G. Ledyard Stebbins - UC Berkeley

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. 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.