Jens Clausen

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Jens Christen Clausen (March 11, 1891 - November 22, 1969) was an Danish-American botanist, geneticist, and ecologist. He is considered a pioneer in the field of ecological and evolutionary genetics of plants.

Clausens Masters and PhD work looked an hybrids formed between Viola arvensis and Viola tricolor.

Clausen was born in Eskilstrup, Denmark. His parents were famers and at age 14 he took responsibility for the family farm and was largely educated at home with the assisatnce of a local school teacher. He studied Mendel's genetics and Darwinian evolutionary theory. In 1913 entered the University of Copenhagen, where he studied botany, genetics, and ecology. Chresten Raunkier suggested he undertake graduate studies and Clausen chose to study the genetics and ecology of the Violaceae, He studied hybridization patterns across a range of environments described introgression of genes between species. He completed his masters degree in 1920 and he was appointed appointed assistant professor to geneticist Øvjvind Winge at the Royal Agricultural College in Copenhagen.

In 1926 Clausen was awarded his PhD for his work on the Violaceae, his monograph was one of the first publications that combined systematics, ecology, and genetics for any plant group. In 1927-1928 Clausen received a Rockefeller scholarship to study at the University of California, Berkeley where he worked on the genetics of the genus Crepis with E. B. Babcock. During this time he met Californian botanist Harvey Monroe Hall, who invited Clausen to return to the United States to Stanford University to work on the ecological genetics of Californian native species. Calusen returned to the US in 1931, and with taxonomist David D. Keck and physiologist William Hiesey formed the first interdisiplinary effort to combine genetics, ecology, and systematics in order to understand the ecological genetics of the evolutionary process in California plants. The project lasted 20 years and they performed some of the classic experiments in plant ecology where they looked at species formation across altitudinal gradients using experimental plots at Stanford (near sea level), at Mather (at about 4,600 feet), and at Timberline in the Sierra Nevada (at about 10,000 feet).

Clausen, Keck and Hiesey worte two books on their work, Experimental Studies on the Nature of Species. I. Effect of Varied Environments on Western North American Plants published in 1940 and Experimental Studies on the Nature of Species. III. Environmental Responses of Climatic Races of Achillea in 1948. He produced one additaional book about his work based on the Messenger Lectures he gave at Cornell and published in 1951 as Stages in the Evolution of Plant Species.

He was elected a member of the National Academy of Sciences, the American Academy of Arts and Sciences, and the Royal Swedish Academy of Science; he was also a member of the Botanical Society of America, and the Society for the Study of Evolution. He was a fellow of the California Academy of Sciences and of the American Association for the Advancement of Science. He was knighted as knight of Danneborg by Frederick IX of Denmark. Clausen was a Christian and was a member of the First Baptist Church of Palo Alto.