So, you have a story that's set millions or billions of years in the future. It's safe to assume that there are still humans, or possibly other organic life forms, still reproducing.
One would think that, even if natural selection has ceased to be a factor because of a static environment, mutation and genetic drift would still be factors. Even the proponents of Designer Babies rarely intend to micromanage everything. Genetic drift will happen in the parts that aren't controlled, after all. And even cloning gets mutations. And if any other species has survived, then they should be affected by evolution, too.
But writers tend to ignore any possible evolution unless it's directly related to the plot. It may be on purpose, to save the trouble of imagining new species; Worldbuilding is a major distraction from writing, after all. And it's commonly believed that humans as we know them are more relatable than barely recognizable semi-humanoid beasts who don't even resemble any other known species. The ones that do resemble known species generally have undergone an Anthropomorphic Shift, which generally did not happen naturally. A nonhuman human might as well be an alien, from a storytelling perspective.
Or maybe the writers didn't do their homework. Even if the fact that there will be genetic changes is possible to research, the nature of the changes would be a good deal harder.
This also happens with Lost Worlds, where the dinosaurs and other prehistoric species look exactly like the ones that were around hundreds of millions of years ago (and in especially egregious examples, may include species that are further apart from each other in time than we are from any of them).
Keep in mind that modern-day humans aren't that different physically from prehistoric Homo sapiens. If you took a human from 50,000, or even 100,000 years ago and raised them with modern nutrition and medicine, they'd probably be indistinguishable from any of us.
- Now and Then, Here and There is set 5 billion years in the future and humans haven't changed at all. It's possible the human population was brought to the future by time travel, the way Shu is, but that's never implied in the series. Other modern-day animals, such as cats, cows, and crows, are also seen in the future. However, there are also some "futuristic" creatures that do seem to have evolved from those of the present, such as some weird bird-like bipeds and a massive Antlion Monster.
- Seele from Neon Genesis Evangelion believe this to be the case; that human evolution has stagnated, and that humans must be forced to evolve through the Human Instrumentality Project, which involves merging the souls of all humans with the Eldritch Abomination Lilith, progenitor of all terrestrial life.
- Johnny Future, from Alan Moore's Tom Strong, lives in an age when the Sun has become a red giant, yet he looks perfectly human.
- Despite being born millennia in the past, the immortal Vandal Savage still looks identical to the modern Homo sapiens. An easy Hand Wave for this is that Vandal is probably a common ancestor for most of humanity by this point.
- A recurring plot point over the years with Marvel Comics species the Kree, who have retained the same basic look for millions of years, and their DNA has stubbornly resisted any attempt to get it to budge, despite some very, very spirited efforts from their leaders (up to and including blowing up a chunk of their home galaxy to give them a kick in the metaphorical ass). ... of course, it would probably help if a lot of the Kree weren't also fanatically racist and hell-bent on maintaining "genetic purity". One plotline, during Avengers Forever, had them actually breaking the stasis... and then this got promptly ignored by every other writer afterward.
- Green Lanterns: Taken to absolutely absurd extremes during a time-travel arc, when Simon Baz and Jessica Cruz go back four billion years to when the Green Lanterns were first founded. Half the first seven choices are of species who are still around in the modern day, and who look exactly identical.
- King Kong (1933): Despite being a Lost World where the Dinosaurs still roam, none of Skull Islands wildlife look any different from what we thought the Dinosaurs looked like back in 1933. The 2005 remake averts this by populating Skull Island with fictional dinosaur species descended from real ones.
- Zig-Zagging Trope in Planet of the Apes (1968). It's notably averted with the apes, with the famous twist revealing that the planet was Earth All Along and its inhabitants the evolutionary descendants of orangutans, chimpanzees and gorillas (who have all conveniently converged towards very similar, more humanoid forms). However, over the same timescale, the surviving homo sapiens have remained anatomically identical, albeit regressing culturally and losing the ability to speak.
- Averted in The Underland Chronicles in that the underlander humans' eyes turned purple and their hair turned a very light blond. Although how living underground could cause a change in melanin levels that quickly could be an example of artistic license. The serpents seem largely unchanged from plesiosaurs though, aside from losing their eyes from living underground.
- Discussed in Isaac Asimov's The End of Eternity. Twissell, from beyond the 30,000th century, is virtually undistinguishable from a modern human (to make no mention of Nöys, who's from beyond the 100,000th century and still there's no way to tell). Twissell is convinced that this stagnation is due to some machination from beings in the future, but it's revealed that Eternity itself has deprived mankind of any evolutionary drive.
- In Robert E. Howard's "The Hyborian Age", the Back Story of Conan the Barbarian, he inverts this, regarding cultural evolution as fixed, the failure of which, even in a short period of time, needs explaination.
To the south of them the Picts remain savages, apparently defying the laws of Nature by neither progressing nor retrogressing.
- But he seems to think that evolution is a one lane road that life can go both ways, but never adapting into new branches. In his world humans repeatedly regress back into apes, and then evolve to humanity again. Conan's ancestors were Atlanteans who became apes who became people, for example.
- In The Hitchhiker's Guide to the Galaxy, it is mentioned that the Vogons stopped evolving shortly after they stopped being an aquatic species. The planet Vogsphere has created many other life forms far more appealing than the Vogons, which they inevitably destroy for their own amusement.
- To lampshade it, the book even states that evolution threw up its metaphorical hands in horror at the sight of the Vogons in daylight, refused to let them evolve again, and produced the other, amazingly beautiful creatures of Vogsphere in compensation for the Vogons.
- Averted in Seveneves, where humans five thousand years in the future have evolved to live underwater, underground and on the moon and are drastically different from contemporary humanity.
- In the Dinotopia books, the dinosaurs on the island still look exactly like they did millions of years ago (or at least, how we thought they looked when the books were written), even though evolution has continued to progress normally in the rest of the world.
- The Dying Earth books take place so far in the future that the sun is on the verge of going cold. Yet if humans look any different from the way they do now, the changes aren't noted.
- Downplayed in A World Out of Time, which is mostly set three million years in the future. Humans have subtly changed; mostly, they have different hair patterns and increased intelligence (that's discounting artificial changes, such as genetically-engineered pets and a Longevity Treatment that allows the existence of separate populations of Boys and Girls). Still, considering that in three million years we evolved from Australopithecus, the changes are very minor.
- Doctor Who:
- In "New Earth" and "Gridlock", taking place 5 billion years in the future, pure humans are extinct, but the descendant species look identical.
- In "Utopia", taking place trillions of years in the future, humans still look identical. At some point in between they spent time as clouds and data, but they apparently keep returning to human form.
- In the BBC series Jekyll, the antagonists justify their attempts to kidnap Tom Jackman and experiment on his other personality on the basis that humanity is doomed to this trope, without Hyde's unique genetics.
- Warhammer 40,000:
- The major genetic evolution in humanity has been the emergence of psykers, and even then they're around one in a million. Somewhat enforced, however, in that FTL travel is far from easy: the populations of planets become known for very particular traits (every Catachan looks like Rambo and the Predator squad) and there is little opportunity for interbreeding. The other major modification, which involves taking the cloned genes of a long-dead superhuman warrior and injecting them into a human until they're seven feet tall and bulletproof also makes them sterile.
- This is actually averted with the various "Abhumans" who sometimes show up as Imperial Guard auxiliaries. In contrast to the various mutants created by Chaos energies and genetically altered supersoldiers, Abhumans are new races who have evolved naturally on planets with different environments than Earth, most notably the Squats, who evolved on a high-gravity planet.
- In Chrono Trigger, humans haven't changed one iota in 65 million years aside from the development of magic, and there's a number of other species that haven't changed at all either except for color. Justified when you fight the final boss - turns out Lavos has personally controlled humanity's evolution since its crash landing in 65 Million BC. Apparently the original form of man serves it best for its meal.
- The vorcha from Mass Effect have DNA that has been unchanged for millions of years. This is due to their ability to adapt rapidly to any environment as individuals, which removes their need to evolve as a species.
- An Exploited Trope in Xenoblade Chronicles. The Big Bad Zanza hid the DNA of the Telethia, draconic monsters that serve as macroscopic digestive bacteria for his true body, the Bionis, inside of the High Entia, a sentient, technologically-advanced species created specifically for the role. By hiding them inside beings who rely on technology rather than adapting to their environment, the Telethia genes would be protected from the ravages of natural selection in order to prevent them from changing too much to be usable before Zanza was ready to consume all life en masse. Fortunately this didn't work 100%, as the High Entia royal family somehow learned the truth and set up a Breeding Cult using intermarriages with humans to try and breed out the Telethia genes.
- This is often assumed to be the case in so-called "living fossils". Examples include horseshoe crabs, coelacanths, and triops shrimp. However, it's actually subverted; these animals have undergone evolutionary change, just not to a visible degree compared to what other animals have been through.