vines, succulents, and perennial and annual herbs, which grow from tidal to alpine zones, from the equator nearly to the poles. He concluded from such cases that “the obvious adaptive changes that can take place mostly occur so easily and frequently that they tend to mark species and genera rather than larger groups” (Cronquist, 1968).
Ledyard Stebbins, whose 1974 book on the macroevolution of flowering plants dominated discussions for decades, held much the same view. That is, he argued that owing ultimately to limited functional and developmental integration in plants, vegetative traits related to climate tolerances were highly labile and only rarely marked higher taxa. In fact, his main thesis depended directly on the rapidity with which transitions between major climate zones could occur. He argued that major evolutionary changes occurred in ecotones or climatically marginal zones, and that tropical rain forests were therefore “museums,” not “cradles.” As he appreciated, this required “extreme ecological plasticity” and genetic adaptation to moister or drier climates with “relative ease” (Stebbins, 1974).
Growing up with these views, and having passively accepted them, I remember being surprised by several later findings. Whereas I fully expected reproductive traits to show less homoplasy than vegetative traits that seemed to be linked more directly to climate variables (e.g., leaf margins, pubescence), a meta-analysis of homoplasy in published plant phylogenies failed to demonstrate such a difference (Donoghue and Sanderson, 1994). In fact, levels of homoplasy in phylogenetic studies rarely seem to correspond to standard intuitions about lability or selective value. Instead, homoplasy seems to be positively correlated primarily with the number of terminal taxa included in analyses [e.g., Sanderson and Donoghue (1989)] and limitations on the number of character states (Donoghue and Ree, 2000).
Even more surprising was the finding by Campbell Webb (2000) that the trees occupying 0.16-hectare plots in a rainforest in Borneo seemed to be more closely related to one another than expected by chance assembly from the regional species pool of 324 tree species. Given that these plots differed with respect to key environmental variables (some were located in swampy areas, some on ridge tops, etc.), one interpretation is that there are subtle, previously underappreciated, ecological similarities among related plants, both above and below the traditional rank of family. In retrospect, this can be reconciled with Cronquist’s observations simply by allowing that shared ecological niche characteristics exist, but that the phylogenetic distribution of these does not correspond well with particular named clades or taxa assigned to a given taxonomic rank.
In general, comparisons centered on taxonomic ranks have hidden the connections between phylogeny and ecology. In fact, some tradi-