"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.
"Evolution used to ensure the survival of the most intelligent."
Incorrect, evolution ensures the survival and proliferation of the species by favouring by natural selection those best suited to thrive in a concomitant given situation.
Some times this means the survival of morons.