Adaptive and non-adaptive evolution of trait means and genetic trait correlations for herbivory resistance and performance in an invasive plant

The EICA-hypothesis predicts that invading plants adapt to their novel environment by evolving increased performance and reduced resistance in response to the release from natural enemies, and assumes a resource allocation tradeoff among both trait groups as mechanistic basis of this evolutionary change. Using the plant Silene latifolia as a study system, we tested these predictions by investigating whether 1) invasive populations evolved lower resistance and higher performance, 2) this evolutionary change is indeed adaptive, and 3) there is a negative genetic correlation between performance and resistance (i.e. a tradeoff) in native and introduced individuals. Moreover, we sampled eight native and eight invasive populations and determined their population co-ancestry based on neutral SSR-markers. We performed controlled crossings to produce five sib-groups per population and exposed them to increased and reduced levels of enemy attack in a full-factorial experiment to estimate performance and resistance. With these data, we performed trait-by-trait comparisons between ranges with ‘animal models’ that account for population co-ancestry to quantify the amount of variance in traits explained by non-adaptive versus adaptive evolution. Moreover, we tested for genetic correlations among performance and resistance traits within sib-groups. We found significant reductions in resistance and increases in performance in invasive versus native populations, which could largely be attributed to adaptive evolution. While we detected a non-significant trend towards negative genetic performance × resistance correlations in native populations, invasive populations exhibited both significant and non-significant positive correlations. In summary, these results do not support a shift of performance and resistance trait values along a tradeoff line in response to enemy release, as predicted by EICA. They rather suggest that the independent evolution of both traits is not constrained by a tradeoff, and that various selective agents (including resource availability) interact in shaping both traits and in weakening negative genetic correlations in the invaded habitat.