 Relatedly, for heredity to be a part of evolution, there must be  replication  of entities, or at least reproduction (with the former being a special case of the latter that involves copying). In order to allow for a more general theory of evolution, many authors will speak of replicators (or reproducers) and vehicles (or interactors) rather than the more limited and specific terms “genes” and “organisms”. With these terms in hand, one can more easily begin to discuss (as many have) questions over  units and levels of selection:  does selection occur at the level of the gene, the organism, the group, the species, or all of the above? These units of selection (replicators/reproducers or vehicles/interactors) are often taken to be biological individuals (see the entry on  the biological notion of individual)  as a necessary condition for being units of selection at all. 