1 The expansion histories of two South American species of Cortaderia, simi
lar in morphology but differing profoundly in their breeding systems, were
compared in California, USA.
2 Both species were introduced to California in the mid-1800s, but herbariu
m records indicate that the sexual C. selloana has expanded spatially at tw
ice the rate of the asexual C. jubata.
3 The invasiveness of C. selloana has increased over time, whereas that of
C. jubata has remained relatively constant. Populations of C. selloana now
occupy more vegetation types and more non-ruderal habitats than C. jubata.
4 Populations of C. selloanu have experienced directional morphological cha
nge, whereas the morphology of C. jubata has been constant over the 90 year
s for which preserved specimens are available.
5 The invasion of an alien species appears to be a malleable process, rathe
r than a singular event. Species traits, such as inbreeding, can be advanta
geous at some stages but disadvantageous at others. Alien species also adju
st over time to the novel and diverse selective regimes that they encounter
as they expand spatially. Sexual species may have a greater ability to adj
ust to diverse selective landscapes relative to asexual species.