Data from: Herbarium specimens reveal a historical shift in phylogeographic structure of common ragweed during native range disturbance

Invasive plants provide ample opportunity to study evolutionary shifts
that occur after introduction to novel environments. However, although
genetic characters pre-dating introduction can be important determinants
of later success, large-scale investigations of historical genetic
structure have not been feasible. Common ragweed (Ambrosia artemisiifolia
L.) is an invasive weed native to North America that is known for its
allergenic pollen. Palynological records from sediment cores indicate that
this species was uncommon before European colonization of North America,
and ragweed populations expanded rapidly as settlers deforested the
landscape on a massive scale, later becoming an aggressive invasive with
populations established globally. Toward a direct comparison of genetic
structure now and during intense anthropogenic disturbance of the late
19th century, we sampled 45 natural populations of common ragweed across
its native range as well as historical herbarium specimens collected up to
140 years ago. Bayesian clustering analyses of 453 modern and 473
historical samples genotyped at three chloroplast spacer regions and six
nuclear microsatellite loci reveal that historical ragweed’s
spatial-genetic structure mirrors both the paleo-record of Ambrosia pollen
deposition and the historical pattern of agricultural density across the
landscape. Furthermore, for unknown reasons this spatial-genetic pattern
has changed substantially in the intervening years. Following on previous
work relating morphology and and genetic expression between plants
collected from eastern North America and Western Europe, we speculate that
the cluster associated with humans’ rapid transformation of the landscape
is a likely source of these aggressive invasive populations.