[CobaltBlue]

"Survival of the fittest" isn't circular.

"Survival of the survivors" is circular.

"Survival of the fittest" argues that there is something about the survivors that is different that those which don't survive. That difference is called "fitness" - which doesn't mean that the survivors are physically fit from working out in the gym, but that they "fit" the environment better than those which don't survive.

If the environment is a round hole, and they are square pegs, they are rounder than the ones that don't survive.

[Doctor Stochastic]

It might be better just to say: "Survival of the adequate."