scales historical factors such as speciation, extinction, and biogeographic boundaries become highly relevant.
A less well appreciated role of phylogenetic niche conservatism, and the one that I especially want to highlight here, concerns the composition of the regional species pools from which local communities become assembled. In my view the most compelling studies to date along these lines have been carried out in Mediterranean climate systems, by Miguel Verdu et al. (2003) and David Ackerly (2004), both building on the seminal work of Carlos Herrera (1992).
I will focus here on Ackerly (2004), as his study concerned a single region, the California chaparral. His analyses of a number of signature woody chaparral clades suggested that in many cases the characteristic adaptive traits were evolved well before the advent of the Mediterranean (summer dry) climate some 3 million to 5 million years ago. Plants with these characters (or their similar precursors) were either present in the vegetation of that region beforehand, or they moved in from outside the region, perhaps from dry areas further to the east and south (“mexical” vegetation), as opposed to evolving in situ in response to the changing climate. That is, many characteristic elements of the flora (e.g., Ceanothus, Arctostaphylos), and consequently many of the traits that we associate with chaparral (e.g., thick, sclerophyllous leaves), resulted from the sorting of species with relevant adaptations already in place, which then adjusted to the new rainfall regime by shifts in the timing of growth and reproduction. In a few lineages (e.g., Adenostoma, Heteromeles), however, chaparral leaf features probably did evolve in situ. Interestingly, many of the elements that filtered into the flora have Madrean-Tethyan affinities (Axelrod, 1975) and were preadapted to hotter, arid conditions, whereas those showing in situ adaptation derive mainly from temperate Northern Hemisphere lineages. In other words, it appears that the composition, structure, and function of the chaparral as we know it today reflects the characteristics of prechaparral vegetation and the biogeographic source areas that were accessible as the climate materialized.
These studies suggest what may be a general principle, namely that under many circumstances it may be easier for species to migrate into an area with at least some of the relevant adaptations having already evolved, than it is for those adaptations to evolve in place. This formulation highlights two key variables, namely the rate of trait evolution in relation to