the rate of dispersal. But, the existence of relevant geographic/climatic connectivities through the relevant time period is critical to the general model and to specific outcomes. If, when climate change occurs, suitable corridors exist for the dispersion of species with relevant traits, these lineages will tend to track the habitat to which they are adapted and move into the region relatively quickly, perhaps preempting the in situ evolution of these traits among the natives. Under these circumstances, it is the sorting or filtering of species into the region that will dominate the assembly of the regional species pool.

On the other hand, if plants with relevant adaptations do not already exist in an area undergoing environmental change, and if corridors for the movement of relevant plants are missing at the right times and places, then adaptations will presumably evolve within the resident lineages. Oceanic islands, of course, provide classic cases of such isolation and of the ecological radiations of those species that did manage to establish [e.g., Carlquist (1974) and Baldwin and Sanderson (1998)]. But, as Ackerly (2003) has argued, isolation by distance can also occur in continental settings (e.g., isolated mountaintops, peculiar edaphic circumstances), and “environmental islands” that are discontinuous in realized environmental space from surrounding habitats can similarly promote adaptive divergence.

Major physiological/morphological adaptive shifts certainly have occurred, sometimes frequently, and sometimes even rapidly, perhaps especially in relative isolation. My argument is that under circumstances of habitat continuity that promote migration, movement into an area experiencing change is often likely to occur before resident species adapt to the circumstances. It is a question, then, of the interplay of historical circumstances either favoring or disfavoring the movement of lineages from habitats outside of a region, with the relative ease or difficulty of evolving the suitable ecological characters. If movement into the area is difficult, and the relevant adaptive shifts are easy, then in situ evolution will predominate. My assertion is simply that corridors have often been present, and that adaptations to major new climates have often been somewhat difficult.

The existence of corridors at the right times and places not only provides a mechanism for community assembly but also helps to explain PNC itself. Under these circumstances, stabilizing selection may be a key factor in maintaining ecological tolerances. If corridors did not exist and isolation were correspondingly increased, in situ evolution and local extinction would probably increase and PNC would be less commonly observed. That is, there is a connection between environmental connectivity and niche conservatism, on the one hand, and between isolation and convergent ecological adaptation on the other [cf. Wiens et al. (2006)]. In this sense, historically contingent connectivities have played a more