Cospeciation (2 of 3)
When the gopher lineage splits into lineages A and B, lice have few opportunities for gopher-switching, and lice on gopher lineage A don't mate with lice living on gopher lineage B. This "geographic" isolation of the louse lineages may cause them to become reproductively isolated as well, and hence, separate 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.