Biotic responses to Pleistocene climatic fluctuations have traditionally been analyzed in the context of glacial-interglacial cycles on the scale of 10000-100 000 years. However, emerging evidence indicates that short-term, high-amplitude, climatic 'flickers', close to the limits of the resolving power of the fossil record, occurred within the glacial and interglacial substages. Because species shift geographically in response to the climate flickers, community structures are fluid, with changes absorbed ecologically and not mediated macroevolutionarily. The rapidity of these shifts may also explain anomalous fossil assemblages.