The evolution of phylogenetic specificity among parasitic organisms.

It has long been recognized that, in some cases, the evolution of parasites seems to follow closely that of their hosts, yet in the literature dealing with host-parasite evolution, the causes behind this Phylogenetic specificity (co-evolution) are rarely addressed, the term 'parasite' is seldom clearly defined, and the diversity inherent in the nature of parasitism means that our 'parasitophyletic rules' have often come under fire. In this article, Ian Humphery-Smith tries to give order to the discussion of this topic and outlines what sort of host-parasite relationships are most likely to produce Phylogenetic specificity.