G. Ledyard Stebbins | |
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| Born | George Ledyard Stebbins Jr. (1906-01-06)January 6, 1906 |
| Died | January 19, 2000(2000-01-19) (aged 94) |
| Education | Cate School,Carpinteria, California;Harvard University (Ph.D. 1931);UC Berkeley |
| Known for | Variation and Evolution in Plants |
| Awards |
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| Scientific career | |
| Institutions | UC Davis |
| Academic advisors | Merritt Lyndon Fernald,E. C. Jeffrey,Karl Sax |
| Author abbrev. (botany) | Stebbins |
George Ledyard Stebbins Jr. (January 6, 1906 – January 19, 2000) was an Americanbotanist andgeneticist who is widely regarded as one of the leadingevolutionary biologists of the 20th century.[1] Stebbins received his Ph.D. in botany fromHarvard University in 1931. He went on to theUniversity of California, Berkeley, where his work withE. B. Babcock on the genetic evolution of plantspecies, and his association with a group of evolutionary biologists known as theBay Area Biosystematists, led him to develop a comprehensive synthesis of plant evolution incorporating genetics.
His most important publication wasVariation and Evolution in Plants, which combinedgenetics andDarwin's theory ofnatural selection to describe plantspeciation. It is regarded as one of the main publications which formed the core of themodern synthesis and still provides the conceptual framework for research in plant evolutionary biology; according toErnst Mayr, "Few later works dealing with the evolutionary systematics of plants have not been very deeply affected by Stebbins' work."[2] He also researched and wrote widely on the role ofhybridization andpolyploidy 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 theUniversity of California, Davis, and was active in numerous organizations involved in the promotion of evolution, and of science in general. He was elected to theNational Academy of Sciences and theAmerican Philosophical Society,[3][4] was awarded theNational 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.
Stebbins was born inLawrence, New York, the youngest of three children. His parents were George Ledyard Stebbins, a wealthy real estate financier who developedSeal Harbor, Maine and helped to establishAcadia National Park, and Edith Alden Candler Stebbins; both parents were native New Yorkers andEpiscopalians. 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 contractedtuberculosis and the Stebbins moved toSanta Barbara, California to improve her health. In California, Stebbins was enrolled at theCate School inCarpinteria where he became influenced byRalph Hoffmann, an Americannatural history instructor and amateurornithologist andbotanist.[5] After graduating from high school, he embarked on a major in political studies atHarvard. By the third year of his undergraduate study, he had decided to major inbotany.[6]
Stebbins started graduate studies at Harvard in 1928, initially working on floweringplant taxonomy andbiogeography—particularly that of the flora of New England—withMerritt Lyndon Fernald. He completed hisMA in 1929 in biological sciences and continued to work toward hisPh.D. He became interested in usingchromosomes for taxonomic studies, a method that Fernald did not support. Stebbins chose to concentrate his doctoral work on thecytology of plant reproductive processes in the genusAntennaria, with cytologistE. C. Jeffrey as his supervisor and Fernald on his supervisory panel. During his Ph.D. candidature, Stebbins sought advice and supervision from geneticistKarl 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.[7]
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 atColgate University. While at Colgate, he continued his work in cytogenetics; in particular, he continued to study the genetics ofAntennaria and began to study the behaviour of chromosomes in hybridpeonies 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 byThomas Hunt Morgan andBarbara McClintock, who spoke aboutchromosomal crossover. Stebbins reproduced McClintock's crossover experiments in the peony, and published several papers on the cytogenetics ofPaeonia, which established his reputation as a geneticist.[7]
In 1935, Stebbins was offered a genetics research position at theUniversity of California, Berkeley working with geneticistE. B. Babcock. Babcock needed assistance with a largeRockefeller-funded project characterizing the genetics and evolutionary processes of plants from the genusCrepis and was interested in developingCrepis into amodel plant, to enable genetic investigations similar to those possible in the model insectDrosophila melanogaster. Like the genera that Stebbins had previously studied,Crepis commonly hybridized, displayedpolyploidy (chromosome doubling), and could make seed without fertilization (a process known asapomixis). The collaboration between Babcock and Stebbins produced numerous papers and twomonographs. The first monograph, published in 1937, resulted in splitting off the AsiaticCrepis species into the genusYoungia. The second, published in 1938, was titledThe American Species of Crepis: their interrelationships and distribution as affected by polyploidy and apomixis.
InThe American Species of Crepis, Babcock and Stebbins described the concept of thepolyploid complex, and its role in plant evolution. Some genera, such asCrepis, 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 thatallopolyploid types formed from the hybridization of two different species always have a wider distribution than diploid orautotetraploid 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 thehistory of a genus. This monograph was described by Swedish botanistÅke Gustafsson as the most important work on the formation of species during that period.[8]
Stebbins's review, "The significance of polyploidy in plant evolution", published inAmerican 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 plantfamilies, he argued that polyploidy was only common in herbaceousperennials and infrequent in woody plants andannuals. 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.[9] 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 ofpaleopolyploidy in angiosperm evolution, where he argued that chromosome number may be a useful tool for the construction of phylogenies.[10] 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 theBay Area Biosystematists, which included botanistJens Clausen, taxonomistDavid D. Keck, physiologistWilliam Hiesey and the evolutionary geneticistTheodosius Dobzhansky. During this time he also became friends with the botanistHerbert Baker.[11] With the encouragement of this group of scientists, Stebbins directed his research towards evolution. He became involved with theSociety 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 thePoaceae and publishing numerous papers on the subject though the 1940s. He produced an artificial autotetraploid grass from the diploid speciesEhrharta erecta through treatment with the chromosome doubling agentcolchicine. 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.[12]
Columbia University's Jesup Lectures were the starting point for many of the most important works of themodern evolutionary synthesis. The presenters introduced the connection between two important discoveries—the units of evolution (genes) withselection as the primary mechanism of evolution. In 1941,Edgar Anderson (whose work on hybridization in the genusIris had interested Stebbins since they met in 1930) andErnst Mayr co-presented the lecture series and Mayr later published his lectures asSystematics 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 asVariation and Evolution in Plants, which proved to be one of the most important books in 20th-century botany.[13] 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.[14]
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 1937Genetics and the Origin of Species and provided the conceptual framework to organize a disparate set of disciplines into a new field: plant evolutionary biology.[15] 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 individualvariation within an interbreeding population; second, that of thedistribution 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.[16] He used the work of biosystematists Clausen, Keck, Hiesey, andTuresson to show that it was possible to distinguish betweengenotypic andphenotypic 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 ofC. D. Darlington to show that genetic systems like hybridization and polyploidy were also subject to selection.[7]
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."[17] The book effectively ended any serious belief in alternative mechanisms of evolution in plants, such asLamarckian evolution orsoft inheritance, which were still upheld by some botanists.[18] 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."[19]
Stebbins took an appointment at theUniversity 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.[20] At Davis, the focus of his research changed to incorporate newer areas, such asdevelopmental morphology andgenetics in crop plants, includingbarley. 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 on the importance of hybridization in adapting to new environments. They proposed novel adaptations would facilitate the invasion of habitats not utilized previously by either parent and that novel adaptations may facilitate the formation of stabilized hybrid species.[21] Following this paper, Stebbins developed the first model ofadaptive radiation.[22][23] He proposed that a high degree of genetic variability was necessary for major evolutionary advances, that because of slow mutation rates,genetic recombination was the most likely source of this variation, and that variation could be maximised though hybridization. As of 2006, research is ongoing regarding whether hybridization is an accidental consequence of evolution or if it is necessary for the creation and evolution of plant species;[24] it has been argued that contemporary studies are part of an intellectual lineage that started with the work of Stebbins and Anderson.[25]
Stebbins wrote several books during his time at UC Davis. These included his follow-up toVariation and Evolution,Flowering Plants: Evolution Above the Species Level,[26] which was published in 1974, following his delivery of the Prather Lectures at Harvard. Stebbins discusses the origins, genetics and developmental biology of theangiosperms. He argues for the role of adaptive radiation in the diversification of the angiosperms and the usefulness of applying our current understanding of species' genetics and ecology to gain knowledge about the evolution of ancient species.[27] He also wroteProcesses of Organic Evolution,The Basis of Progressive Evolution,Chromosomal Evolution in Plants and the textbookEvolution with co-authors Dobzhansky,Francisco Ayala andJames W. Valentine. His last book,Darwin to DNA, Molecules to Humanity was published in 1982.
Stebbins was passionate about teaching evolution, advocating during the 1960s and 70s the teaching of Darwinian evolution in public schools. He worked closely with theBiological Sciences Curriculum Study to develop high school curricula based on evolution as the central unifying principle in biology. He also opposed scientific creationism groups.[18] Stebbins was active in numerous science organizations—including the International Union of Biological Sciences, the Western Society of Naturalists, the Botanical Society of America, and the Society for the Study of Evolution—and served as President of the American Society of Naturalists. He was elected to theNational Academy of Sciences in 1952. Stebbins received numerous awards for his contributions to science: theNational Medal of Science, the Gold Medal from theLinnean Society of London, theAddison Emery Verrill Medal from the YalePeabody Museum of Natural History, and theJohn Frederick Lewis Award from theAmerican Philosophical Society.[28] He was elected a Fellow of theAmerican Academy of Arts and Sciences in 1952.[29] He was awarded the 1983Leidy Award from the Academy of Natural Sciences of Philadelphia.[30]
Stebbins was active in conservation issues in California during his later life. He established aCalifornia Native Plant Society branch in Sacramento in the early 1960s. Through the society, he created an active field trip program to increase interest in the native flora of California and to document rare plants. Stebbins was the state President of the Society during 1966. The society was instrumental in preventing the destruction of a beach on theMonterey Peninsula that he referred to as "Evolution Hill"—the area is now known as the S.F.B. Morse Botanical Area and is managed by the Del Monte Forest Foundation.[31] He was a major contributor to the Society's 1996 bookCalifornia's Wild Gardens: A Living Legacy. Stebbins was instrumental in the establishment of theInventory of Rare and Endangered Vascular Plants of California by the California Native Plant Society; it is still used by state and federal bodies in the United States for conservation policy-making.[32] Stebbins was also a member of theSierra Club.
During his tenure at UC Davis, he trained more than 30 graduate students in genetics, developmental biology and agricultural science. In 1973, Stebbins gave his last lectures at UC Davis and was madeprofessor emeritus. Following his retirement, he travelled widely, taught, and visited colleagues for the next 20 years. His last paper, "A brief summary of my ideas on evolution", was published in theAmerican Journal of Botany in 1999. The same year he was co-recipient with Ernst Mayr of the Distinguished Service award from the American Institute of Biological Sciences. A colloquium was held by the National Academies of Science in 2000 to commemorate the 50th anniversary of the publication ofVariation and Evolution in Plants. Stebbins died in his home in Davis the same year from a cancer-related illness. Stebbins was honored at aUnitarian memorial service—he had been active in the church in his later years following his 1958 marriage to his second wife, Barbara Monaghan Stebbins.[33][34] His ashes were scattered atStebbins Cold Canyon Reserve.[35]
Stebbins made an enormous contribution to scientific thought and botany by developing an intellectual framework for studying plant evolution including modern concepts of plant species and plant speciation. His contributions to the literature of plant evolutionary biology, in addition to his seven books, include more than 280 journal articles and book chapters, a compilation of which were published in 2004—The Scientific Papers of G. Ledyard Stebbins (1929–2000) (ISBN 3-906166-15-5). Betty Smocovitis, a historian of science who is preparing a book-length biography on Stebbins,[36] described Stebbins's scientific contribution as follows:
In science as in everything, small-scale synthesizers usually get credit from all constituent parties, but truly great synthesizers can fall between the cracks in the cycle of scientific credit. Ledyard Stebbins was in the latter category; neither fish nor fowl, he frequently failed to receive credit for work in some areas, usually at the hands of narrower colleagues. Few, however, have challenged his contributions to plant evolutionary biology, nor questioned his ability to synthesize disparate literature into a coherent framework. His ability to read quickly, recognize novel insights, digest new material, and then integrate the knowledge were the hallmarks of his scientific work style. He was a masterful synthesizer and master of the review essay or synthetic thought piece.[33]
In 1980, the University of California, Davis, named a parcel of land nearLake Berryessa, California, theStebbins Cold Canyon Reserve in recognition of his contributions to conservation and evolutionary science. The reserve is part of theUniversity of California Natural Reserve System. The UC Davis Herbarium maintains a G. Ledyard Stebbins student grant program, established in celebration of his 90th birthday.
Calystegia stebbinsii,Lomatium stebbinsii,Harmonia stebbinsii,Elymus stebbinsii,Lewisia stebbinsii and others are named in honor of Stebbins.
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