What is the main conflict in The Botany of Desire by Michael Pollan?

The text, usually categorized as a Nature and Gardening book, presents the argument that four plants have shaped human evolution at least to the same extent that humans have shaped those plants' evolution. The text uses the standard biological term "co-evolution" to describe this synergistic process. The four plants considered are the Apple, consisting of Malus domestica and M. seiversii; the Tulip, consisting of about 100 species in the genus Tulipa; Marijuana, consisting of the species Cannabis sativa and C. indica, as well as hybrids; and the Potato, or Solanum tuberosum. The text does not consider, or even apparently realize, the problematical approach of discussing co-evolution of species at generally the genus level. The bulk of the narrative consists of anecdotal experiences, personal observation, opinion and summarized topical history—which does not particularly support the major thesis.