Herbivores and molecular clocks as tools in plant biogeography

Historical biogeography plays an important role in understanding evolutionary processes and the history of life, with fossil data, plate tectonics, and palaeoclimatology offering major data bases for biogeographic analyses. Here we suggest that specialized interspecific interactions, in combination with molecular data, can play an important role in such analyses. We use the interaction between Bowlesia incana (Apiaceae) and the host-specific herbivore Greya powelli (Lepidoptera: Prodoxidae) in California to demonstrate the utility. Distributed in disjunct temperate parts of North and South America, B. incana has been proposed to have been introduced in historical time (<250 yrs BP) into North America. Three lines of evidence together suggest that the plant is of a far older age in North America. First, G. powelli is not known from the South American range and the genus is very unlikely to exist there, making introduction with the plant in North America unlikely. Second, divergence of mtDNA among members of the genus Greya suggests that the lineage leading to G. powelli originated 2.3-3.8 Mya, thus predating a proposed introduction by several orders of magnitude. Third, host shifts are consistently linked with species divergence within the genus Greya, suggesting that the G. powelli lineage has utilized Bowlesia since a time near its origin. We conclude that B. incana has been present in western North America for a long period of time, and that it did not arrive by human transport. The use of specialized herbivores and molecular data adds a powerful tool to historical plant biogeography.