“A little over a decade after he published “On the Origin of Species,” in which he described his theory of natural selection shaped by “survival of the fittest,” Darwin published another troublesome treatise — “The Descent of Man, and Selection in Relationship to Sex.” This expanded on an idea he mentioned only briefly in “Origin.” Sometimes, he proposed, in organisms that reproduce by having sex, a different kind of selection occurs: Animals choose mates that are not the fittest candidates available, but the most attractive or alluring. Sometimes, in other words, aesthetics rule.”