Splitting or synonymizing – genus concept and taxonomy exemplified by the Parmeliaceae in the Nordic region.

The current use of a 30 million year dateline for accepting genera in the Parmeliaceae is evaluated. Its value is doubtful since evolution obviously went quicker in some lichen groups compared with others. Parallel evolution is also very common among lichens. Polyphyletic genera should always be split, while sister groups may remain split or be combined after considering non-molecular characters. Researchers dealing with the same groups should consult each other in an endeavour to achieve consensus. The most recent changes in taxonomy in the Nordic Parmeliaceae relate to the cetrarioid group, the last large clade to be revised. The division into two large genera, Cetraria and Nephromopsis, is supported, with a recommendation to retain Masonhalea due to its unique non-molecular characters. Three cases of parallel evolution in Cetraria and Nephromopsis are illustrated. Synonymization of Gowardia with Alectoria is supported, whereas synonymization of the parasitic genera Raesaenenia and Nesolechia with Protousnea and Punctelia respectively seems hardly necessary. Segregation of Montanelia from Melanelia is strongly supported, in contrast to the morphologically similar sister groups of Usnea, Dolochousnea and Eumitria, being recognised as separate genera