Evolution and plasticity can drive population-level phenotypic change (e.g., changes in the mean phenotype) on time scales comparable to changes in population densities.  However, it is unclear if phenotypic change has the potential to be just as fast as changes in densities, or if comparable rates of change only occur when densities are changing slow enough for phenotypes to keep pace. Moreover, it is unclear if this depends on the mode of adaptation. Using scaling theory and fast-slow dynamical systems theory, we develop a method for comparing maximum rates of density and phenotypic change estimated from population-level time series data.  We apply our method to 30 published empirical studies where changes in morphological traits are caused by evolution, plasticity, or an unknown combination.  For every study, the maximum rate of phenotypic change was 0.5 to 2.5 times faster than the maximum rate of change in density.  Moreover, there were no systematic differences between systems with different modes of adaptation.  Our results show that plasticity and evolution can drive phenotypic change just as fast as changes in densities.  We discuss the implications of our results in terms of the strengths of feedbacks between population densities and traits.

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