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

The book conclusively demonstrates that humans have manipulated plants through agriculture and cultivation throughout recorded history. All the plants mentioned are economically significant plants that have been extensively cultivated for prolonged periods of time. Most of the book, in fact, demonstrates and illustrates how extensive humans' influence over individual plant groups has been. The apple has been domesticated—in fact is named Malus domestica—for thousands of years and its natural range has been vastly expanded by human activity. The book discusses John Chapman's activity as an extreme example of how far the apple's range can be extended by vigorous cultivation.