Quantification of environmental variation
To quantify environmental variation all over the potential range of the
species, we selected an area that included its actual distribution range
(based on Bons and Geniez 1996 [for north-west Africa], Pleguezuelos
1997 [for the Iberian Peninsula] and IUCN web [for the rest of
north Africa]) plus a 450-850 km wide perimeter belt around it (width
variation depended on the distance from the edges of a non-quadrilateral
range to the rectangle that was used to set the subsequent inference
models). Within this area, we used data from the Bioclim 2.0 dataset
(cell resolution = 1x1 km; Booth et al. 2014) to compute the score of
each cell on a principal component analysis that combined all Bioclim
environmental variables using R core (R Core Team, 2013). This PCA
yielded a single principal axis that opposed hot areas with low
precipitation to temperate ones with high precipitation (Fig. 1B).