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The impact of natural selection on the adaptive divergence of invasive populations can be assessed by testing the null hypothesis that the extent of quantitative genetic differentiation ( Q) would be similar to that of neutral molecular differentiation ( F). Using eight microsatellite loci and a common garden approach, we compared Q and F among 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 for five traits (height, total biomass, reproductive allocation, above- to belowground biomass ratio, and days to flowering). Although low F indicated weak genetic structure and strong gene flow among populations, we found significant diversifying selection ( Q > F) for reproductive allocation that may be closely related to fitness. It suggests that abiotic conditions may have exerted selection pressure on A. artemisiifolia populations to differentiate adaptively, such that populations at higher altitude or latitude evolved greater reproductive allocation. 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, promoted the rapid evolution and adaptation of this invasive species. [ABSTRACT FROM AUTHOR]

•An improved inference of the evolutionary history of invasive species may be achieved by analyzing the genetic variation and population differentiation of recently established populations and their ancestral (historical) populations. Employing this approach, we investigated the role of gene flow in the post-invasion evolution of common ragweed ( Ambrosia artemisiifolia). •Using eight microsatellite loci, we compared genetic diversity and structure among nine pairs of historical and recent populations in France. Historical populations were reconstructed from herbarium specimens dated from the late 19th to early 20th century, whereas recent populations were collected within the last 5 yr. •Recent populations showed greater allelic and genetic diversity than did historical populations. Recent populations exhibited a lower level of population differentiation, shorter genetic distances among populations and more weakly structured populations than did historical populations. •Our results suggest that currently invasive populations have arisen from active gene flow and the subsequent admixture of historical populations, incorporating new alleles from multiple introductions. [ABSTRACT FROM AUTHOR]