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Tuesday, October 2, 2012

Theodosius Dobzhansky

 

Theodosius Dobzhansky (1900-1975) synthesized field study and laboratory experimentation in the study of natural selection, laying a foundation for Darwinian evolutionary theory.
Few biologists have made more important contributions to 20th-century evolutionary theory than Theodosius Dobzhansky. His work represents a major part of the synthesis of field study, laboratory experimentation, and classical Mendelian theory that became a powerful foundation for Darwinian theory. Dobzhansky's writings were prolific and influential, comprising over 550 papers and some dozen books. He wrote not only about the technical details of evolution in natural populations, but also about the social and philosophical sides of evolution--including the future evolution of the human species. He lived according to his own dictum, the title of a paper in 1972: "Nothing in biology makes sense except in the light of evolution."
Born on January 25, 1900, in Nemirov, Russia, a small town 200 kilometers southeast of Kiev in the Ukraine, Dobzhansky was the only child of Sophia Voinarsky and Grigory Dobzhansky (sic, the precise transliteration of the Russian name), a high school mathematics teacher. An avid butterfly collector, young Dobzhansky decided to become a biologist at about the age of 12. The family lived on the outskirts of Kiev during the tumultuous years of World War I and the Bolshevik Revolution. During this period Dobzhansky managed to complete his high school and university studies, graduating with a degree in biology from the University of Kiev in 1921.
As an instructor in zoology at the Polytechnic Institute in Kiev (1921-1924), he met Yuri Filipchenko, head of the newly-created Department of Genetics at the University of Leningrad and a strong advocate of T. H. Morgan's work at Columbia University in New York with the small fruit fly, Drosophila melanogaster. Dobzhansky went to work in Filipchenko's lab in 1924, where he began his first studies in genetics. In that same year he married Natalia Sivertzev, a geneticist in her own right working with the famous Russian evolutionist I. I. Schmalhausen in Kiev.
In 1927 Dobzhansky travelled to the United States under the auspices of a fellowship from the International Education Board (Rockefeller Foundation) to work in Morgan's laboratory at Columbia University. Here he began learning the techniques of cytogenetics, particularly the study of chromosome banding structures, that were to be so valuable in his later field studies of evolution in the wild Drosophila population. In September 1928 Dobzhansky moved to the California Institute of Technology, where Morgan had gone to organize and direct the newly-created division of biology. Dobzhansky was named assistant professor of genetics at Caltech in 1929 and professor in 1936. In 1940 he returned to New York as professor of zoology at Columbia University, where he remained for the next 22 years. In 1962 Dobzhansky was appointed professor at the Rockefeller Institute (now Rockefeller University), and in 1971 adjunct professor in the Department of Genetics at the University of California, Davis, a position he held until his death on December 18, 1975.
Dobzhansky's contributions to evolutionary theory relate to five major issues: the amount of variation that exists in natural populations; genetic changes in wild populations due to natural selection; speciation; laboratory studies of fitness under controlled conditions; and human variation and evolution.
Genetic Variation Studies
When Dobzhansky came to the United States in 1927 the predominant view of genetic variation was that established by the work of T. H. Morgan and H. J. Muller in mutant laboratory stocks. Mutations were thought to be relatively rare and other variations in most cases deleterious. Since an organism's overall genetic make-up was the result of natural selection, with deleterious mutations weeded out, wild populations were assumed to harbor few mutations, or variations. As a result, evolution would be--as this was in line with what Darwin had predicted--a relatively slow process.
One of Dobzhansky's major contributions was to show that this view was incorrect. Applying the cytological methods of the Morgan group to the analysis of chromosome structure in wild populations of Drosophila pseudoobscura, Dobzhansky discovered a surprising amount of hidden variability--that is, variations not readily observed in the appearance of individual organisms. Dobzhansky suggested that preservation of extensive variation would allow populations to evolve rapidly as environmental conditions change. Dobzhansky published his findings in Genetics and the Origin of Species in 1937. This book was an important landmark in the evolutionary synthesis: the union of Mendelian genetics and Darwinian theory.
In Morgan's laboratory at Caltech Dobzhansky learned the cytological techniques involved in studying chromosome structure from two of Morgan's most important co-workers, A. H. Sturtevant and C. B. Bridges. In the mid- and late-1930s he had collaborated with Sturtevant on a series of papers using chromosome inversions (where a chromosome segment has been accidentally excised and reinserted in the chromosome upside down) as a way of tracing phylogenetic relationships among species and subspecies of Drosophila. Since inversions are inherited, two separate populations of varieties that showed similarities in chromosome inversion patterns ought to be more closely related than one having fewer similarities.
This work led to a subsequent series of papers of great importance. In the early and mid-1940s, Dobzhansky examined chromosome inversion patterns in populations of Drosophila pseudoobscura from Santa Barbara to central Texas. Each population, he found, had a different frequency for each of several inversion patterns. Moreover, noting that from one season to the next certain inversion patterns increased and others decreased within the same population, Dobzhansky correlated these changes with climatic and other environmental differences associated with changing seasons. He found that one inversion pattern predominated during warmer seasons, while another predominated during colder seasons. Bringing samples from each population back into the laboratory, Dobzhansky showed that he could vary environmental conditions so as to produce the same changes in frequency of inversion patterns that were observed with changing seasons in the field. Dobzhansky concluded that such seasonal fluctuations were the result of natural selection at work, with temperature acting as the selecting agent. These masterful studies provided concrete support for the theory of natural selection, at the same time illustrating the fruitfulness of combining field and laboratory work in the study of evolution.
Speciation and Evolution
As part of the larger question of how speciation occurs, Dobzhansky initiated a number of investigations into the basis of hybrid sterility--that is, the inability of the offspring of many hybrids (especially among animals) to be reproductively fertile. By studying the specifics of sexual, physiological, and behavioral isolating mechanisms in Drosophila pseudoobscura and Drosophila paulistrorum, Dobzhansky showed that varying degrees of reproductive isolation represented speciation in the process. Like many of his other studies, Dobzhansky's work on reproductive isolation was aimed at studying the process of evolution in action.
Although not primarily a human geneticist or paleontologist, Dobzhansky wrote frequently on human evolution, including the biology of race and the future of human evolution. His Mankind Evolving of 1962 was a highly influential work in directing attention to human variation and adaptation. Several works of a similar nature, such as The Biological Basis of Human Freedom (1956), The Biology of Ultimate Concern (1967), and Genetics of the Evolutionary Process (1970), all reflect Dobzhansky's wide-ranging and philosophical turn of mind.
Although plagued by a form of leukemia in his later years, Dobzhansky remained vigorous and active until the day before his death on December 18, 1975. During his lifetime he was the recipient of many honors and awards. He was a member of the U.S. National Academy of Sciences, the American Academy of Arts and Sciences, the American Philosophical Society, the Royal Society (Great Britain), the Academia Leopoldina (Leipzig), and the Academia Nazionale dei Lincei (Florence). He received the Daniel Giraud Elliot Medal (1946) and the Kimber Genetics Award (1958) from the National Academy of Sciences, the Darwin Medal from the Leopoldina (1959), the A. E. Verrill Award from Yale University (1966), the Gold Medal Award for Distinguished Achievement in Science from the American Museum of Natural History (1969), and the National Medal of Science (1964). In addition, Dobzhansky was awarded honorary degrees by over 20 institutions, including the Universities of São Paulo (Brazil), Münster (Germany), Sydney (Australia), Oxford (England), Padua (Italy), and Chicago, Columbia, Michigan, Syracuse, Berkeley, and Northwestern in the United States.


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