Box 2: The demography of biological invasion
Demographic bottlenecks (in which the census population size decreases and then increases again) are expected to occur under simple models of biological invasion and may be common among invasive species. The consequences of demographic bottlenecks are well-established and far reaching in terms of population genetic models (reviewed by Gattepaille, Jakobsson, & Blum, 2013). Tajima’s D will depend on the magnitude and number of generations since the invasion, and will ultimately show greater than expected variance among loci (Stajich & Hahn, 2005). After a sufficiently extreme bottleneck, all lineages will coalesce to form a star-like genealogy with an excess of rare variants; however if the bottleneck is more moderate and multiple lineages persist, an excess of intermediate-frequency may be observed (Depaulis, Mousset, & Veuille, 2003). These scenarios are depicted below (adapted from Gattepaille et al., 2013).