Genetic Bottlenecks in Alien Plant Species INFLUENCE OF MATING SYSTEMS AND INTRODUCTION DYNAMICS

versity through genetic drift, founder effects, and genetic bottlenecks. Such events can reduce a population’s evolutionary potential and increase the risk of extinction. In this chapter, we use plant immigrants as a model system to examine the circumstances in which genetic bottlenecks do (and do not) occur and where in the hierarchical partitioning of genetic diversity such bottlenecks take place. We assess the amount and distribution of genetic diversity among plant species in their native (or donor) and introduced ranges. Two factors are important in influencing genetic bottlenecks for alien plants: their mating system, and the circumstances of their introduction. Among outcrossing alien species (e.g., Echium plantagineum, Epipactis helleborine, Trifolium hirtum), the likelihood of a genetic bottleneck can be low because even a few immigrants may represent much of the species’ genetic variation. Similarly, multiple introductions can reduce the likelihood and severity of bottlenecks among selfing alien species (e.g., Bromus tectorum and Avena barbata). The loss of genetic diversity most often takes place across populations and not at the within-population level. Within-population genetic diversity can be higher in naturalized than in native populations if the naturalized populations are a byproduct of genetic admixtures from multiple sources. As a result, multiple inGenetic Bottlenecks in Alien Plant Species