Is there evidence for the post-invasion evolution of increased size among invasive plant species?

Many plant species grow taller and have higher reproductive capacity where they are nonindigenous invaders than where they are native components of the flora. Traditionally, it has been accepted that this is a plastic response to a benign environment, though recently this assumption has been challenged and a genetic basis for increased plant size has been invoked. We tested the hypothesis that the increased size of certain weed species is genetically, rather than environmentally, based. A common environment growth experiment revealed no significant differences in the size of Carduus nutans, Digitalis purpurea, Echium vulgare or Senecio jacobaea sampled from alien (Australia and New Zealand) or native (Britain and continental Europe) habitats. We conclude that post-invasion genetic changes associated with increased size may be unusual and that the phenomenon, where it occurs, generally reflects a plastic response to a novel environment.