Figure 1. Processes in a host–parasite association. The host and parasite may speciate at the same time (a) or the parasite may speciate independently of its host (b, c). If independent speciation occurs, then one or more of the descendant parasites may colonize a new host (b) or the parasite may remain on the original host (c). Absence of a parasite from a host where it would be expected to occur may be due to extinction of that parasite (d), or the ancestors of the host lineage may not have inherited the ancestral parasite (e).

Figure 2. Three null hypotheses concerning host–parasite cospeciation. First (a) the topologies of the host and parasite trees are identical, although the rates of evolution and speciation times may differ. Second (b) given that the topologies are the same, the relative speciation times are identical, even if the rates are different in the two clades. Third (c) is the topologies, speciation times, and rates are identical in host and parasite. Huelsenbeck et al. (1997).

Figure 3. Phylogenies for pocket gophers and their chewing lice parasites with branch lengths proportional to number of substitutions at the third codon position in the mitochondrial DNA COI gene. Pocket gopher genera are Orthogeomys, Pappogeomys, Cratogeomys, Geomys, Thomomys and Zygogeomys; louse genera are Geomydoecus and Thomomydoecus. Thin lines connect gophers with their species-specific parasite. Inset shows a louse clasping a gopher hair shaft. From Hafner et al. (1994).