The authors offer a selectionist explanation, which is nevertheless uncertain. Note the repeated use of the subjunctive.
This is by no means an all-or-nothing selection (as the impression is sometimes given). Rather, the alleles are retained.
In a series of posts starting today, I offer some notes on the question of whether macroevolution is happening on the Galápagos Islands.
Lang and Rice cite a number of articles to show that loss-of-function mutations are just a small minority of those found in studies of organisms.