The concepts of niche conservatism and adaptive radiation have played central roles in the study of evolution and ecological diversification. With respect to phenotypic evolution, the two processes may be seen as opposite ends of a spectrum; however, there is no straightforward method for the comparative analysis of trait evolution that will identify these contrasting scenarios. Analysis of the rate of phenotypic evolution plays an important role in this context and merits increased attention. In this article, independent contrasts are used to estimate rates of evolution for continuous traits under a Brownian motion model of evolution. A unit for the rate of phenotypic diversification is introduced: the felsen, in honor of J. Felsenstein, is defined as an increase of one unit per million years in the variance among sister taxa of lntransformed trait values. The use of a standardized unit of measurement facilitates comparisons among clades and traits. Rates of diversification of three functional traits (plant height, leaf size, and seed size) were estimated for four to six woody plant clades (Acer, Aesculus, Ceanothus, Arbutoideae, Hawaiian lobeliads, and the silversword alliance) for which calibrated phylogenies were available. For height and leaf size, rates were two to Ϸ300 times greater in the Hawaiian silversword alliance than in the other clades considered. These results highlight the value of direct estimates of rates of trait evolution for comparative analysis of adaptive radiation, niche conservatism, and trait diversification.adaptive radiation ͉ leaf size ͉ seed size ͉ plant height ͉ niche conservatism O ne of the central goals at the intersection of ecology and phylogenetics is to understand the tempo and mode of evolution for traits related to coexistence, niche, and ecological strategies. How fast do ecological differences between species evolve? Under what conditions is rapid versus slow evolution observed? What is the relative influence of climate change, isolation, and biotic interactions on ecological diversification? What is the sequence of ecological changes during speciation and adaptive radiation? These questions lie at the core of two closely related research areas: the study of adaptive radiations and the study of niche conservatism. In many ways, adaptive radiation and niche conservatism represent two ends of one spectrum, describing patterns (and underlying processes) of rapid divergence and convergent evolution versus slow divergence and phylogenetic conservatism, respectively. The two phenomena may also be seen as outcomes of the same underlying factors, albeit acting in opposite directions. Adaptive radiations are promoted by ecological opportunities that may arise after release from biotic pressures (especially competition), transition into novel adaptive zones, or colonization of island and islandlike environments (1). (In this article I focus on the adaptive component of adaptive radiation-the diversification of ecological traits-and not on the rates of lineage diversification.) Ni...