system ecologists [however, see Edwards et al. (2007)]. It is possible that knowledge of phylogeny is less relevant in these areas, but it seems more likely that the most productive intersections have not yet crystallized. Recent work on community phylogenetics, and emerging ideas on the integration of historical biogeography in studies of biodiversity, may be yielding a key principle governing the historical assembly of communities, which could in turn provide the basis for a new synthesis of phylogeny and ecology. Put simply, it may often be easier for lineages to move than it is for them to evolve. My essay is an attempt to better characterize this principle and explore some of its consequences for global diversity patterns and the future of biodiversity.
When I began studying phylogenetic systematics in the late 1970s, it was widely believed that ecologically important traits were too labile to be of much use in phylogenetic inference. The feeling was that such characters were so prone to homoplasy that they would be positively misleading about relationships; instead, one should concentrate on characters that lack obvious functional value [Mayr (1969) called this the “Darwin principle”]. The rapid rise of the use of molecular data was partly a function of the belief that molecular characters were less subject to selection and would therefore better reflect evolutionary history. Arguments were made against this view on the grounds that it was hard to judge the phylogenetic value of characters at the outset of an analysis [e.g., see Hennig (1966)]. However, in general, such arguments had rather little impact on the overall mindset; homoplasy was viewed as bad for phylogenetics and “ecological traits” were viewed as especially prone to homoplasy.
Where did this view come from? Within systematic biology one line of reasoning was that groups that were placed at higher taxonomic ranks (e.g., families, orders) showed little uniformity with respect to the environments that they occupied. This was said to be especially true in plants. For example, Arthur Cronquist, the prime architect of angiosperm classification in that era (from the 1960s through the 1980s), pointed out repeatedly that higher taxa were not readily characterized by particular ecological roles: “Each of the obvious ecological niches for land plants is occupied by species representing diverse families and orders…. Conversely, a single family may fill widely varying ecological niches” (Cronquist, 1968). Instead, higher taxa tended to be more uniform in minute details of their flowers and fruits, which remained constant through transitions into disparate environments. Cronquist referred especially to the Asteraceae (sunflower family) for support, noting that its members are marked by totally obvious reproductive characters, but vary from being trees, to shrubs,