tional taxonomic families are generally associated with particular environments; for example, Poaceae (the grasses) in grasslands, Ericaceae (the heaths) in heathlands, and Zosteraceae (the sea grasses) in the sea. The plant clade I know the best, the Dipsacales, ancestrally occupied northern temperate forests, but Dipsacaceae have specialized in drier habitats, especially around the Mediterranean basin, and Valerianaceae have adapted to alpine habitats (Donoghue et al., 2003). But there are also ecologically distinctive clades within traditional families: multiple mangrove lineages, dry-adapted Euphorbiaceae, and bamboos and multiple C4 lineages within grasses, to name just a few examples. Likewise, there are ecologically distinguishable clades comprised of a number of related families. Core Caryophyllales provide an example, mostly being adapted to arid or high-salinity habitats (e.g., the “portulacaceous alliance” of families, which includes the Cactaceae). The several insectivorous families within Carophyllales sensu lato present another clear case, as do those of the “aquatic clade” within Alismatales, and the parasitic plants of Santalales.
Moreover, it is important to appreciate that the findings of Webb (2000) and others (see below) do not depend on all members of a clade occupying the same habitat, but rather on a tendency for members of a clade to be more similar with respect to the environments that they occupy. From this perspective the link between phylogeny and ecology becomes even clearer. After all, major clades within angiosperms, despite significant ecological diversity, hardly occupy all possible environments, but instead are restricted to one or a few major biomes, such as tropical rainforests, temperate forests, grassland, or deserts. Despite the great variety of environments occupied by the Asteraceae, for example, they are far more common in arid environments than they are in tropical forests. Such tendencies are well known to field botanists, but have only recently been subjected to analysis. In the most comprehensive study to date, Prinzing et al. (2001) demonstrated far higher than expected levels of niche conservatism in the plants of Central Europe.
Such ecological correlations (especially those at the level of major clades) imply that evolutionary shifts from one ecological setting into another, where these require substantial physiological adjustments, are not so readily accomplished as one might have imagined, homoplasy in ecological traits notwithstanding. Consider one example: the evolution of cold tolerance. Many plant lineages have managed to adapt to cold, highly seasonal climates, but it is also true that only a subset of ancestrally tropical plant lineages have succeeded in making this transition (Judd et al., 1994). That is, many tropical plant groups are strictly tropical [e.g., half of the families of flowering plants have no temperate representatives; Ricklefs and Renner (1994)] and have not spread out of the tropics despite