Figure 2: Diversification of Borrelia garinii and Borrelia bavariensis in Eurasia demonstrates how evolutionary changes impact the ecological interactions.

A. The sister species B. garinii and B. bavariensis diverged at node i as a result of specialization on different sets of host species with B. garinii specializing on bird species and B. bavariensis specializing on small mammals. Subsequent B. bavariensis divergence into two populations at node ii occurred due to adaptation to a novel vector, Ixodes ricinus, allowing B. bavariensis to invaded western Europe. B. The distributions of I. ricinus (blue) and I. persulcatus (yellow) overlap in eastern Europe (green) where the eastern B. bavariensis population predominates. The host and vector switches that have occurred during the evolutionary history of these Borrelia species has facilitated range expansion and increased the diversity of their ecological interactions. (Species ranges as described in Stanek et al. 2012¹⁵⁸. Adapted with permission from the European Concerted Action on Lyme Borreliosis. Available at: http://www.eucalb.com/.)