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 ]