opportunities, these plants maintained their tropical physiological tolerances and moved out of northern regions as climates shifted, as opposed to adapting to colder, seasonal climates and maintaining a northern distribution. The same sort of explanation, involving tropical niche conservatism, applies to other major tropical disjunctions, including subgroups within Melastomataceae (Renner et al., 2001) and Lauraceae (Chanderbali et al., 2001), the latter probably moving from east to west through the boreotropics [for possible additional examples see Krutzsch (1989), Lavin and Luckow (1993), Richardson et al. (2004), Pennington et al. (2006)].
A second example is in some ways a mirror image of the first. This concerns plants that were able to adapt to cold, highly seasonal environments in the north. The lineages that did manage this transition were able to move (often repeatedly and at different times) around the Northern Hemisphere, especially as a major corridor through Beringia opened and closed with fluctuating climates (Donoghue and Moore, 2003). Ultimately, these movements yielded the very common disjunctions between the temperate forests of eastern Asia and eastern North America (Wen, 1999; Milne and Abbott, 2002; Donoghue and Smith, 2004.
Here, too, it is important to appreciate the dual role played by PNC. First, recall that only some originally tropical lineages managed to make the transition to temperate climates (Latham and Ricklefs, 1993; Judd et al., 1994). Malpighiales provide an example (Davis et al., 2005). As noted above, Malpighiaceae remained restricted to tropical climates, and the same is true of most other major lineages within Malpighilaes. Only a few lineages made it out of the tropics and have been successful in the northern temperate zone, including violets (Violaceae), willows (Salicaceae), and St. John’s worts (Hypericaceae). As suggested above, this may relate to the complexity of the cell-level adaptations necessary to tolerate frost. Second, just as only some tropical lineages adapted to temperate climates, only some temperate plant lineages adapted to even colder climates that would have allowed their continuous distribution through Beringia, even today. It is also noteworthy that few temperate-adapted plant lineages appear to have moved back into truly tropical climates, although in this case competition may also have played an important role.
One final aspect of this case is noteworthy. There is still little phylogenetic evidence for the argument [see Latham and Ricklefs (1993)] that the majority of transitions from the tropics into the northern temperate zone took place in Asia. However, it is noteworthy that “out of Asia” biogeographic inferences have been the most commonly reported to date and appear to well outnumber originations in North America and subsequent movement to Asia (Donoghue and Smith, 2004). The continuous connection that existed in Asia between tropical and temperate climates and vegetation types throughout the Tertiary, and the great complexity of