainly ocean, they figured that an amphibian form would work best there - while failing to notice intelligent life evolving on dry land at the same time.
It's also referenced in Freefall. Florence, an uplifted wolf, is a 'proof of concept' for a process designed to uplift and educate a species from a fertile-but-biologically-hostile world called Pfouts - rather than altering humans to be alien, it's a case of altering aliens to be human.
In Dreamwalk Journal and sequels, although never stated directly, it's heavily implied that this is how the planet Cyeatea's human/arthropod hybrids originated.