What is this book's thesis?

Michael Pollan's introduction to this fascinating book describes the moment when he had the idea and insight for what the book would explore. He was planting fingerling potatoes, thinking of how they presented different challenges from growing other varieties of potatoes, and had the thought that plants force humans to do their bidding. He wondered: was this how plants actually evolve alongside humans, by convincing us to do the things they want? This made him think about how plants have evolved over the years via human intervention, to develop qualities that made them more desirable to humans, so that humans would engage in activities to help the plants propagate and survive. 
Pollen then chose four plants and one specific quality of each plant, and in four chapters then explores in detail the various ways in which that quality defines the plant's evolution and history of being cultivated by human beings. For example, the quality chosen for the apple is sweetness. Wild apples are not known for being sweet; and each wild apple tree is a blend of two different apple trees. Humans discovered how to graft branches and buds of sweeter and better quality apples onto existing trees, and were able to then produce many more apples to satisfy the desire for a sweeter tasting fruit. The book explores many fascinating aspects of history, folklore and science to answer this intriguing question.