Danish botanist and geneticist whose experiments in plant (plant breeding) heredity offered strong support to the mutation theory of the Dutch botanist Hugo de Vries (that changes in heredity come about through sudden, discrete changes of the heredity units in germ cells). Many geneticists thought Johannsen's ideas dealt a severe blow to Charles Darwin's theory that new species were produced by the slow process of natural selection.

Johannsen studied in Copenhagen, Germany, and Finland and was a professor first at the Institute of Agriculture in Denmark and then at the University of Copenhagen. After working initially in plant physiology, he devoted himself entirely to experimental research in heredity, becoming a leading authority in that subject. He found in studies with princess beans that in the succession of individuals arising from a single seed there existed what he termed a “pure line,” in which the hereditary units of all individuals are the same. About 1905 he demonstrated that he could produce large or small plants from beans of corresponding size. He concluded that although the plants differed in external characteristics, or in their “phenotype,” they nevertheless carried identical hereditary units or, in other words, preserved a common “genotype”; his terms phenotype and genotype are now a part of the language of genetics. Johannsen supported de Vries' discovery that variation in genotype can occur by mutation; that is, as a sudden, spontaneous appearance of a new species character. The new character, while independent of natural selection in its initial occurrence, is then subject to natural selection, as described by Darwin, as it either survives or disappears in future generations.