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.
How could the authors suddenly do this? Some of the following points may be considered.
Some hares turn white in winter where it’s snowy, but remain brown in winter where it’s mild. What does that have to do with Darwinian evolution?