Jens (Christian) Clausen

Botanist, born in North Eskilstrup, Denmark. He carried out genetic research in Copenhagen (1921–31), where his study of violets led to his recruitment by taxonomist Harvey Monroe Hall to join the Carnegie Institution's laboratory at Stanford University (1931–69). There Clausen became the cytologist, geneticist, and unofficial leader of climatological transplant studies over two decades. With colleagues David Keck and William Hiesey, he demonstrated that new species can result from rapid experimental environmental changes. He made major contributions to the study of hybridization, race ecology, plant evolution, and studies of world forest compositions.

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. 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.

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. 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. 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. Effect of Varied Environments on Western North American Plants published in 1940 and Experimental Studies on the Nature of Species. 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;