[9][10] The diversification of various Parmelioideae lineages may have been a result of gaining innovations that provided adaptive advantages, such as melanin production in the genus Melanohalea.

The most dramatic period of generic splitting occurred in the 1970s and 1980s, when Mason Hale and others proposed numerous new genera based primarily on morphological features such as lobe shapes, rhizine types, and cortical chemistry.

[14] The advent of molecular phylogenetics techniques in the late 1990s provided new tools for evaluating which morphological and chemical characters were most reliable for defining genera.

For example, molecular data revealed that the brown-fruited genus Neofuscelia needed to be merged into Xanthoparmelia, while confirming that groups like Parmotrema and Cetrelia represented distinct evolutionary lineages.

Characters of the ascomata (especially exciple anatomy and ascospore characteristics), conidial types, and cell wall polysaccharides have proven particularly valuable for defining natural groups.

[14] Modern molecular studies have established that approximately 75% of Parmeliaceae species belong to well-defined major clades, including groups like Xanthoparmelia, Parmotrema, and their close relatives.

Rather than being defined by single diagnostic features, most genera are now recognised as monophyletic groups characterised by unique combinations of multiple morphological, chemical, and anatomical traits.

[14] Although fossil records of extant lichen species are scarce, the existence of some amber inclusions has allowed for a rough estimate of the divergence of the Parmeliaceae from its most recent common ancestor.

Pycnidia are immersed or rarely emergent from the upper cortex, are produced along the lamina or margins, pyriform in shape, and dark-brown to black in colour.

[18] Members of the Parmeliaceae exhibit a diverse chemistry, with several types of lichenan (Xanthoparmelia-type, Cetraria-type, intermediate-type), isolichenan and/or other polysaccharides being known from the cell walls of many species.

The compounds usnic acid and atranorin, which are found exclusively in the Parmeliaceae, are of great importance in the systematics of the family, and the presence or absence of these chemicals have been used in several instances to help define genera.

Parmelia and Usnea are the best chemically characterized genera, while the species Cetraria islandica and Evernia prunastri have attracted considerable research attention for their bioactive compounds.

[19] A study of three parmelioid lichens (Bulbothrix setschwanensis, Hypotrachyna cirrhata, and Parmotrema reticulatum) collected from high-altitude areas of Garhwal Himalaya, showed considerable variation in the chemical content with the rising altitude.