final accepted version Natural Enemies Drive Geographic Variation in Plant Defenses

Plants defend themselves against attack by natural enemies and these defenses vary widely across populations. However, whether communities of natural enemies are a sufficiently potent force to maintain polymorphisms in defensive traits is largely unknown. Here, we exploit the genetic resources of Arabidopsis thaliana, coupled with 39 years of field data on aphid abundance to: (i) demonstrate that geographic patterns in a polymorphic defense locus (GS-ELONG) are strongly correlated with changes in the relative abundance of two specialist aphids; and (ii) demonstrate differential selection by the two aphids on GS-ELONG, using a multi-generation selection experiment. We thereby show a causal link between variation in abundance of the two specialist aphids and the geographic pattern at GS-ELONG, which highlights the potency of natural enemies as selective forces. Intraspecific genetic variation is essential in enabling species to respond rapidly to evolutionary challenges such as changing environmental conditions (1) or the emergence of novel pests and pathogens (2). This diversity often reflects the balance between the strength of local selection and the current and historical levels o