Species of the genus Populus are widespread in temperate latitudes and some subtropical regions of the northern hemisphere. Their inherent morphological variability, tendency to hybridization and introgression often complicate the identification of species and make the genus one of the most complex and problematic groups in plant classification. Knowledge of Populus species has passed from mythical ideas in antiquity to the emergence of a scientific interpretation of the genus since the description of the genus by K. Linnaeus. In the 17th–18th centuries, new species began to be imported to Europe from other regions of the world, which was often accompanied by their spontaneous hybridization with a number of native species. At the same time, they were also scientifically described. In the 19th century, the taxonomy of the genus Populus was established with the allocation of units of a rank higher than species, in particular, sections. The creation of simple classifications began, built on the principle of “genus–section–species”, recognizing a limited number of polymorphic species in the genus. In parallel with them, at the beginning of the 20th century, against the background of the development of ideas about the biological species and, in particular, with its monotypic interpretation, complex classifications of the genus appear and develop, built on the principle of “genus–subgenus–section–species”. All these systems were built on the basis of the analysis of morphological features, therefore the stage under consideration can be called morphological.

Keywords: evolution of Populus, origin of the genus Populus, paleobotany, phylogeny, Populus, systematics of Populus