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.