In Guns, Germs, and Steel, how does comparing mutations help trace the spread of agriculture?

Mutations in plants helped farmers to domesticate wild
crops. Some crops, such as wheat, had evolved to drop their seeds and so reproduce, but
farmers harvested and replanted the mutation of wheat that kept its seeds in the air;
the result was that the mutated wheat became the dominant species. Diamond comments
that:

If one
carries out such a genetic analysis for major ancient New World crops many of them prove
to include two or more of those alternative wild variants, or two or more of those
alternative transforming mutations.
(Diamond, Guns, Germs, and
Steel, Google
Books)

By examining the
mutations that are unique to domesticated crops, agriculturalists can see how those
mutations were spread around the world; some wild plants without the mutations still
exist, and so it can be estimated both how long ago the mutation was adopted, and how
far it spread by human interference. Non-indigenous plants were carried to new areas and
planted by farmers, and they became indigenous through this activity; therefore, the
study of domesticated plants versus wild plants shows how farmers exploited mutation to
create new crops.