The role of natural selection in the evolution and adaptation of invasive species can be assessed by testing the null hypothesis that the extent of quantitative genetic differentiation (Q_ST) would be similar to that of neutral molecular differentiation (F_ST). Using eight microsatellite loci and common garden approach, we compared Q_ST and F_ST of ten populations of an invasive species Ambrosia artemisiifolia (common ragweed) in France. In a common garden study with varying water and nutrient levels, we measured Q_ST for five traits (height, total biomass, reproductive allocation, above-/belowground biomass ratio, and days to flowering).

Results/Conclusions

Although low F_ST indicated weak genetic structure and strong gene flow among populations, we found significant disruptive selection (Q_ST > F_ST) for reproductive allocation that may be closely related to fitness. It suggests that abotic environments (water and nutrients) may have exerted selection pressure sufficient for A. artemisiifolia populations to locally adapt. However, we did not detect any genetic x environmental interaction in all traits. We cannot exclude the possibility that the adaptive divergence of populations could have been invoked by other selection pressures (e.g. natural enemies, disturbance). As previous studies indicate multiple introductions from various source populations of A. artemisiifolia in North America, our results suggest that the admixture of introduced populations may have increased genetic diversity and additive genetic variance, and in turn, have resulted in the rapid evolution and adaptation of this invasive species.