[en] Plant population differentiation may play a role in decreasing the ability to predict whether, where, and when an introduced species will invade. However, few studies have addressed the level of genetic change an alien species may undergo during range expansion, e.g. in response to climatic variation with altitude. In the present study, we tested the hypothesis that invasive populations of Senecio inaequidens (Asteraceae) differentiated during migration from two independent introduction sites into divergent altitudinal and climatic zones. We carried out two years of common garden experiments with eight populations from a Belgian altitudinal transect and ten populations from similar French transect. Climatic analysis revealed that the Belgian transect followed a temperature and precipitation gradient. A temperature and summer drought gradient characterized the French study site. We evaluated differentiation and clinal variation in the following characters: days to germination, days to flowering, height at maturity, final plant height and aboveground biomass. Results showed that S. inaequidens populations differentiated in growth traits during invasion. First year of experiment, regressing population growth trait means against source population altitude indicated the presence of clinal variation along both transects. Second year, similar results were found along the French transect, i.e. a reduction in height at maturity, plant height and aboveground biomass with increasing altitude. Including seed mass as a covariate did not change the outcome of the analysis. The possible evolutionary causes for the differentiation observed are discussed.
