Thanks to Adam Greenwood for discovering this science fiction short story called Spacemaster, published in 1965 by James H Schmitz - which very clearly foresees a Mouse Utopia scenario of mutation accumulation, meltdown and extinction of humans, due to insufficient natural selection.
But, added to what he had been shown from Vinence's incredible ship, there had been enough he understood to present the story of the genetic collapse of Man -- or Spacemaster's version of it. It was not too implausible. The death seed of multitudinous abnormal genes had been planted in the race before it set out to explore and inhabit the galaxy, and with the expansion their rate of development increased. For another long time, improving medical skills maintained the appearance of a balance; it had become very much less easy for civilized Man to die even under a heavy genetic burden. But since he continued to give short shrift to any government audacious enough to make the attempt of regulating his breeding preferences, that burden also continued to grow. A point regularly came where medical knowledge, great as it might be, was suddenly shown to be no longer capable of the human repair work needed now to keep some specific civilization on its feet. The lethal genes, the innumerable minor mutations, had established at last a subnormal population, chronically sick and beginning to decrease rapidly in numbers. Spacemaster's charts indicated that this period, once entered, was not prolonged. When there were simply not enough healthy minds and bodies left to attend to the requirements of existence, the final descent became catastrophically swift and was irreversible. On Liot, Haddan had been living through the last years of such a period, modified only by Spacemaster's intervention. Spacemaster, with its supermachines and superscience, had come into existence as an organization almost too late to act as more than humanity's undertakers. Liot had been the last of all islands of galactic civilization. In less than fifteen centuries, the race had gone everywhere from its peak of achievement and expansion to near-extinction.
Chapter 6 Spacemaster from
http://whatcanimake.noip.me:8000/mike/secdir/books/scifi/pdf/James%20H.%20Schmitz/James%20H.%20Schmitz%20-%20Eternal%20Frontier.pdf
