1.2 Dispersal evolution during range expansions
Here, we consider traits affecting dispersal separately from
reproductive life history traits (Bonte & Dahirel 2017). Range
expansion theory predicts that a population at the expanding edge will
evolve increased dispersal ability relative to a population at the core
through the process of spatial sorting (Shine et al. 2011).
During range expansion, individuals with greater dispersal ability are
more likely to arrive at the range edge and disperse to new territory,
resulting in populations at the expanding edge being a non-random
selection of better dispersers. Since dispersal ability is a heritable
trait in many species (Dällenbach et al. 2018), this gradient is
further reinforced by assortative mating between individuals on the
edge. Spatial sorting has been predicted using mathematical models
(Fisher 1937; Kot 1996; Travis & Dytham 2002; Bénichou et al.2012) and demonstrated using model organisms (Simmons & Thomas 2004;
Fronhofer & Altermatt 2015; Van Petegem et al. 2016; Ochocki &
Miller 2017; Szücs et al. 2017; Weiss-Lehman et al. 2017).
The evolution of increased dispersal at range edges has also been
documented empirically, both in species whose ranges are shifting due to
climate change (Cwynar & MacDonald 1987; Thomas et al. 2001;
Simmons & Thomas 2004; reviewed in Hill et al. 2011), and in
invasive species (Phillips et al. 2006, 2010a; Monty & Mahy
2010; Berthouly-Salazar et al. 2012; Lombaert et al. 2014;
Merwin 2019; but see Ashenden et al. 2017).
External conditions, such as population density, can be important
signals to individuals about the potential costs and benefits of
emigration (Clobert et al. 2009; Endriss et al. 2019) and
can influence dispersal evolution along expansion fronts (Traviset al. 2009). Species that exhibit positive density-dependent
dispersal (increased dispersal at high densities) may be less likely to
evolve increased dispersal ability at the edge of the range expansion
where population density can be low (Travis & Dytham 2002; Fronhoferet al. 2017), while species with negative density-dependent
dispersal (increased dispersal at low densities) may be more likely to
evolve increased dispersal abilities and generate accelerating expansion
fronts (Altwegg et al. 2013). For many species, high population
density can signal strong intraspecific competition, which may increase
emigration. Alternatively, high population density can signal high mate
availability, which may decrease emigration.
How an individual incorporates external conditions into dispersal
behavior also depends on internal state, such as whether an individual
has previously mated (Clobert et al. 2009). A mated individual
may increase its fitness by dispersing from high density environments to
reduce competition and inbreeding (Clobert et al. 2009), while an
unmated individual may increase its fitness by dispersing from low
density environments to increase the chances of finding a mate. Here, we
combine the predictions from spatial sorting theory (Shine et al.2011) with those from informed dispersal theory (Clobert et al.2009) to develop the refined predictions shown in Fig. 1C. We predict
edge populations will disperse more often or further than core
populations, but dispersal will also depend upon mating status and
density (Fig. 1C), thus we can only be confident in our dispersal
comparisons across a range when controlling for those contexts. Since we
seek to apply evolutionary theory to natural range expansions, these
refined predictions will enable us to evaluate the evolution of
dispersal during range expansion and how expression of such evolutionary
shifts might depend upon both external and internal factors that
organisms experience.