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.
