The Relative Importance of Genetic Diversity and Phenotypic Plasticity in Determining Invasion Success of a Clonal Weed in the USA and China

Plant invasions often involve rapid evolutionary change. Founder effects, hybridization, and adaptation to novel environments cause genetic differentiation between native and introduced populations and may contribute to the success of invaders. An influential idea in this context has been the Evolution of Increased Competitive Ability (EICA) hypothesis. It proposes that after enemy release plants rapidly evolve to be less defended but more competitive, thereby increasing plant vigour in introduced populations. To detect evolutionary change in invaders, comparative studies of native versus introduced populations are needed. Here, we review the current empirical evidence from: (1) comparisons of phenotypic variation in natural populations; (2) comparisons of molecular variation with neutral genetic markers; (3) comparisons of quantitative genetic variation in a common environment; and (4) comparisons of phenotypic plasticity across different environments. Field data suggest that increased vigour and reduced herbivory are common in introduced plant populations. In molecular studies, the genetic diversity of introduced populations was not consistently different from that of native populations. Multiple introductions of invasive plants appear to be the rule rather than the exception. In tests of the EICA hypothesis in a common environment, several found increased growth or decreased resistance in introduced populations. However, few provided a full test of the EICA hypothesis by addressing growth and defence in the same species. Overall, there is reasonable empirical evidence to suggest that genetic differentiation through rapid evolutionary change is important in plant invasions. We discuss conceptual and methodological issues associated with cross-continental comparisons and make recommendations for future research. When testing for EICA, greater emphasis should be put on competitive ability and plant tolerance. Moreover, it is important to address evolutionary change in characteristics other than defence and growth that could play a role in plant invasions. Show more

Despite the increasing biological and economic impacts of invasive species, little is known about the evolutionary mechanisms that favor geographic range expansion and evolution of invasiveness in introduced species. Here, we focus on the invasive wetland grass Phalaris arundinacea L. and document the evolutionary consequences that resulted from multiple and uncontrolled introductions into North America of genetic material native to different European regions. Continental-scale genetic variation occurring in reed canarygrass' European range has been reshuffled and recombined within North American introduced populations, giving rise to a number of novel genotypes. This process alleviated genetic bottlenecks throughout reed canarygrass' introduced range, including in peripheral populations, where depletion of genetic diversity is expected and is observed in the native range. Moreover, reed canarygrass had higher genetic diversity and heritable phenotypic variation in its invasive range relative to its native range. The resulting high evolutionary potential of invasive populations allowed for rapid selection of genotypes with higher vegetative colonization ability and phenotypic plasticity. Our results show that repeated introductions of a single species may inadvertently create harmful invaders with high adaptive potential. Such invasive species may be able to evolve in response to changing climate, allowing them to have increasing impact on native communities and ecosystems in the future. More generally, multiple immigration events may thus trigger future adaptation and geographic spread of a species population by preventing genetic bottlenecks and generating genetic novelties through recombination. Show more