Hieracium L. (hawkweeds) is a highly diverse genus (500-10000 species depending on taxonomic concept) known for its taxonomic complexity, which is associated with variation in ploidy levels and breeding systems. It consists of perennial herbs distributed mainly in temperate areas of Europe, Asia and North America, and in mountains of Central and South America; several species were introduced to Australia and New Zealand. Main centres of Hieracium diversity are located in the European mountains, in westernmost Asia, and in the American mountains.
Hieracium plants can reproduce sexually or apomictically, i.e., by producing seeds without fertilisation. Three subgenera are recognised: the American subgenus Chionoracium and the two Eurasian subgenera Hieracium and Pilosella whose centres of diversity are in the European mountains. The subgenera differ in their mode of reproduction: as far as known, Chionoracium species are all sexual diploids, Hieracium (s.str.) species are either polyploid obligate apomicts of the diplosporous type or diploid sexuals, and subgenus Pilosella is characterized by a mixture of sexual and facultatively apomictic taxa of the aposporous type.
In the two important monographs on Hieracium from the late 19th and early 20th century, a distinction is made between 'basic' species (Hauptarten / species principales) and 'intermediate' species (Zwischenarten / species intermediae). Most of the basic species comprise diploid plants whereas the intermediates are generally restricted to higher ploidy levels. In this definition, intermediate species show a combination of the morphological characters of at least two basic species. They are usually considered to be of hybridogenous origin. In contrast to this Central European view, North-Eastern European authors adopt a microspecies concept of smallest distinguishable units, which is more useful for the distinction of mainly apomictic North-Eastern European taxa. The British school uses the Central European system for Pilosella, but follows the North-Eastern tradition for Hieracium s.str.