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).