3.2 Population bottlenecks as a paradox to resolve
A central aim in invasion genetics is to understand the general impact of genetic drift on invasion success beyond individual case studies (Bock et al., 2015). Debates about the role of genetic drift in invasion success are as old as the field of invasion genetics (Baker & Stebbins, 1965; Barrett, 2015). Even today researchers seek to resolve the “paradox of biological invasion” by explaining how invasive species rapidly adapt to new environments despite a loss in genetic diversity, a reduction in the efficiency of natural selection and an increased risk of inbreeding depression (Estoup et al., 2016). Additionally, if an introduced population can overcome (or avoid) these challenges, it must then somehow endure expansion load (see Box 2). WGR data are well suited to examining this apparent paradox.
First, WGR can be used to reliably test whether an introduced population has experienced a population bottleneck, and to distinguish among different demographic scenarios that that explain the observed level of genetic diversity (Smith & Flaxman, 2020; Welles & Dlugosch, 2018). For example, higher genetic diversity was seen in invasive populations of P. xylostella compared to their native range, despite clear evidence of a population bottleneck, apparently as a result of admixture among introduced populations (You et al., 2020). Second, WGR allows quantification of the genomic landscape of genetic diversity, which can be used to better understand the processes that mediate diversity. Comeault et al. (2020) show that, although introduced populations of Zaprionus indianus (African fig fly) have lower genetic diversity than native populations, diversity is proportionally lower in regions of low recombination. This result indicates the effect of linked selection and suggests that the reduction in diversity in the introduced range is not solely due to demography. Finally, WGR can be used to observe subtle detrimental effects of population bottlenecks, such as expansion load (see Box 2). Thus, population genetic analyses applied to WGR data can shed light on longstanding fundamental questions in invasion biology.
[ BOX 3 ]