the landscape, may have promoted both the generation and the maintenance of higher species diversity in eastern Asia as compared with North America or Europe (Latham and Ricklefs, 1993; Qian and Ricklefs, 1999, 2000; Xiang et al., 2004).
My third example concerns plants adapted to high-elevation alpine conditions. Historical biogeographic analyses of several clades that have succeeded in mountainous areas around the world indicate that these originated in the Northern Hemisphere, often in Asia, moved around the Northern Hemisphere, and then, much more recently, moved into South America, where they have radiated in the Andes. Valerianaceae provide an example (Bell and Donoghue, 2005; Moore and Donoghue, 2007). These plants appear to have originated in the Himalayas, to have moved to North America, and then into South America, perhaps on the order of 6 million to 8 million years ago, not long before the physical connection was established. In the Andes there are now ≈150 species of Valeriana sensu lato. Similar patterns of movement and diversification have been documented in Gentianella (von Hagen and Kadereit, 2001), Halenia (von Hagen and Kadereit, 2003), and Lupinus (Hughes and Eastwood, 2006), to name a few cases.
Two aspects of this case warrant comment. First, this pattern provides perhaps the clearest case of the movement of preadapted plants into a region as physical changes created new climates. Owing to the formation of the corridor and the dramatic rise of the Andes in the same time frame, movement into these habitats was evidently easier than the evolution of alpine adaptations in situ. However, it remains unclear how many resident South American plant lineages living at lower, warmer elevations, gave rise to Andean alpine plants (e.g., Espeletia, Asteraceae; Puya, Bromeliaceae). Second, it is noteworthy that a number of these movements into the Andes appear to have been accompanied by an upward shift in diversification rate. One obvious factor is geographical complexity and its promotion of allopatric speciation, at least in part as a function of PNC (Wiens, 2004). But, another important factor may have been the relative lack of occupancy by South American plants, owing in part to the relative difficulty of evolving the necessary traits in situ. In exploring this case further, however, it will be important to bear in mind differences between adapting to tropical highlands versus cold climates at high latitudes [see Janzen (1967) and Körner (2003, 2007)].
Explanations for major patterns in the distribution of biodiversity have traditionally tended to focus on environmental correlations and local determinism. With the rapid rise of phylogenetic knowledge, a growing