Cospeciation (1 of 3)
A species of louse lives on a species of gopher. When the gophers get together to mate, the lice get an opportunity to switch gophers and perhaps mate with lice on another gopher. Gopher-switching allows genes to flow through the louse species.
Cospeciation (3 of 3)
When lineages have cospeciated, the parasite phylogeny will "mirror" the host phylogeny. This example is somewhat idealized rarely do scientists find hosts and parasites with exactly matching phylogenies. However, sometimes the phylogenies indicate that cospeciation did happen along with some host-switching.