This probably isn't news... even to Darwin.
It is the most adept in the given the selection circumstances, that survive, and not the "fittest". If a snake-filled cavern suddenly gets sealed due to a landslide, it will be the thinnest ones that manage to escape through the cracks, that survive.
That's what the "fittest" means: those most fit to survive under the circumstances. This "fitness" may be a genetic predisposition, innate qualities, size, color, etc., or even something learned...but whoever or whatever has it will have a greater chance to survive under specific circumstances.