Jens Clausen (1891 - 1969)

Courtesy Carnegie Institution for Science Administrative Archives

Jens Clausen was a pioneer. As the leader of a historic trio that included William M. Hiesey and David D. Keck, Clausen’s group established an interdisciplinary approach to study the evolution of plant species. They were responsible for establishing the concept of ecotypes, providing experimental evidence of natural selection, and helping to unite Darwin’s natural selection with Mendelian genetics. In a span of twenty years, their research made significant waves in the field of genetics, ecology, and evolutionary biology.

Jens Clausen was born on March 11, 1891 in Eskilstrup, a small Danish town on the island of Falster. During his youth, Clausen was homeschooled and was taught Mendelian genetics along with Darwinian theory of evolution. He was an observant and curious farm boy who became interested in the genetics of the violets near his home. His desire to investigate living things eventually led him to attend the University of Copenhagen in 1913, where he pursued genetics and received his master’s degree in 1920. He dove into studying the ecology and genetics of Viola, examining hybridization patterns across different violet species. Clausen was awarded his Ph.D. in 1926 for his work with Viola tricolor L. and Viola arvensis, as he was among one of the first to publish plant research that integrated ecology and genetics while demonstrating a relationship between taxonomic characters and genetics. Because of his work on Viola, Clausen became widely known as one of the leading botanists of Denmark.

Clausen’s work at the Carnegie Institution of Washington began in 1931, where he studied the ecological genetics of native plant species as a cytologist. He was initially invited to collaborate with Harvey Hall, a botanist at Carnegie who had begun a project on transplanting plants into different environments. Along with Hall’s two students, William M. Hiesey and David D. Keck, the team transplanted native species at three different elevations -- near sea level, 4,500 feet, and 10,000 feet. Clausen would take over as project director a mere two months after joining the team, as Hall passed away unexpectedly from cancer during a conference in Washington D.C.

Clausen would prove to be a strong leader and delightful colleague. He was full of vigor and enthusiasm, often spending long hours in the field. Advancing the project, his trio combined cytology with systematics and ecology to understand the influence of environmental factors on genetics and evolution of plant species -- an approach that was relatively novel in the field of genetics. In 1940, they published Experimental Studies on the Nature of Species. I. Effect of Varied Environments on Western North American Plants, which showed how adaptations to altitude changes resulted in distinct ecotypes. Within this book, Clausen’s team had provided some of the first experimental confirmations of natural selection in action -- a heavy blow to Lamarckism. 

Clausen, Keck and Hiesey’s project would end up spanning 20 years, during which the trio would continue their work on ecotypes. During these two decades, Clausen’s team would provide some of the most detailed evidence for the principles of evolutionary synthesis, focusing on natural selection in the wild and the genetic structure of populations. Researchers today credit them with the first experimental analysis of genetic differentiation in adjacent populations and for using transplantation as a means to distinguish between genetic and environmental effects.

Clausen’s accomplishments extended beyond his work at Carnegie; for most of his career he lectured at multiple universities across the United States and was elected to multiple scientific academies. Even after his retirement from Carnegie, Clausen continued to work and preferred to use the word “pensioned” rather than retired. An independent and enthusiastic thinker, Clausen was a scientist who relentlessly pursued his curiosity of the world.

Photo Credit: "Courtesy Carnegie Institution for Science Administrative Archives"



--by David Hoang (2019), Carnegie Summer Intern in the Rhee lab

Other Carnegie Historical Figures

“A Mythic Collaboration.” Centennial History of the Carnegie Institution of Washington, by
Parratt. Craig, vol. 4, Cambridge University Press, 2005, pp. 99–118.