"Variation and Evolution in Plants" was the first book to provide a wide-ranging explanation of how evolutionary mechanisms operated in plants at the genetic level. It brought concepts related to plant evolution into line with animal evolution as it emerged from Dobzhansky's 1937 "Genetics and the Origin of Species" and provided the conceptual framework to organize a disparate set of disciplines into a new field: plant evolutionary biology. In the book Stebbins argued that evolution needed to be studied as a dynamic problem and that evolution must be considered on three levels: first, that of individual variation within an interbreeding population; second, that of the distribution and frequency of this variation; and third, that of the separation and divergence of populations as the result of the building up of isolating mechanisms leading to the formation of species. He used the work of biosystematists Clausen, Keck, Hiesey, and Turesson to show that it was possible to distinguish between genotypic and phenotypic variation—that is, genetically identical plants could have different phenotypes in different environments. One of the book's most original chapters used the cytogenetics work of C. D. Darlington to show that genetic systems like hybridization and polyploidy were also subject to selection.