A central tenet of evolutionary biology is that environmental change is the prime driving force behind speciation and the genesis of evolutionary novelty. While gradual, long-term environmental change may be an important stimulus for speciation, it is not clear that such change would generate highly divergent phenotypic, physiological, and life history attributes, and disparate ecological tolerances, as described in numerous angiosperm genera. I propose that niche shifts rather than long-term environmental change are the primary drivers of evolutionary novelty within genera. This argument is based on the differences in the dynamics of speciation through gradual change and in the dynamics of speciation through niche transitions. Ecological speciation is a prospective process that involves the invasions of new niches in space. Geographical or vicariant speciation is a responsive process involving changes dictated selectively or randomly by environmental gradients in time. Niche conservatism may limit major adaptive shifts in time.