  Simpson (1943) and others built even more historicity into the concept. These are all, of course, intended as definitions of the species category, and they attempt to provide a test (or a ‘yardstick’: Mayr 1976 479) that in principle will permit a researcher to decide whether a group of individuals should all be identified by a single species-level concept such as ‘homo sapiens’. The test for species membership is the capacity to interbreed; the test distinguishing two species is incapacity to interbreed. Dobzhansky makes the importance of this test transparent—the transition from a single interbreeding population to two reproductively isolated ones is the process of speciation.