in facilitating speciation among vertebrate species. This concept of refugia has been applied to numerous groups of species and their biogeographic distributions. Glaciation and subsequent retreat caused speciation in many boreal forest birds, such as with North American sapsuckers (Yellow-bellied, Red-naped, and Red-breasted); the warblers in the genus Setophaga (S. townsendii, S. occidentalis, and S. virens), Oreothlypis (O. virginiae, O. ridgwayi, and O. ruficapilla), and Oporornis (O. tolmiei and O. philadelphia now classified in the genus Geothlypis); Fox sparrows (sub species P. (i.) unalaschensis, P. (i.) megarhyncha, and P. (i.) schistacea); Vireo (V. plumbeus, V. cassinii, and V. solitarius); tyrant flycatchers (E. occidentalis and E. difficilis); chickadees (P. rufescens and P. hudsonicus); and thrushes (C. bicknelli and C. minimus). As a special case of allopatric speciation, peripatric speciation is often invoked for instances of isolation in glaciation refugia as small populations become isolated due to habitat fragmentation such as with North American red (Picea rubens) and black (Picea mariana) spruce or the prairie dogs Cynomys mexicanus and C. ludovicianus. Superspecies Numerous species pairs or species groups show abutting distribution patterns, that is, reside in geographically distinct regions next to each other. They often share borders, many of which contain hybrid zones. Some examples of abutting species and superspecies (an informal rank referring to a complex of closely related allopatrically distributed species, also called allospecies) include: Western and Eastern meadowlarks in North America reside in dry western and wet eastern geographic regions with rare occurrences of hybridization, most of which results in infertile offspring. Monarch flycatchers endemic to the Solomon Islands; a complex of several species and subspecies (Bougainville, white-capped, and chestnut-bellied monarchs and their related subspecies). North American sapsuckers and members of the genus Setophaga (the hermit warbler, black-throated green warbler, and Townsend's warbler). Sixty-six subspecies in the genus Pachycephala residing on the Melanesian islands. Bonobos and chimpanzees. Climacteris tree creeper birds in Australia. Birds-of-paradise in the mountains of New Guinea (genus Astrapia). Red-shafted and yellow-shafted flickers; black-headed grosbeaks and rose-breasted grosbeaks; Baltimore orioles and Bullock's orioles; and the lazuli and indigo buntings. All of these species pairs connect at zones of hybridization that correspond with major geographic barriers. Dugesia flatworms in Europe, Asia, and the Mediterranean regions. Dichromatic toucanets of the genus Selenidera may be a superspecies that arose by the refugia hypothesis in the Amazon basin. In birds, some areas are prone to high rates of superspecies formation