In the anime Vandread, the antagonists (earthlings) had a goal that I find interesting. So I am going to use them as the premise and hopefully expand it well enough to work for a world I am working on.
There are earthlings and their are settlers. Settlers are basically humans that have left Earth to settle other planets and earthlings are humans that didn't. I know that's pretty complicated but bear with me.
The earthlings, being that they are from earth and have the name of the planet in their names, feel that they are above the settlers and, over the generations, pull their best impersonation of European royal families circa formation of European royal families to the Red Revolution and wind up inbred and lacking in genetic diversity. Also since this is the far-ish future, humans have since learned that building their civilization based on lighting dead dinosaurs on fire to boil water to cook their flatulating bovines raised entirely on fields of grains grown the way nature intended them to be grown complete with natural selection and non-human intervention was not a great way to maintain said civilization.
Luckily, their version of Rasputin, who shall be known as Space Rasputin, was something of a mad scientist/possible space ghoul. He figured that the settlers were human and earthlings were humans but better so the settlers would be more than happy to help out the Earthlings survival.
Space Rasputin, not being the worlds most diplomatic thinker but the self-titled world's greatest evolutionary biological humanist, figured that they should just sort of take the settlers' organs from them and attach them to the earthlings' bodies. Because that's how genetics works.
But continuing forward on that path to its "logical" conclusion: he figured they should take the best organs from the best settlers. And since natural selection is probably a thing on other planets too, they should harvest the organs from the settlers who have the best organs because evolution and natural selection. Even if they had to tweak the whole natural part.
For this reason, his merry band of human harvesters went out into space to collect body parts from the best human planets for those body parts. For example, they harvested skin from a planet that was basically just slowly and painfully poisonous to humans, the thinking being that the skin would get stronger as humans survived and mated and managed to live into their thirties. They harvested sex organs from planets that only contained that sex, because that makes sense somehow. So basically all earthlings are Frankenstein's monster.
The Question (tl;dr)
So basically, given some amount of terraforming to make alien planets liveable and apply tweaks to the environment but otherwise no other earthling interference, is it possible to raise a population of humans to have one organ that is super fitted to its purpose even at the expense of some or all of their other organs?
You can assume enough time has passed that the settlers have forgotten that they are settlers and believe they are "X-Planet"-lings and have no real knowledge of earth. However, they must still look at least mostly human. The idea is that those harvested are still considered human.
Science-based is good enough, but hard science is better.
Edit for clarity: by super fit organ I mean an organ that does the task it is designed for extremely well. Like a liver that is highly resistant to damage caused by poisons it tries to filter out and processes them very fast or a heart that is super efficient and able to do in 1 heart beat what the average human heart might do in 5 or 6. However, things like lungs that breathe water or a spleen that makes you fly and plays the banjo are not considered "super" fit. And you can limit it to human organs.