Speciation
The greenish warbler (Phylloscopus trochiloides) has a number of subspecies, of  which P. t . viridianus is the most familiar in Europe. It is a ring species with populations diverging east and westwards of the Tibetan Plateau, later meeting on the northern side. Their relationships are fairly confusing.Alström, Per 2006. Species concepts and their application: insights from the genera Seicercus and Phylloscopus. Acta Zoologica Sinica 52 (Supplement): 429-434.