We consider four well-defined co-evolutionary event types: cospeciation - where the host and parasite speciate together (and which is generally to be maximised, in accordance with Farenholz’s rule), duplication - where the parasite lineage speciates independently of its host, lineage sorting, where a parasite lineage does not follow an available host lineage (including extinctions), and host switching - where a parasite moves from one host lineage to another. This is equivalent in the gene/organism question to horizontal transfer.
