Environmental variability
By using the scree plot criterion, the environmental PCA yielded a single axis (eigenvalue = 0.679) that retained four variables: annual mean temperature (BIO1), max temperature of the warmest month (BIO5), mean temperature of the warmest quarter (BIO10), and annual precipitation (BIO12) (Llanos-Garrido et al. 2021). The warmest/driest cells were located near the lowland population (El Pardo), while the opposite was true for the montane population (Navacerrada). We found significant environmental differentiation between both populations (one-way ANOVA, F1,38 = 4802,014, p-value << 0.0001), as well as relatively high levels of environmental variation along the cells that an individual would have to cross in a theoretical straight-line migration following valley bottoms (mean = -1.563, sd = 0.725). In fact, such variation was much greater than the one found in the geographical cells within any of the two populations, either lowland (mean = -1.069, sd = 0.048; Levene test: F1,28 = 12.048 , p = 0.002) or montane (mean = -2.121, sd = 0.043; Levene test: F1,42 = 25.807, p < 0.001).