Evolutionary diversifications of plants on the Qinghai-Tibetan Plateau

Abstract

The Qinghai-Tibetan Plateau (QTP) is the highest and one of the most extensive plateaus in the world. Phylogenetic, phylogeographic, and ecological studies support plant diversifications on the QTP through multiple mechanisms such as allopatric speciation via geographic isolation, climatic oscillations and divergences, pollinator-mediated isolation, diploid hybridization and introgression, and allopolyploidy. These mechanisms have driven spectacular radiations and/or species diversifications in various groups of plants such as Pedicularis L., Saussurea DC., Rhododendron L., Primula L., Meconopsis Vig., Rhodiola L., and many lineages of gymnosperms. Nevertheless, much work is needed toward understanding the evolutionary mechanisms of plant diversifications on the QTP. Well-sampled biogeographic analyses of the QTP plants in the broad framework of the Northern Hemisphere as well as the Southern Hemisphere are still relatively few and should be encouraged in the next decade. This paper reviews recent evidence from phylogenetic and biogeographic studies in plants, in the context of rapid radiations, mechanisms of species diversifications on the QTP, and the biogeographic significance of the QTP in the broader context of both the Northern and Southern Hemisphere biogeography. Integrative multidimensional analyses of phylogeny, morphological innovations, geography, ecology, development, species interactions and diversifications, and geology are needed and should shed insights into the patterns of evolutionary assembly and radiations in this fascinating region.

Keywords: QTP; Qinghai-Tibetan Plateau; allopatric speciation; biogeography; evolutionary radiations; vicariance.

FIGURE 1

Geographic location of the Qinghai-Tibetan Plateau (A) and representative plant groups: (B) Rhododendron roxieanum Forrest ex W.W.Sm.; (C) Rhododendron wardii W.W.Sm.; (D) Rheum nobile Hook.f. & Thoms., showing “glasshouse-like” morphology; (E) Saussurea laniceps Hand.-Mazz., showing “snow-ball” morphology;(F) Meconopsis betonicifolia Franch.; (G) Pedicularis siphonantha D.Don var. delavayi (Franch. ex Maxim.) P.C.Tsoong; and (H) Rhodiola alsia (Fröd.) Fu (photo credit: B,C,G by H. Sun; D,F by J. Wen; E by B. Song; and H by J. Q. Zhang).

FIGURE 2

Major patterns of biogeographic diversification of plants on the Qinghai-Tibetan Plateau: (A) the QTP as a biogeographic source area in Eurasia with examples of Gentiana sect. Cruciata (Wang et al., 2004) and Hippophae rhamnoides (Jia et al., 2012); (B) close biogeographic connections of the QTP with Mediterranean Eurasia, as exemplified by Mandragora (Tu et al., 2010); (C) Hengduan–Himalayan forest regions of the QTP showing close biogeographic relationships with Arcto-Tertiary floristic elements in eastern Asia, as exemplified by the radiation of Maianthemum on the QTP following the migration from the Sino-Japanese floristic region (Meng et al., 2008); (D) the QTP and western Cordillera forest region of western North American, as shown in Kelloggia (Nie et al., 2005b); (E) migration of the QTP plants from Central Asia, as inferred from Solms-laubachia (Yue et al., 2009); and (F) the QTP as a major biogeographic barrier for plant diversification in Eurasia, as shown by examples of two Mediterranean–eastern Asian disjunctions in Pistacia (Yi et al., 2008; Xie et al., 2014).