"Survival of the fittest" is often misinterpreted as "Survival of the strongest" (especially from non english speakers who think "fit" means "fitness").

But it comes from "to fit" like "suited". Therefore "Survival of the fittest" means that those will survive who fit best into a changed environment. And in many cases in Earths history that were not the strongest, often it were the weaker and defensive ones who fitted best.