"Survival of the fittest" has been misquoted and misunderstood from the start. People thought Darwin meant the "strongest" (physically). This has led to much misunderstanding of evolution.

Darwin meant that it's the most adaptable species who survive, and that can mean very small (and relatively weak) species indeed.

it's always "the most adaptable", just people saying the wrong thing. Intelligence is just a single possibility as to why one species survives and another dies out. (beside man's helping their extinctions along).

The "fittest" include the most powerful, preditory, rapists, tribal chiefs enjoying executive conjugal privileges, etc. For some cultural reason Northern Europeans in recent years have risen above that. The U.S. is still in it's teen chaos.