In Robert Heinlein’s book Starship Troopers, he mentioned a planet called Sanctuary. It was retarded: given favorable conditions and similar amounts of time, life had not gotten very far. The most advanced animal was a proto-insect, the most advanced plant something like a primitive fern. The explanation given in the book was that the planet had a very low level of natural radiation: with very few mutations, evolution dawdled.
The planet had been settled by humans, who did well. For example, Terran wheat easily displaced the local plants. But a local biologist tells our hero that in the long run, Sanctuary is a trap. The settlers will hardly evolve at all, and eventually – we’re talking the long run - they’ll be obsolete, replaced by fast-changing outsiders.
Heinlein was right on some of this, wrong on more. In other words, he knew a lot more biology than the average science fiction writer. Mutations are indeed the seed corn of adaptive evolution: you have to have some. But we’re pretty sure that radiation is not the only way of producing them, not even the most common way. It takes a lot of radiation to double the mutation rate – something like a lethal dose, way more than you get from background radiation in a lifetime. I’m not sure how well that was understood in 1959.
But assume it anyway. what would happen if people moved somewhere where the mutation rate was far lower?
Their genetic load would decrease with time, assuming that they were still subject to much selection. Today, everybody has hundreds of nicked or broken genes: selection keeps eliminating them, while mutation keeps creating them. The suspicion is that their effect is quite large. This hypothetical population would have fewer and fewer. In a few thousand years, they would lose most of the variants that decrease fitness by 1% or more.
If the theory of mutational load is correct, it wouldn’t be the people of Sanctuary who would have to worry about becoming obsolete.