The genetic diversity within the foliose form of Ricasolia amplissima from Europe and North America was studied using molecular phylogenetic analysis of the nuclear ITS and RPB2, and mitochondrial SSU. Boundaries between closely related species were also examined using morphological and chemical patterns. Four species of the recently reinstated lichen genus Ricasolia De Not. were phylogenetically verified which necessitated a new combination, Ricasolia japonica (Asah.) Cornejo. Analyses suggest that the generic type taxon R. amplissima (Scop.) De Not. belongs to a species complex that shows two evolutionary centres, one in Europe, North Africa, Asia Minor and the Macaronesian Islands, the other from north-western North America on exposed shores of mainly forested marine islands in south-eastern Alaska, where it shows strong habitat specificity. The Alaskan lineage is very similar to the European lineage but it differs by the lack of scrobiculin and other chemical substances. It is described here as R. amplissima subsp. sheiyi Derr & Dillman.
