balderdash writes:
Like, first you have this population with all bearded faces
-Next, you get one individual with a mutation that makes their face look hairless.
I don't think this is what was described.
What was described was that, after genetic isolation, more and more of the male population started preferring females with faces appearing to have less-and-less hair, because females with less hair started looking more attractive. Those less-hair-is-better-faces-for-females genes were spread this way in the population.
It's like those people born with tails today. They don't look too attractive to the opposite sex; therefore those genes won't be spread too extensively. Although, nowadays, those tails can be cut-off, safely.