presumably having had ample opportunity to do so with the expansion of temperate climates (or the contraction of the tropics) during the Tertiary (Fine and Ree, 2006). The implication is that it is not so easy to evolve tolerance to freezing temperatures and highly seasonal environments. In fact, such adaptations do appear to require a coordinated set of physiological adjustments, including (in most cases) biochemical changes to maintain a fluid lipid layer for the rapid transfer of water out of cells to avoid ice crystals forming in the cytoplasm, and the deployment of special proteins and sugars to stabilize membranes when cells become desiccated and condensed (Sakai and Larcher, 1987; Körner, 2003).

Overall, my impression of ecological traits is that they show the same wide range in evolutionary lability as do morphological traits. On one end of the spectrum there are extremely labile traits of the sort that ecologists and population geneticists have tended to concentrate on. On the other end there are ecological traits of the sort I have highlighted above, which are evolutionarily much more conserved, perhaps because their evolution entails the modification of complex, highly integrated physiological systems. Recent phylogenetic studies have simply focused new light on the existence and the global importance of key ecological traits at the more conservative end of this distribution. My concentration here on the conserved end of the distribution is in no way meant to question the reality or significance of highly labile ecological traits, the evolution of which surely underlie many ecological adjustments.

What Harvey and Pagel (1991) termed PNC refers to the expectation that, all else being equal, related species will tend to occupy the same sorts of environments (Wiens and Graham, 2005). PNC is not meant to imply that ecological barriers are insurmountable, or even that niche shifts are rare. Niche evolution obviously occurs, and may even be the norm under certain circumstances [e.g., Losos et al. (2003)]. However, PNC, to the extent that it exists, does imply that speciation can occur regularly without major niche shifts [e.g., Peterson et al. (1999)]. Furthermore, under the assumption that opportunities to undergo major shifts have regularly presented themselves, PNC implies that these opportunities have not been seized so readily that high-level eco-phylogenetic correlations are completely overridden by ongoing low-level niche shifting. Although, strictly speaking, it is not necessary to link PNC with the view that there are constraints on niche evolution, I believe that it is the relative difficulty of making major ecological shifts that explains the patterns I have highlighted above, and which, as discussed below, render PNC especially consequential in explaining the distribution of biodiversity.