The speciation of parasites follow the general rules of speciation: if two populations of the same species are isolated from each other, they can diverge genetically because of different selective pressures or simply by genetic drift if the populations are of small size. Isolation is always necessary to prevent gene flows.
The isolation may be achieved by a physical barrier (mountain range, sea, etc.) which causes allopatric speciation. This process is certainly the more common.
When a parasite – host system is fragmented in two or more parts, for instance by geologic events, does the host populations diverge faster than the parasite, or the parasite faster than the host, or do they diverge synchronously? There are no satisfactory answers to these questions. One can only suppose that, in the majority of cases, the parasites diverge (and finally speciate) faster than the hosts, because, during the time the populations are separated, the number of generations of the parasite is greater...