Survival of the fittest was a concept in biology, in which those lifeforms that were the healthiest and hardiest survived, while weaker members died out, eliminating them from the gene pool.
In 2268, Doctor Leonard McCoy attributed the survival of some of the Omega IV natives for more than a thousand years to survival of the fittest, saying their ancestors had to have the ability to resist the Omega IV virus. (TOS: "The Omega Glory")
In 2375, Quark, irate at the thought of the reforms Grand Nagus had instituted due to his mother's influence, asked what had happened to survival of the fittest. (DS9: "The Dogs of War")