Study system
Psammodromus algirus is a ground-dwelling, heliothermic lizard from the Western Mediterranean region whose distribution range encompasses a wide variety of habitats, from arid shrublands to temperate forests (Díaz and Carrascal 1991). In the Iberian Peninsula, where P. algirus is the most abundant and widespread lizard species, climatic heterogeneity is mirrored by broad changes in vegetation patterns: forests dominate in the west of its range, whereas shrublands prevail in the east. The genetic diversity of the species is broadly structured in two mtDNA lineages, eastern and western, which diverged ca. 3-3.5 mya (Carranza et al. 2006; Verdú-Ricoy et al. 2010). These lineages show some degree of ecologically-driven divergence, because eastern lizards typically display a striped dorsal pattern absent among western ones, and striped and unstriped phenotypes seem to be adaptively linked to crypsis to the predominant habitat where lizards live (Díaz et al. 2017).
We sampled 95 lizards in five populations along a broad environmental gradient in the center of the Iberian Peninsula (Fig. 1B). Three sampling sites housed populations of eastern mtDNA adscription: 1) Lerma (L; 42.058 ºN, -3.611 ºE; 900 m asl; N = 18), a fragmented mixed forest interspersed with grassland patches, 2) Aranjuez (A; 40.016 ºN, -3.586 ºE; 594 m asl; N = 20), a hot, dry site with a high cover of herbs and shrubs and no trees, and 3) Brihuega (B; 40.778 ºN, -2.911 ºE; 1,009 m asl; N = 18), a deciduous open forest with a mosaic of grassland and woodland patches. The two other sampling sites had populations of the western lineage: 4) El Pardo (P; 40.511 ºN, -3.755 ºE; 658 m asl; N = 19), a xeric, lowland evergreen forest, and 5) Navacerrada (N; 40.726 ºN, -4.023 ºE; 1,230 m asl; N = 20), a montane location covered by deciduous forest. Several particularities of these populations make them representative of a wide range of selective pressures gathered around the core of this species’ range: 1) lizards from Lerma inhabit a very fragmented forest archipelago that resembles the typical habitat of western lizards (although they belong to the eastern lineage; see Díaz et al. 2005; Santos et al. 2008; Telleria et al. 2011; and Pérez-Tris et al. 2019 for further information about habitat fragmentation effects in this system); 2) Aranjuez lizards inhabit the typical hot and dry habitat of eastern lizards, and although this locality is very close to the western populations included in this study (El Pardo and Navacerrada), it receives very little gene flow from them (Díaz et al. 2017), so that its isolated condition promotes the accumulation of genetic divergence subject to selection (Llanos-Garrido et al. 2019); and 3) the two western populations (El Pardo and Navacerrada) are separated by a significant altitudinal gradient, and although lizards from both populations show little genetic differentiation (Díaz et al. 2017), they differ in important phenotypic traits such as escape tactics, sexual dimorphism, sexual ornaments, ectoparasite loads and other life history traits (Iraeta et al. 2006, 2010, 2011; Llanos-Garrido et al. 2017).