While “most fit” seems tautological it’s also subtly wrong. There’s a great deal of randomness in survival and a great number of genes involved at the same time.

Extremely harmful mutations have massive selective pressure, but things with minimal negative impact can spread as long as they fall below the noise floor. There’s nothing pushing any specific negative mutation but the space of minor negative mutations is so large some of them spreading becomes quite likely.

Thus “sufficiently fit” is a more accurate model. Especially when you consider extinction is a common outcome.