can allopatric speciation of terrestrial vertebrates occur on large continents?

are populations separated by geographic barriers often enough to allow differentiation in allopatry and subsequent secondary contact?

continental glaciations have caused large changes in the distributions of habitats in the northern hemisphere -- Pleistocene (from >1 million to 20,000 years ago) included four major glaciations -- continental glaciers extended as far south as Long Island and Ohio River -- sea level dropped 100 m -- rainfall decreased worldwide -- forest zones migrated far southward ahead of glaciers -- coastal NC was covered by spruce forest somewhat like forests now on highest mountains in NC or extending across central Canada

during periods when glaciers advanced, forests of North America were divided into two separate areas -- eastern and western -- widely separated by grasslands in the south and glaciers in the north -- consequently, populations of forest birds were also isolated in these eastern and western forest refuges -- when glaciers retreated, forests spread from the east westward across the continent north of the Great Plains -- populations of forest birds that had been allopatric during a glacial period came into secondary contact during the subsequent interglacial period

tropical forests probably also divided and fused as a result of climate changes during the Pleistocene

current distributions of North American birds suggest that these repeated cycles of glacial advance and retreat (and thus geographical isolation of forests followed by secondary contact) produced repeated speciation

warblers (subfamily Parulinae) provide some good examples -- there are many other cases of birds and other organisms in which a widespread eastern species is closely related to 3 or 4 western species (or subspecies) with mostly allopatric distributions in the western mountains . . .

Black-throated Green Warbler is a widespread breeding bird in cool coniferous forests from NC northward along the Appalachians and westward almost to the Rockies in Canada -- there its breeding range almost meets that of a similar species, Townsend's Warbler, which breeds in coniferous forests from Alaska to the northwestern states -- it meets (and in some places hybridizes with) another similar species, Hermit Warbler, in the coniferous forests of the west coast -- a fourth species, Black-throated Gray Warbler, breeds in drier coniferous forests in the mountains from Colorado to the west coast -- a fifth species, Golden-cheeked Warbler, now endangered, breeds only in cedar forests in mountains of central Texas -- apparently each western species originated as western populations of the widespread eastern species were isolated from eastern populations by the advance of continental glaciers -- glaciers advanced and retreated four times -- as a result four species now inhabit the western mountains but only one occupies the entire east

Yellow-rumped Warbler has distinct eastern and western subspecies that meet in the Canadian Rockies and interbreed in a hybrid zone -- despite distinct changes in coloration, isolation by glaciers did not result in reproductive isolation between eastern and western populations

this hypothesis has been challenged recently!

molecular differences between the species of warblers discussed above suggest that they separated in early Pleistocene or even earlier -- none in late Pleistocene coinciding with the last glaciation -- provided we can assume a "molecular clock" (accumulation of genetic differences between populations at a constant rate -- so the genetic difference between two populations indicates the time since they separated)

so there is discrepancy between the hypothesis of speciation as a result of continental glaciation and the hypothesis of times of speciation indicated by a molecular clock

how might this discrepancy be resolved?

• changes in forests during the Pleistocene might not explain speciation in birds (or other organisms with similar patterns of distribution)

• the molecular clock might need recalibration for these phylogenetic lineages

• genetic differentiation of populations in a cline might begin before geographic isolation results in the evolution of distinct species