Just to clarify: survival ofthe fittest absolutely does not mean survival of the strongest.
Rabbits, cockroaches, mice...
Sometimes it's the wiliest: sneaky male chimps get the gals while the alpha are busy showing off
sometimes it's the fastest breeders, as in rabbits, mice and the like
sometimes its the fanciest...the most garish male peacocks get selected by females for mating.
I'm sure that you can think of dozens of other examples.
I do agree that "strength" can be found in unlooked for places, and measured in many ways.
The human being is a prime example, small in relative physical strength, but endowed with enough brain power to overcome the entire animal kingdom. So far, anyway.