 An alternative approach to stating conditions for natural selection involves attention to replicators, of which genes are the paradigm instance. This approach was motivated by the discovery of genetic variations that spread despite not being conducive to the reproduction of the organisms that bear them, for instance, genes that exhibit meiotic drive. Replicator selectionism, which makes genes/replicators the units of selection, was developed in contrast with the Lewontin-style approaches, which make larger entities, organisms and even species, units of selection (see entry on  units and levels of selection).