Phylogeny and classification, origins and evolution of host associations of lice

Lice are highly successful ectoparasites. Most species of mammal and bird are infested by at least 1 but up to 6 species of lice. Current opinion is that lice evolved from free-living Psocoptera (booklice, barklice and psocids). It is generally agreed that there are four main groups of lice: Anoplura, Amblycera, Ischnocera, and Rhyncophthirina. In contrast there is no agreement on the phylogenetic relationships of these groups and their classification. In particular, there is much debate over the validity of the taxon Mallophaga, which is almost certainly paraphyletic. For many years the sister group of the Boopiidae, which almost certainly exclusively infest Australian marsupials, was thought to be a group of lice that now infest marsupials in South America. This, however, is almost certainly incorrect; the sister group of the boopidae almost certainly contains bird infesting lice from the Menoponidae (Amblycera). Thus, menoponid lice transfered from birds to mammals and from these arose the Boopiidae. Transfers of lice between mammals and birds have occured on other occasions during the evolution of the lice.; 2 of the 4 main groups of lice, the Ischnocera and the Amblycera, contain families that infest birds and families that infest mammals. Strict cospeciation and coevolution was thought to predominate amoung the lice; however, detailed studies show this to be incorrect. Consequently the axiom that lice and their hosts invariably coevolve should be abandoned. Ironically biologists may learn more about the evolutionary biology of hosts when host switching has occured. Some evidence exists for competition between species of lice; this interaction may determine whether or not the transfer of a species of louse to an atypical host (a potential host switch) is successful. Thus the extinction of populations of lice (that result in uninfested hosts) may fascilitate host switching and perhaps the evolution of new taxa of lice. In contrast the extinction of hosts unfortunately often leads to the extinction of species of lice.