Range inference
To infer the distribution range of the species, we followed a two-step
procedure. Firstly, we included all the geographical cells that
presented the same environmental scores as the sampled populations
(predicted range #1). This first approach provides a baseline
prediction with no genetic information that can be used to quantify the
improvement in predictive ability supplied by GEAM. Secondly, we
extrapolated such GEAM to all possible combinations of alleles for the
outlier loci significantly associated with environment (i.e., all
possible genotypes under selection), so that we could predict all
environmental values that were suitable for at least one genotype. By
fulfilling all grid cells with those environmental values, we could
extrapolate our prediction to the overall distribution range of the
species. Finally, we removed from the inferred range a few disconnected
patches (in central France, coastal Italy, and the Mediterranean
islands) that were too far from the main distribution range of P.
algirus (>8 km from the nearest inferred distribution
limit, which was the largest distance between adjacent but disconnected
patches within the inferred range), whose low dispersal rate (Santos et
al. 2009) is supported by the fact that genetic differentiation can be
detected even among forest fragments separated by 350 m of unsuitable
arable land (Pérez-Tris et al. 2019). This produced our second (and
final) inferred distribution range (predicted range #2). The extent of
overlap between real and predicted distribution ranges was estimated
using QGIS v2.18.16 (QGIS Development Team 2018).
The delineation of a real distribution range that could be compared with
our inferences #1 and #2 was based on two previously published
atlases: Bons and Geniez (1996) for north-west Africa, and Pleguezuelos
(1997) for the Iberian Peninsula. To make such delineation as
reproducible as possible, we digitalized these previous maps to get a
grid of 10x10 km geographical cells, which was the original scale in the
atlases. We subsequently added all the areas devoid of lizards, but
embedded within the previous range, that were tagged with an artificial
land use in the Corine database (European Environment Agency under
Copernicus program, 2018). This was done to correct for the fact that
absences from such areas should be attributed to land use (unsuitable
for P. algirus ) and not to the environmental variables included
in our model. On the other hand, our inferred ranges were based on
environmental variation measured across a grid of 1x1 km geographical
cells, because a scale of 10x10 km cells would have been too rough to
properly model the distribution range of the species. Thus, and for the
sole purpose of estimating the accuracy of our inferences using a
criterion as similar as possible to the one used in the atlases, we
lowered their resolution by including all 10x10 km grid cells with at
least 25% of its surface corresponding to inferred presences.