A concept originally proposed by Ernst Mayr (1904-2005) in 1942 in his discontinuous theory of genetic revolution by allopatric speciation to account for rapid biological evolution in a way that the phyletic gradualism of the Modern synthesis could not. The starting point is that a few individuals within a gene pool (i.e., the founders) become geographically isolated (and it could be even just one pregnant female). These isolated and marginal individuals subsequently evolve a gene pool via genetic drift that is sufficiently distinct from the original one to prevent interbreeding between pools, with behavior forming an important isolating mechanism in this respect. In this way, a new species might evolve. The effect, while demonstrated in laboratory studies of Drosophila flies, has been subjected to a number of criticisms (e.g., it is a relatively infrequent event in biological evolution).
See Biological evolution, Cultural evolution, Genetic drift (or random walk), Lineage, Modern synthesis, Phyletic gradualism and punctuated equilibrium, Speciation