central role than we might have imagined in mediating the tradeoff of dispersal and evolutionary change, and, thereby, in generating the patterns of biodiversity that we see today.
A key question that has still received only limited attention, concerns the circumstances in addition to isolation that will favor adaptive niche evolution. Ackerly (2003), for example, highlighted the evolutionary opportunities that may present themselves along the “trailing edge” of a species that is shifting its geographic range in response to changes along a single environmental axis (e.g., temperature). This mechanism depends to some extent on a barrier preventing immediate occupancy by species that are appropriately adapted to the new conditions. Here, too, it is the balance of the ease of movement in relation to the ease of adaptation that determines the outcome.
I believe that the simple idea outlined above provides the basis for a much fuller understanding of global biodiversity patterns, and that phylogeny and PNC will therefore play a much larger role in explaining such patterns. Here, I briefly highlight three biodiversity phenomena in flowering plants that variously reflect the interaction of niche conservatism with environmental changes through the Tertiary (Fig. 13.1).
The first example concerns the disjunction of clades among the major tropical regions of the world. Although many of these cases, especially involving South America and Africa, have traditionally been attributed to the breakup of Gondwana, recent studies have shown that in many (perhaps most cases) the relevant lineage-spliting events are too young. Instead, there must have been movements among these regions after the breakup was well along. An example is provided by Malpighiaceae, which is most diverse in the New World tropics but has several lineages in tropical Africa (and Madagascar) and Asia. Analyses by Davis et al. (2002, 2004) demonstrated that this distribution reflects several disjunctions, in widely separated clades, and that these were, on the whole, too young to have been caused by the spreading of the Atlantic. The presence of fossils in several places in the Northern Hemisphere, where Malpighiaceae no longer occur, combined with evidence on paleogeography and the greatly expanded northern limit of tropical climates and vegetation in the Eocene [e.g., Fine and Ree (2006)], favor the movement of several lineages from the New World tropics into the Old World tropics through a Laurasian corridor of tropical climates in the early Tertiary.
The modern disjunct distribution is largely, therefore, a result of the southward retreat of these plants as tropical climates receded in the Oligocene and Miocene. The key point is that, perhaps despite “trailing edge”