Number of adapted genotypes per cell across the species’ range
Besides the general pattern by which fewer genotypes were adapted to range boundaries than to core areas, we found two distinct scenarios within the Iberian Peninsula (Fig. 6). On the one hand, in the northern half of the peninsula there were many areas suitable for a high number of genotypes. In such context, there is a high probability of finding the few genotypes that are adapted to the challenging environmental conditions characteristic of northern boundaries, which should facilitate the establishment of the marginal populations that shape the corresponding edge (Kawecki 2008; Hardie and Hutchings 2010; Halbritter et al. 2015). Conversely, most of the southern half of Iberia seemed to be suitable only for a small number of genotypes, despite the fact thatP. algirus is abundant in this area. However, several small areas locally suitable for many genotypes were interspersed all across the region. Such areas could therefore play an important role as sources of specific genetic diversity adapted to the demanding, singular environments that surround them (Holt and Keitt 2005; Sagarin et al. 2006). The genotyping of populations that inhabit demanding environments, suitable for a small number of allele combinations, would be crucial to sustain this assertion (Eckert et al. 2008; Gallet et al. 2018). Similarly, a model simulating the intensity of selection in both sources and sinks of genetic diversity should be useful to test whether the genetic variants adapted to demanding environments arise with higher probability in source populations with more relaxed selection regimes (Alleaume-Benharira et al. 2006).