Molecular evolution is questioning the way we think about the evolution of complex traits:

DNA evidence challenges the assumption that parasitic lice share a single common ancestor, suggesting that parasitism evolved twice independently in this group of insects. Parasitic lice infecting mammals and birds were believed to be the only major group of insects in which all members were permanent parasites. New data shows that parasitic lice evolved twice independently from a non-parasitic bark louse ancestor. This new group includes members with wings, raising the intriguing possibility of flying lice. This challenges two common assumptions - that parasitism evolved just once in lice, and that all lice are flightless.

A major fraction of the diversity of insects is parasitic, being plant herbivores, parasitoids, or vertebrate ectoparasites. Understanding this diversity requires information on the origin of parasitism in various insect groups. Parasitic lice (Phthiraptera) are the only major group of insects in which all members are permanent parasites of birds or mammals. Lice are classified into a single order but are thought to be closely related to or derived from book lice and bark lice (Psocoptera). Here we use sequences of the nuclear 18S rDNA gene to investigate the relationships among Phthiraptera and Psocoptera and identify the origins of parasitism in this group (termed Psocodea). Maximum likelihood, Bayesian maximum likelihood, and parsimony analyses of these data indicate that lice are imbedded within the psocopteran infraorder Nanopsocetae, making the order Psocoptera paraphyletic (i.e. does not contain all descendents of a single common ancestor). Furthermore, one family of Psocoptera, Liposcelididae is identified as the sister taxon to the louse suborder Amblycera making parasitic lice (Phthiraptera) a polyphyletic order (i.e. descended from two separate ancestors). We infer from these results that parasitism of vertebrates arose two times independently within Psocodea, once in the common ancestor of Amblycera and once in the common ancestor of all other parasitic lice.