American botanist and geneticist (1906-2000)
George Ledyard Stebbins Jr. (January 6, 1906 – January 19, 2000) was an American botanist and geneticist who is widely regarded as one of the leading evolutionary biologists of the 20th century. Stebbins received his Ph.D. in botany from Harvard University in 1931. He went on to the University of California, Berkeley, where his work with E. B. Babcock on the genetic evolution of plant species, and his association with a group of evolutionary biologists known as the Bay Area Biosystematists, led him to develop a comprehensive synthesis of plant evolution incorporating genetics.
His most important publication was "Variation and Evolution in Plants", which combined genetics and Darwin's theory of natural selection to describe plant speciation. It is regarded as one of the main publications which formed the core of the modern synthesis and still provides the conceptual framework for research in plant evolutionary biology; according to Ernst Mayr, "Few later works dealing with the evolutionary systematics of plants have not been very deeply affected by Stebbins' work." He also researched and wrote widely on the role of hybridization and polyploidy in speciation and plant evolution; his work in this area has had a lasting influence on research in the field.
From 1960, Stebbins was instrumental in the establishment of the Department of Genetics at the University of California, Davis, and was active in numerous organizations involved in the promotion of evolution, and of science in general. He was elected to the National Academy of Sciences and the American Philosophical Society, was awarded the National Medal of Science, and was involved in the development of evolution-based science programs for California high schools, as well as the conservation of rare plants in that state.
Early life and education.
Stebbins was born in Lawrence, New York, the youngest of three children. His parents were George Ledyard Stebbins, a wealthy real estate financier who developed Seal Harbor, Maine and helped to establish Acadia National Park, and Edith Alden Candler Stebbins; both parents were native New Yorkers and Episcopalians. Stebbins was known throughout his life as Ledyard, to distinguish himself from his father. The family encouraged their sons' interest in natural history during their periodic journeys to Seal Harbor. In 1914, Edith contracted tuberculosis and the Stebbins moved to Santa Barbara, California to improve her health. In California, Stebbins was enrolled at the Cate School in Carpinteria where he became influenced by Ralph Hoffmann, an American natural history instructor and amateur ornithologist and botanist. After graduating from high school, he embarked on a major in political studies at Harvard. By the third year of his undergraduate study, he had decided to major in botany.
Stebbins started graduate studies at Harvard in 1928, initially working on flowering plant taxonomy and biogeography—particularly that of the flora of New England—with Merritt Lyndon Fernald. He completed his MA in 1929 in biological sciences and continued to work toward his Ph.D. He became interested in using chromosomes for taxonomic studies, a method that Fernald did not support. Stebbins chose to concentrate his doctoral work on the cytology of plant reproductive processes in the genus "Antennaria", with cytologist E. C. Jeffrey as his supervisor and Fernald on his supervisory panel. During his Ph.D. candidature, Stebbins sought advice and supervision from geneticist Karl Sax. Sax identified several errors in Stebbins's work and disapproved of his interpretation of results that, while in accordance with Jeffrey's views, were inconsistent with the work of contemporary geneticists. Jeffrey and Sax argued over Stebbins's dissertation, and the thesis was revised numerous times to accommodate their differing views.
Stebbins's Ph.D. was granted by Harvard in 1931. In March that year, he married Margaret Chamberlin, with whom he had three children. In 1932, he took a teaching position in biology at Colgate University. While at Colgate, he continued his work in cytogenetics; in particular, he continued to study the genetics of "Antennaria" and began to study the behaviour of chromosomes in hybrid peonies bred by biologist Percy Saunders. Saunders and Stebbins attended the 1932 International Congress of Genetics in Ithaca, New York. Here, Stebbins's interest was captured by talks given by Thomas Hunt Morgan and Barbara McClintock, who spoke about chromosomal crossover. Stebbins reproduced McClintock's crossover experiments in the peony, and published several papers on the cytogenetics of "Paeonia", which established his reputation as a geneticist.
UC Berkeley.
In 1935, Stebbins was offered a genetics research position at the University of California, Berkeley working with geneticist E. B. Babcock. Babcock needed assistance with a large Rockefeller-funded project characterizing the genetics and evolutionary processes of plants from the genus "Crepis" and was interested in developing "Crepis" into a model plant, to enable genetic investigations similar to those possible in the model insect "Drosophila melanogaster". Like the genera that Stebbins had previously studied, "Crepis" commonly hybridized, displayed polyploidy (chromosome doubling), and could make seed without fertilization (a process known as apomixis). The collaboration between Babcock and Stebbins produced numerous papers and two monographs. The first monograph, published in 1937, resulted in splitting off the Asiatic "Crepis" species into the genus "Youngia". The second, published in 1938, was titled "The American Species of Crepis: their interrelationships and distribution as affected by polyploidy and apomixis".
In "The American Species of Crepis", Babcock and Stebbins described the concept of the polyploid complex, and its role in plant evolution. Some genera, such as "Crepis", have a complex of reproductive forms that center on sexually diploid populations that have also given rise to polyploid ones. Babcock and Stebbins also observed that allopolyploid types formed from the hybridization of two different species always have a wider distribution than diploid or autotetraploid species, and proposed that polyploids formed through hybridization have a greater potential to exploit varied environments, because they inherit all traits from both parents. They also showed that hybridization in the polyploid complex could provide a mechanism for genetic exchange between diploid species that were otherwise unable to breed. Their observations offered insight into species formation and knowledge of how all these complex processes could provide information on the history of a genus. This monograph was described by Swedish botanist Åke Gustafsson as the most important work on the formation of species during that period.
Stebbins's review, "The significance of polyploidy in plant evolution", published in "American Naturalist" in 1940, demonstrated how work done on artificial polyploids and natural polyploid complexes had shown that polyploidy was important in developing large, complex, and widespread genera. However, by looking at the history of polyploidy in plant families, he argued that polyploidy was only common in herbaceous perennials and infrequent in woody plants and annuals. As such, polyploids played a conservative role in evolution since problems with fertility prevented the acquisition and replication of new genetic material that might lead to a new line of evolution. This work continued with the 1947 paper "Types of polyploids: their classification and significance", which detailed a system for the classification of polyploids and described Stebbins' ideas about the role of paleopolyploidy in angiosperm evolution, where he argued that chromosome number may be a useful tool for the construction of phylogenies. These reviews were highly influential and provided a basis for others to study the role of polyploidy in evolution.
In 1939, with Babcock's support, Stebbins was made a full professor in the Department of Genetics at UC Berkeley, after the Department of Botany failed to promote him. Stebbins was required to teach a course on evolution, and during his preparation he became excited by contemporary research combining genetics and evolution. He became associated with a group known as the Bay Area Biosystematists, which included botanist Jens Clausen, taxonomist David D. Keck, physiologist William Hiesey and the evolutionary geneticist Theodosius Dobzhansky. During this time he also became friends with the botanist Herbert Baker. With the encouragement of this group of scientists, Stebbins directed his research towards evolution. He became involved with the Society for the Study of Evolution in 1946, and was one of the few botanists involved with the new organization.
His research on plant evolution also progressed during this period; he worked on the genetics of forage grasses, looking at polyploidy and the evolution of the Poaceae and publishing numerous papers on the subject though the 1940s. He produced an artificial autotetraploid grass from the diploid species "Ehrharta erecta" through treatment with the chromosome doubling agent colchicine. He was able to establish the plant in the field, and after 39 years of field trials was able to show that the autopolyploid was not as successful as its diploid parent in an unchanging environment.
"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 the modern synthetic theory] to speak of. I just modified things so that people could understand how things were in the plant world."
UC Davis and later life.
Stebbins took an appointment at the University of California, Davis in 1950, where he was a key figure in the establishment of the university's department of genetics; he was the department's first chairman and held the position from 1958 to 1963. At Davis, the focus of his research changed to incorporate newer areas, such as developmental morphology and genetics in crop plants, including barley. He continued to publish widely and extensively on plant evolution, writing over 200 papers and several books after 1950.
Stebbins and Edgar Anderson wrote a paper in 1954