Evidence of Rapid Evolution in Herbivory Defense Responses With Conserved Trade-Offs in Populations of Medicago polymorpha

Theories explaining the evolution of plant defensive strategies are difficult to experimentally test. Biological invasion scenarios can serve as helpful natural experiments for examining the evolutionary dynamics of plant defenses when plants become established as potential hosts in new environments. This study uses a historical invasion by Medicago polymorpha (Burr Clover) to test the predictive power of the Shifting Defense Hypothesis (SDH) by investigating variation in plant defenses to herbivorous insects. We compared the feeding preferences of a generalist and a specialist herbivore on native and invasive populations of M. polymorpha. We document a shift in herbivore preference patterns for constitutive versus herbivore-induced tissues when comparing plants from native and invaded ranges. However, specific biochemical defenses showed a conserved negative correlation between constitutive and inducible defenses across both ranges, indicating a fundamental trade-off in defense strategy that persists despite allocation differences, suggesting defense evolution that was not revealed by tests in this study. These results provide evidence of evolutionary shifts in plant palatability that are consistent with predictions of the SDH, which predicts evolutionary shifts in defense allocation. Our findings reveal complex evolutionary dynamics that underlie invasion success and demonstrate that invasive M. polymorpha have undergone evolutionary adaptation in defense strategy beyond any immediate ecological advantages of enemy release, providing insight into how invasive plants successfully adapt to novel herbivore communities over time.