Subgenus Hieracium (= Hieracium s.str., sometimes considered as a genus and including Chionoracium) is distributed in temperate regions of Europe, Asia, northernmost Mediterranean Africa, and North America (and introduced to several other regions, e.g. to New Zealand). The species occupy forests, forest margins, various grasslands and rocks. Polyploid (triploid and tetraploid, very rarely pentaploid) taxa with asexual reproduction via parthenogenetic development of the unreduced egg cell (Antennaria-type diplospory) prevail. Sexual reproduction is rare and restricted to a few diploid species. Diploids are recently mostly confined to never glaciated refuges, polyploids are also widespread in areas that had been covered by ice sheets. Although the vast number of polyploids is thought to have originated by hybridization events in the past, nowadays gene flow is severely restricted. Vegetative spread by means of short rhizomes is of little importance in Hieracium s.str. (in contrast to both under- and above-ground stolons in many species of the closely related (sub)genus Pilosella). A mentor effect, i.e., induction of self-compatibility in otherwise incompatible pollen when it is mixed with foreign pollen, was recently discovered. As the diploid species are usually strictly self-incompatible, this mentor effect may represent a very effective hybridization barrier in this diplosporic (sub)genus.

The recent morphological variability in the genus likely reflects an immense reticulate evolution in the past. Hybridization processes in Hieracium are often followed by a rise in ploidy level and by apomictic reproduction as an "escape from sterility". Because of agamospermy with full omission of female meiosis, the ability of polyploids serving as mother plants is highly limited. Present-day gene flow is more likely either among diploid taxa, or between diploids (as mother plants) and pollen-producing polyploids (as pollen donors).

About Hieracium