One of the main impediments in studies of the evolution and phylogeny of Siphonaptera is morphological specialization related to their ectoparasitic way of life (Whiting, 2002a). In particular, this specialization is reflected in the peculiar morphology of the head, thorax and genitalia. On the one hand, specialized characters are not especially informative for use in phylogenetic reconstructions. On the other hand, the sharing of these specialized characters by practically all flea species strongly suggests monophyly of the order. Indeed, the monophyletic origin of fleas is supported by both morphological (e.g. Medvedev, 2003a, b) and molecular evidence (Whiting, 2002a, b). Although many authors agree on the monophyly of Siphonaptera, there is still no consensus on other questions related to their phylogeny and the origin of their parasitism. What are the relationships of fleas with other insect taxa? Did specialized features such as winglessness, laterally compressed body and locomotory apparatus allowing jumping originate as adaptations to ectoparasitism or, alternatively, were they characteristic of flea ancestors and are thus pre-adaptations to parasitism (see Medvedev, 2005)? Did fleas coevolve with their hosts? What was the main driver of diversification of this order? In this chapter, I review these questions and summarize the relatively limited knowledge on the evolutionary history of fleas.

Ancestral and sister taxa

Although fossil fleas are extremely rare, there are several finds of fleas from the Baltic and Dominican amber.