The biogeography of the holarctic Nephrotoma dorsalis species-group (Diptera, Tipulidae)

A biogeographic analysis is presented for the Holarctic Nephrotoma dorsalis-group in relation to the palaeogeographic, climatic and floristic history of the Holarctic region in the Cenozoic. Holarctic intercontinental connections across the Atlantic and by way of Beringia are discussed as well as the biogeographic developments in both the Palaearctic and Nearctic regions. Much attention is paid to a comparison with other insect groups. It is concluded that the history of the dorsalis-group was enacted largely in the Tertiary, besides Pleistocene influences on diversity and distribution. The early Tertiary origin of the group is located in the eastern Palaearctic, and its members are supposed to have reached Europe not prior to the late Miocene. It is concluded that exchange between the Palaearctic and Nearctic was entirely by way of Beringia where two vicariance events are postulated, namely in the Oligocene and in the late Pliocene. Other vicariances within the dorsalis-group could be correlated with the formation of the Japanese Sea (Miocene), the Himalayan Orogeny (middle Miocene), the development of boreal habitats (middle to late Miocene), the separation of eastern and western North America by climatic and orogenic factors (middle to late Miocene) and the Pleistocene glaciations. Late Pleistocene range extensions across Beringia in both directions are apparent in two species of the group.