1.1 Life history evolution during range expansions
Life history traits, such as fecundity and age at first reproduction, are predicted to either increase or decrease during range expansions, depending upon whether selection or non-adaptive evolutionary processes are dominant at the expansion front (Phillips et al. 2010b). Selection at the expansion front is hypothesized to differ from the core due to differences in density between edge and core (Burton et al. 2010; Fronhofer & Altermatt 2015). Species undergoing range expansion generally have lower population densities at the edge, where individuals are dispersing to uncolonized territory, and higher densities at the core (Burton et al. 2010; Altwegg et al.2013). This means that individuals at the edge will experience exponential, or density-independent, population growth, while those at the core experience logistic, or density-dependent, population growth (Phillips et al. 2010b). Thus, selection at the low density edge will favor increased fecundity and younger age at first reproduction relative to the core (Fig. 1A) (Phillips et al. 2010b).
In contrast, when non-adaptive processes are dominant, reduced fitness, or expansion load, is predicted at the edge relative to the core due to accumulation of deleterious alleles (Fig. 1B) (Travis et al.2007; Peischl et al. 2013; Peischl & Excoffier 2015). This can occur when edge populations are so small that they experience strong genetic drift and deleterious alleles ‘surf’ the wave of expansion during repeated founder events (Klopfstein et al. 2006). Expansion load has been predicted using models (Peischl et al.2013) and demonstrated in one plant (González-Martínez et al.2017). This pattern of reduced fecundity at the edge relative to core might also be generated through trade-offs between dispersal, reproduction, and competitive ability (Burton et al. 2010; Phillips et al. 2010b; Fronhofer & Altermatt 2015).