Clausen, Jens Christen

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(b. Eskilstrup, Denmark. II March 1891: d. Palo Alto. California, 22 November 1969)

plant genetics.

The son of Christen Augustinus and Christine (Christensen) Clausen, Jens was brought up on a farm. From the ages of fourteen to twenty-two, he read extensively in genetics and plant breeding, and learned the principles of physics and chemistry through self-organized experiments. A schoolmaster named Thorgille interpreted Mendelisim and Darwin’s Origin of Species for him. While a student at the University of Copenhagen (1913–1920). Clausen supported himself by teaching part-time. After receiving the master of science degree in 1920 and being appointed assistant professor in 1921. he began research on ecological genetics under Wilhelm Johannsen, a leading Danish geneticist. He investigated two species of wild pansy (Viola arvensis and Viola tricolor) found in northern Europe. His monograph (1926) was the first publication to analyze simultaneously, for any group of plant species, variation within populations, ecological adaptations, chromosome numbers, and the behavior of interspecific hybrids and their progeny.

Clausen was invited to spend the period from 1927 to 1928 in California as a Rockefeller fellow, to collaborate with Ernest B. Babcock on investigations of chromosome behavior in hybrids of the genus Crepis Harvey M. Hall, director of a research program organized by the Carnegie Institution of Washington that was designed to explore the ecological genetics of plants native to California, had become much interested in Clausen’s work and, after their meeting in 1927, in Clausen himself. In 1928 Clausen returned to Copenhagen, where Hall visited him and saw his research firsthand. Realizing that cytological data such as Clausen had obtained were essential to his program. Hall added him to his team, which included the taxonomist David Keck and the physiologist William Hiesey.

In the autumn of 1931, Clausen had hardly arrived at Stanford, where the team was established in a recently constructed laboratory, when Hall was taken ill (he died in March 1932). Clausen became the senior member (in age) of the team and its leader. They continued the technique of cloning plants belonging to several species in distantly related genera and growing clonal divisions in the three gardens that Hall had already established: at Stanford, at Mather, and at Timberline (the latter two in the Sierra Nevada, at 4, 600 and 10, 000 feet, respectively). The results of this research, published in 1940. showed clearly that adaptation to different altitudes and climates had been accompanied in each species by profound changes that established a series of genetically different races or ecotypes.

Continuing this line of research. Clausen hybridized timberline with lowland races of Potentilla glandulosa and analyzed progeny of first and second generations by the same clonal method. The team showed (1) that each character difference between ecotypes is governed by several different gene pairs: (2) that partial correlations between many characters exist in the F2 generation; and (3) that genetic recombination confers upon some of the F2 segregates adaptive properties not found in either parent. The latter results were reported in 1958.

Clausen, Keck, and Hiesey simultaneously continned a second line of research, also begun by Hall, on the nature of plant species. They analyzed progeny from about three hundred hybridizations between the approximately eighty-five annual species and named varieties of California tarweeds (Compositae subtribe Madiinae). Although their results were equal in importance to those obtained by the transplant method in Potentilla glandulosa and other perennial species, they are less known. This is because Clausen was too much of a perfectionist. He did not publish short papers that reported results as they were obtained, for fear that they might later need to be qualified, but almost exclusively long monographs in which a whole series of experiments were presented and analyzed together.

Since he never felt that the data on Madiinae were complete enough to warrant this kind of publication, much of the information was published only in summary fashion, or not at all. Its scope and the major conclusions drawn from it are in a slim volume, Stages in the Evolution of Plant Species (1951), presented at Cornell University as his Messenger Lectures, He showed (1) that a few characteristics used by taxonomists to differentiate species are inherited in simple Mendelian fashion and vary within populations, so that they are not diagnostic of biological species; (2) that most of the species are separated from each other by hybrid sterility based upon gene-controlled barriers of reproductive isolation, but that similar barriers may exist between two populations that are genetically connected via other populations with which they are interfertile; (3) that strong genetic barriers sometimes exist between populations that are virtually indistinguishable morphologically or ecologically: (4) that evolution is often reticulate below the level of the genus, but branching or furcate above this level; and (5) that some (but not all) reticulation is due to hybridization accompanied by chromosome doubling or polyploidy.

The final line of research by Clausen and his team was in response to World War II. He tried to apply his methods to native grasses in order to produce strains that could be used to revegetate overgrazed rangelands and thus increase the supply of native feed available for livestock. This goal was not achieved, Nevertheless, the imesfigations by Clausen and Hiesey of asexual (apomictic) seed production in bluegrasses of the genus Poa were of considerable scientific value.

Clausen’s life was devoted to research. He never taught but did direct the research of graduate students, the most notable of whom was Robert K. Vickery. Deeply religious, he was active in the First Baptist Church of Palo Alto and a trustee of the Berkeley Baptist Divinity School. On 21 October 1921 he married Anna Hansen; they had no children. The Clausens came to the United States in 1931 and were naturalized in 1943. Clausen was named professor of biology at Stanford in 1951 and retired five years later.

Chief among Clausen’s honors were a certificate of merit from the Botanical Society of America (1956); an honorary degree of Doctor of Agronomy from the Royal Agricultural College of Sweden; honorary membership in the Botanical Society of Edinburgh; election to the U.S. National Academy of Sciences, the Royal Swedish Academy of Sciences, and the Royal Danish Academy of Sciences and Letters; and knighthood in the Order of Dannebrog, conferred on him in 1961 by King Frederick IX of Denmark.


I Original Works. Clausen s Writings include’ Genelical and Cytological Investigations on Viola tricolor L. and V. arvensis Murr.’ in Hereditas8 (1926), 1–156; Experimental Studies on the Nature of Species, I, Effect of Varied Environments on Western North American Plants. Carnegie Institution of Washington Publication no. 520 (Washington. D.C., 1940), written with David D. Keck and William M. Hiesey; Experimental Studies on the Nature of Species. II. Plant Evolution Through Amphiploidy and Autoploidy. with Examples from the Madiinae, Carnegie Institution of Washington Publication no, 564 (Washington, D.C., 1945), written with David D. Keck and William M. Hiesey: Experimental Studies on the Nature of Species, III, Enviromental Responses of Climatic Races of Achillea. Carnegie Institution of Washington Publication no, 581 (Washington, D. C. 1948), written with David D. Keck and William M. Hiesey; Stages in the Evolution of Plant Species (Ithaca, N.Y., 1951): and Experimental Studies on the Nature of Species, IV, Genetic Structure of Ecological Races, Carnegie Institution of Washington Publication no. 615 (Washington. D. C. 1958), Written with William M. Hiesey.

II. Secondary Literature. For and obiturary, see Carnegie Institution of Washington Year Book, 69 (1969–1970).

G. Ledyard Stebbins, Jr.