A group of astronauts is set to explore a distant and interesting star. They do a few experiments, one of which is to expose some guinea pigs (or other kind of rodents) to the star's light.
The crew start losing teeth and hair and fingernails, and after a while they start developing telekinesis and mind-reading abilities. The star is eventually found to be the source of "evolution", and the more you are exposed to the star, the further you advance in evolution.
Near the end, one of the group wants to kill the others, only to be stopped by three light blobs, that are then revealed to be the guinea pigs, that in the mean time evolved to the very end of their evolving-line, and are now immaterial and god-like powerful.
The short story is from an old collection, almost surely written before 1970, possibly from some Hugo awards collection, or similar.