Are there many different routes to becoming a global biodiversity hotspot?

A major question in evolution and ecology is why biodiversity is so unevenly distributed across the planet. The most obvious and salient diversity pattern is the order-of-magnitude greater species richness in the tropics compared with the temperate zones. Superimposed on this latitudinal diversity gradient is a much more complex and intricate pattern of regional andmore local biodiversity hotspots (1, 2). These are places with unusually high concentrations of species and especially endemic species. Documenting these patterns is of great significance, most obviously for conservation, with ever more sophisticated and higher-resolution biodiversity hotspot maps becoming available as the world’s biota aremapped in greater detail (3). More fundamentally, we need to document hotspots to understand the underlyingmacroevolutionary and ecological processes shaping the distribution of diversity across the Earth. Most globally significant biodiversity hotspots lie firmly within the tropics or in the Mediterranean climate zones of the world, such as California and the Cape Floristic Region of South Africa. Several coincide with major mountain ranges, including most notably the tropical Andes and the temperate Hengduan Mountains, two of the world’s hottest hotspots (2, 4). It is this Hengduan Mountain hotspot that is the focus of Xing and Ree’s study in PNAS (5), which provides the first integrated analysis of the evolutionary origins and biotic assembly of Hengduan plant diversity. Although formal quantification of biodiversity hotspots really only started 30 y ago (1), the outstanding plant species richness of the HengduanMountains has been known for more than 150 y, having been first revealed by the intrepid 19th century plant collectors—Joseph Hooker, Ernest Wilson, George Forrest, Frank Kingdon-Ward, and others. These explorers penetrated into the deeply dissected and remote mountains and river gorges of Yunnan, Sichuan, Sikkim, and eastern Tibet in search of botanical novelties and especially garden plants, and discovered a veritable treasure house of rhododendrons, primulas, lilies, Himalayan blue poppies, and many other plants that make up the spectacular plant species diversity of the Hengduan hotspot (6) (Fig. 1). Although the species richness and endemism of the Hengduan hotspot is well established, the evolutionary origins of its unique biota remain poorly understood. Biotic assembly is the product of colonization from outside areas, local recruitment via circa situm adaptation, in situ speciation, and extinction. While extinction remains extremely challenging to measure, time-calibrated molecular phylogenies can provide insights into the origins of taxa that make up the biota of a particular area, allowing inferences about geographical origins, colonization, and in situ species diversification, and the timing and tempo of these processes. For the Hengduan region, two problems have hindered such analyses up to now. First, previous studies have relied on single-taxon phylogenies, which can provide only limited insights. Second, the complex relationships and distinct uplift histories of the Qinghai–Tibetan Plateau, the main Himalayan range, and the Hengduan Mountains have been confounded and widely misinterpreted (7). The Fig. 1. Shrubby Rhododendron traillianum and Picea likiangensis var. balfouriana forest at 4,300-m elevation, close to the tree line at Shangri-La Great Snow Pass, in the Hengduan Mountain biodiversity hotspot, Yunnan, China. The genus Rhododendron is one of the 19 plant lineages analyzed by Xing and Ree (5), and includes two species-rich subclades dominated by species from the Hengduan Mountains, which show elevated rates of species diversification dating from the Late Miocene coinciding with the rapid uplift of the Hengduan Mountains. Photo courtesy of Jian Huang (Xishuangbanna Tropical Botanical Garden, Yunnan, China).