Evolution story of the Populus

(OECD 2000)

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Natural distribution

The genus Populus is widely distributed throughout the Northern Hemisphere, in both the temperate and subtropical zones. Representative species are found from Alaska and Labrador south to northern Mexico, as welle as Europe, North Africa, the Himalayas, mainland China and Japan. Some species are very widely distributed. P. tremuloides, for example, is the most broaly distributed tree species in North America, spanning 110° of longitude and 47° of latitude, and the second most widely distributed in the world.

Evolution  and migrational history

It was long felt that Populus was one of the oldest contemporary angiosperm genus, originating in China and Japan during the Triassic; however, these fossil records are now assiciated with other taxa. While the closest relatives in Flacourtiaceae are from tropical Asia, the fossil record now indicates that the genus Populus had tropical origins in North America during the late Paleocene, about 58 million years ago. These early leaf fossils are very similat to the present day P. mexicana in section Abaso. In the late Eocene, the first Eurasian relatives from other sections appeared, with those of Turanga confined to the Old World, and an ancestor of section Leucoides invaded temperate habitats. During the Oligocene, orecursors of Tacamahaca and Aigeiros appeared that would not become distinct sections until the Miocene, at which time members of section Populus also appeared.

The evolution of the advanced sections of Populus has been characterised by rapid speciation during allopatric cycles, but influenced by widespread introgression, both within and between sections. This rapid sequence of events, much conflicting evidence, and the confusion that has characterised the identification of species, has made it difficult to track the recent evolutionary history of poplars in the more advanced sections. While there is evidence of evolutionary divergence among the sections, the sections themselves are very widely distributed. Species within the sections are highly related and many are among the most broadly distributed of any tree species.

It is clear that migration of genes to other section members can occur easily throughout a very large portion of the North Temperate zone. Poplars are pioneering species and migrate quickly. Pollen studies have demonstrated that Populus species frequently dominate the first forest communities following glaciation. In Europe, P. tremula is the early pioneering species. P. nigra occurs along rivers and in pastures, together with Salix alba. Large stands of P. tremuloides in North America are thought to have originated soon after retreat of the Pleistocene ice sheet and have been since maintained asexually from root suckers, making them some of the largest and oldest organisms in the world.

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