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.
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With taxonomist David D. Keck and physiologist William Hiesey, he formed the first interdisciplinary effort to combine genetics, ecology and systematics in order to understand the ecological genetics of the evolutionary process in California plants.
Jens Stoltenberg | Alf Clausen | René Clausen | Jens-Peter Herold | Jens Galschiøt | Jens Frahm | Jens Voigt | Jens Eriksen | Jens Ulltveit-Moe | Jens Munk | Jens Mouris | Jens Lehmann | Jens Juel | Jens Christian Grøndahl | Jimmy Clausen | Jens Staubrand | Jens Peter Jacobsen | Jens Johansson | Walter Jens | Jens Olsen's World Clock | Jens Lindemann | Jens Lekman | Jens Knippschild | Jens Juel (diplomat) | Jens Jensen (landscape architect) | Jens Jensen | Jens Gunderssen | Jens Esmark | Jens Eisert | Frits Clausen |
From 1925 to 1950 he was based at the Carnegie Institute of Washington, at Stanford University where he worked on plant species concepts with Jens Clausen and William Hiesey.