Cospeciation describes the symbiotic relationship of one species that populates in response to another species. This primarily happens through parasite/host relationships (Ricklefs, Fallon & Bermingham, 2004).
For example, when gophers mate, the lice on the gophers get the chance to transport to the other gopher and mate with lice on that gopher.
Ricklefs, R. E., Fallon, S. M., & Bermingham, E. (2004). Evolutionary relationships, cospeciation, and host switching in avian maleria parasites. Syst Biol,53(1), 111-119.