Wilhelm Johannsen (3 February 1857 – 11 November 1927) was a Danish pharmacist, botanist, plant physiologist, and geneticist.

In 1892, he was appointed lecturer at Royal Veterinary and Agricultural University and later became professor of botany and plant physiology.

He was able to show that even in populations homozygous for all traits, i.e. without genetic variation, seed size followed a normal distribution.

Johannsen's findings led him to oppose contemporary Darwinists, most notably Francis Galton and Karl Pearson, who held the occurrence of normal distributed trait variation in populations as proof of gradual genetic variation on which selection could act.

[2] Only with the modern synthesis, was it established that variation needed to be heritable to act as the raw material for selection.