Coevolutionary relationships of lice and their hosts: a test of Fahrenholz's rule

The traditiona view of louse phylogeny as being governed by phyletic tracking of the hosts (Fahrenholz's Rule) underpins much of louse systematics. This relaince on host relationships prevents rather than aids studies of coevolution. To determine the full implication of Fahrenholtz's rule predictions of the phyletic tracking model are tested against observations of louse specificity and host relationships, considering especially a cladogram of 350 louse species derived indipendently of host information. Predictions of strict host cospeciation and failure of lice to colonize novel hosts are shown to be falsified in a number of cases, and the model fails to explain a minimum of 20.7 per cent of louse speciation events in the history of the 350 species analyzed. The resource tracking model, sometimes suggested as an alternative to the phyletic tracking model, is considered as the other extreme to a continuum of host-parasite relationships. The position of any parasite on this continuum is governed by a number of factors , which are discussed. In view of its failure to explain all host-aprasite associations, Fahrenholz's rule can have no value as a precise tool in phylogenetic reconstrution for lice and their hosts.