A quote from Dawkins, R.

Natural selection weeds out the misfits, leaving, besides the fittest, a lot of mediocre individuals...these mediocre individuals put the populations of all species at some degree of risk.

I am currently pondering the evolutionary purpose of mediocrity.
Aside from adding quantity to the gene pool, with the increased probability of extraordinary ability in a random individual, I can see only one other purpose: to provide for the maintenance of the group so as to allow the extraordinary to be even more successful. But what mechanism can do this? (We must not ascribe intent.)