SCHORN, Howard E., and THOMPSON, Anna, Museum of Paleontology, University of California, Berkeley, CA 94720-4780
There are eight (possibly nine) extant species of Pseudotsuga: two species in the western United States, a possible segregate in Mexico, one species each in Japan and Taiwan, and four species in China. Data from DNA restriction fragment and karyotype analyses indicate that the species from China and Taiwan are closely related to each other, and so are the western North American species. The Japanese species is intermediate between the Asian and North American species, but is more closely related to the Asian species. Molecular evidence suggests that the North American species are younger than the Asian species; however, other molecular data support a North American origin for the genus.
All unequivocal fossils of Pseudotsuga occur within the genus' present range. At least six additional fossil species are recognized from the North American Tertiary. The first fossil record of Pseudotsuga is from about 32 Mya in the early Oligocene of Oregon. The next occurrence is not until the Miocene, but then both in Japan and western North America. These fossil taxa are morphologically distinct and possess diagnostic characters of extant Asian and North American clades. In western North America, derived forms related to P. macrocarpa appear around 6 Mya and to P. menziesii around 3.5 Mya.
Taken together this evidence indicates that Pseudotsuga migrated from North America to Asia before the middle Miocene. Asia and North America were connected via the Bering land bridge during the first half of the Miocene, and migration of North American Pseudotsuga into Asia may have occurred during the Miocene climatic maximum (18-15 Mya). Asian and North American lineages were thenisolated by late Tertiary climatic collapse and thus differentiated into the distinct forms recognized today.