Extinction is a dominant force shaping patterns of biodiversity through time however its role as a catalyst of speciation has been overlooked. Here, we synthesize ideas alluded to by Darwin and others into the model of ‘speciation-by-extinction’ in which speciation results from the extinction of intermediate populations within a single geographically variable species. We explore the properties and distinguishing features of speciation-by-extinction with respect to other established speciation models. We demonstrate its plausibility by showing that the experimental extinction of populations within variable species can result in speciation. The prerequisites for speciation-by-extinction, geographically structured intraspecific variation and local extinction, are ubiquitous in nature, indicating that speciation-by-extinction may be a prevalent, but underappreciated, speciation mechanism.