I'm afraid this is going to be one of those longshot questions. I remember reading this story, but I can only recall a few details.
The setting is a new world in which human beings, now capable of FTL travel, have settled. I believe the protagonist is a male, and he has a love interest as well.
The basic plotline is, soon after the first colonist from the ship begin living on the planet, strange mutations begin taking place. People are becoming brute-like, and (IIRC) are reverting to earlier states of evolution. I remember the protagonist, fearing contagion or some other planet-borne threat, orders the remaining colonist and crew to stay aboard the ship, which is still in orbit. His love interest is one of the people ordered to stay aboard.
The revelation that comes late in the story is that it is not the new planet that is causing the mutations. It's far worse than that. It is that Man has reached the end of his evolutionary possibilities. Not just on the colony world, but aboard the ship, back on Earth, and on every other planet, spaceship, etc. where human beings are. Some deep genetic trait in all human beings has been tripped, and it's humanity's inescapable collective fate to de-evolve back into its primate roots.
I recall a line near the end of the story where the protagonist realizes no one aboard the orbiting ship is capable of operating the vessel anymore, and no more people could come down to the surface. Not that it would make a difference.
I think this one came out in the 1970s or 1980s. I read in a sci-fi mag of some sort, I'm almost 100% certain of that. Can't remember anything else about it, unfortunately. Again, this one is going to be a longshot.