 Once it is accepted that more than one inheritance system or, alternatively, more than one independently inherited replicator may be involved in the reproduction of organisms (let alone multiple kinds of replicators), questions about units of selection have to be addressed. As each inheritance system can lead to hereditary variations, there may be multiple lineages related to the production of a single organism and even single phenotypic traits. Evolution may happen in each lineage, and, in particular, each lineage may be “tracked” by natural selection.