The Ixodes ricinus species complex is a group of ticks distributed in almost all 
geographic regions of the world. Lyme borreliosis spirochetes are primarily 
transmitted by tick species within this complex. It has been hypothesized that 
the Lyme vector ticks around the world are closely related and represent a 
monophyletic group. This implies that vector competence in ixodid ticks for Lyme 
agents might have evolved only once. To test this hypothesis, we used a 
molecular phylogenetic approach. Two fragments of mitochondrial 16S ribosomal 
deoxyribonucleic acid were sequenced from 11 species in the I. ricinus complex 
and from 16 other species of Ixodes. Phylogenetic analysis using Bayesian 
methodology indicated that the I. ricinus complex is not a monophyletic group 
unless 3 additional Ixodes species are included in it. The known major vectors 
of Lyme disease agents in different areas of the world are not sister taxa. This 
suggests that acquisition of the ability to transmit borreliosis agents in 
species of Ixodes may have multiple origins.