Conclusion
Many of the key questions in invasion genetics highlighted by Bocket al. (2015) remain unanswered, though our ability to obtain and
interpret genome sequence data has matured substantially in the past
five years. Though WGR data are certainly not a singular solution to
outstanding questions about biological invasion, we are increasingly
appreciating their potential; of the studies we assessed, over one third
were published in the past year and more than two thirds were published
within the past two years (Supplementary Table 1). Whilst some research
questions have more to gain from WGR than others (e.g. ,
quantifying hybridization versus spatial inference), appropriately
designed population genomics studies can address multiple questions
about invasions simultaneously. Indeed, almost without exception, the
examples we have highlighted addressed hypotheses from many areas of
interest.
Based on existing genomic data, processes that maintain adaptive genetic
diversity (i.e. , balancing selection, admixture and adaptive
interspecific introgression) are often key to the success of invasive
species (e.g. , Calfee et al., 2020; Hessenauer et al., 2020;
Stern & Lee, 2020; Valencia-Montoya et al., 2020; Yainna et al., 2020).
In other words, standing genetic diversity that has already been shaped
by natural selection is often repurposed to rapidly overcome adaptive
challenges; invasive species do not have time to re-invent the wheel.
This is not a new observation. Several authors have argued that the same
‘combinatorial’ evolutionary processes known to facilitate major
ecological transitions and adaptive radiations can also enable
biological invasion (Hegarty, 2012; Marques, Meier, & Seehausen, 2019;
Prentis et al., 2008; Rieseberg et al., 2003). Genomic data have
revealed the frequency of this phenomenon among invasive species. It is
now clear that the ‘paradox’ of biological invasion is often explained
not only by the fact that many invasive species avoid the negative
effects of demographic bottlenecks, but also because they avoid the need
for de novo mutation followed by in situ adaptation. This
implies a genic view of biological invasion in which the primary aim of
management strategies should be to minimise the spread of alleles known
to confer invasive success through introduced populations, and
potentially through reproductively compatible native populations.
In the studies we surveyed, the most substantial individual
contributions to our understanding of invasive species did not come from
the largest datasets, but from studies that associated phenotypic or
spatial information with genomic data in a hypothesis-driven design that
incorporated appropriate modelling (e.g. , Calfee, Agra, Palacio,
Ramírez, et al., 2020; Olazcuaga et al., 2020; Stern & Lee, 2020).
Future genomic studies will contribute considerably to our understanding
of pre- and post-introduction adaptation if they adopt such an approach.
Useful inferences will also require development of tailored
non-equilibrium models that can incorporate the full complexity of
invasive species. In particular, an exciting area is understanding the
evolutionary history of loci that contribute to adaptive spread. This
will be easiest when studying recent invasions with samples taken over a
time series. The second, more challenging step will be to quantify the
marginal contribution of positively selected loci to invasive spread.
Increasing use of forward-genetic approaches to dissect the genetic
basis of invasiveness traits will likely make this task easier.
Together, these approaches can be used to test whether the
‘combinatorial’ view of invasion success holds up as a general trend.
Given the pervasive role of hybridization in invasion success, alongside
the declining cost of sequencing, we anticipate that whole genome
sequence data will become a standardised approach for monitoring the
ongoing global redistribution of biodiversity. Comprehensive genomic
datasets will eventually allow invasion events to be consistently
reconstructed at a resolution that is useful for informing management
plans and they will put us in a better position to quantify the
contribution of specific mechanisms to overall invasion success.