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).