Polycarpon is, in its traditional circumscription, a nearly cosmopolitan genus. Its taxonomy, however, is poorly understood. Depending on species circumscription, Polycarpon counts nine or 15 species. Two species are native in parts of Europe (Chater & Akeroyd 1993) but none in Belgium. One however, Polycarpon tetraphyllum, is rather weedy and now occurs far beyond its original distribution range.
Molecular data demonstrated that Polycarpon, as usually circumscribed, is polyphyletic (Kool & al. 2007). Most of the New World members belong in fact to related genera. The Polycarpon tetraphyllum group has its main diversity in the Mediterranean region. It is a polyploid complex with morphologically very similar taxa mostly separated only by partially overlapping characters. The members of this group form a strongly supported, but largely unresolved clade. They are all probably better included in a widely circumscribed P. tetraphyllum (Kool & al. 2007).
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Gereau R.E. & Marticorena C. (1995) The genus Polycarpon (Caryophyllaceae) in South America, with one new species from the Norte Chico of Chile. Novon 5(2): 152-155.
Iamonico D. & Domina G. (2015) Nomenclatural notes on the Polycarpon tetraphyllum aggregate (Caryophyllaceae). Plant Biosystems 149(4): 720-727.
Kool A., Bengtson A. & Thulin M. (2007) Polyphyly of Polycarpon (Caryophyllaceae) inferred from DNA sequence data. Taxon 56(3): 775-782.