A Different Tempo of Evolution in Birds and their Parasitic Lice

A key question in the study of coevolution is the tempo and mode of evolution of the interacting partners. Is the association an ancient one, reflecting a long and intimate interaction between the two organisms, or is it a recent event due perhaps to a parasite colonising a new host? What is the relative rate of evolution between host and parasite? Answering these questions requires the comparison of evolutionary trees (phylogenies) for host and parasite (Figure 1). To the extent that the trees match, host and parasite have cospeciated, that is, parasite and host speciated at the same time. Mismatches between the host and parasite phylogenies signal processes other than cospeciation, such as host switching, speciation by parasites independently of their hosts, and parasite extinction (Page, Clayton et al. 1996). Because cospeciating taxa are, by definition, contemporaneous, we can compare amounts of evolutionary divergence in cospeciating pairs of hosts and parasites to measure relative rates of evolution in the two clades. Because host and parasite are often taxonomically distant (e.g., birds and insects), with differing generation times, population sizes, and metabolic rates, they are ideal systems to investigate the influence of these factors on rates of evolution (Page and Hafner 1996).