Evidence for evolution in the novel range of an invasive weed in multiple common gardens

Background/Question/Methods

Invasions represent excellent opportunities to synthesize evolution, ecology, and genetics over contemporary time scales, and address the role of human-induced evolutionary change as an ecological influence. Adaptation to novel environments should be observable as ecologically important differentiation between populations from the native versus invaded range. Yet simply observing the phenotypes of a single generation in a common environment may be insufficient to demonstrate an evolutionary change in the novel habitat. To test the EICA or trade-off hypotheses, and to look for evidence of rapid adaptation to novel habitats, we conducted two large common garden greenhouse experiments with Centaurea diffusa, one of North America’s worst weedy invaders.  The first included a broad sampling of populations of C. diffusa and the second assessed the importance of maternal effects by using seed produced from greenhouse crosses. Each common garden included both a benign control treatment, and a series of biotic and abiotic stress treatments. These experiments allow us to determine whether there is evidence of rapid evolution in phenotype or resource allocation between the native and invaded ranges, and which specific environmental factors may be responsible.

Results/Conclusions

Under benign common conditions, C.diffusa demonstrates life history trait differences and greater growth and reproductive potential in the invaded compared to the native range, though these ranges have been separated by little more than 100 years. These traits showed a significant effect of origin across broad population sampling, after an attempt to control for maternal environment and latitude, and even, in some cases, across stressful treatment environments. This implies rapid adaptation to the novel invaded range, and that post-introduction evolutionary change may have significantly contributed to the invasion of C. diffusa. Our data (1) provide evidence for genetic differentiation between native and invasive populations for several traits, (2) suggest that post-invasion evolution could have played a significant role in the invasion success of C. diffusa in North America and (3) suggest that variation in the maternal environment associated with field-collected seeds may confound common garden studies involving C. diffusa . Though our data suggest evolution in the novel range, they do not follow the predictions of EICA or the trade-offs hypotheses, at least under the conditions assessed, both biotic (simulated herbivory) and abiotic (drought, flood, and nurtrient deficiency).