G. Ledyard Stebbins - Variation and Evolution in Plants

Variation and Evolution in Plants

Columbia University's Jesup Lectures were the starting point for many of the most important works of the modern evolutionary synthesis. The presenters introduced the connection between two important discoveries—the units of evolution (genes) with selection as the primary mechanism of evolution. In 1941, Edgar Anderson (whose work on hybridization in the genus Iris had interested Stebbins since they met in 1930) and Ernst Mayr co-presented the lecture series and Mayr later published his lectures as Systematics and the Origin of Species. In 1946, Stebbins was invited on Dobzhansky's recommendation to present the prestigious lectures. Stebbins' lectures drew together the otherwise disparate fields of genetics, ecology, systematics, cytology, and paleontology. In 1950, these lectures were published as Variation and Evolution in Plants, which proved to be one of the most important books in 20th-century botany. The book brought botanical science into the new synthesis of evolutionary theory, and became part of the canon of biological works written between 1936 and 1950 that formed the modern synthesis of evolution.

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.

The book offered few original hypotheses, but Stebbins hoped that by summarising the available research on plant evolution the book would "help to open the way towards a deeper understanding of evolutionary problems and more fruitful research in the direction of their solution." The book effectively ended any serious belief in alternative mechanisms of evolution in plants, such as Lamarckian evolution or soft inheritance, which were still upheld by some botanists. Following that publication, Stebbins was regarded as an expert on modern evolutionary theory and is widely credited with the founding of the science of plant evolutionary biology. Variation and Evolution in Plants continues to be widely cited in contemporary scientific botanical literature more than 50 years after its publication.

Stebbins regarded his contribution to the modern synthesis as the application of genetic principles already established by other workers to botany. "I didn't add any new elements to speak of. I just modified things so that people could understand how things were in the plant world."