4.2. A phylogeographic scenario of P. lilfordicolonization of the Balearic Islands
P. lilfordi genetic diversity was largely structured according to
the main geographic distribution, with all analyses supporting the
existence of two major genetic lineages separating the large
archipelagos of Menorca from Mallorca/Cabrera (Figure 5 and 6). The tree
topology was largely consistent with the ones previously reported based
on mtDNA and SNPs data (Bassitta et al., 2021; Brown et al., 2008;
Pérez‐Cembranos et al., 2020; Terrasa et al., 2009). While
Mallorca/Cabrera lineage is further sub-structured in four
well-supported clades (bs>80%, Figure 6), phylogenetic
relationships among Menorcan populations remain largely unresolved, as
also indicated by their low degree of differentiation (Figure 4, Fst
values < 0.1). Menorcan islands were once largely panmictic
(Brown et al., 2008; Terrasa et al., 2009) and a signature of past
admixture can still be observed among most populations of this
archipelago (Figure 5B), with the exclusion of the small islet of
Porros, now representing an independent lineage (Figure 6).
Admixture analysis (K=2, Figure 5B), root inference by IQ-tree (Figure
S2) and root testing all strongly suggest that the origin of the species
occurred in the main island of Mallorca, with most ancestral populations
in our dataset being identified in the North of Mallorca. The highest
rootstrap support (86.13%) specifically points to the small island of
Colomer as the population retaining nowadays most ancestral
polymorphisms, in line with previous proposals (Bassitta et al., 2021;
Terrasa et al., 2009). This root placement is consistent with the
geographic history of both Dragonera and Colomer islands which were once
connected to the Serra of Tramuntana, a mountain chain running up North
of Mallorca, before their detachment from the main Mallorca Island about
2.3 Mya (Brown et al., 2008; Terrasa et al., 2009). The Serra of
Tramuntana is considered to be the last refuge of P. lilfordi(Bailón, 2004) before its extinction in the main Mallorca Island, as
testified by fossil records dating around 2000 years old found in the
Muleta cave, in the central part of the Tramuntana (Alcover, 2000;
Kotsakis, 1981). The Colomer islet is characterized by high lizard
density (Pérez-Mellado et al., 2008) and limited accessibility (due to
high altitude and coastal shape). Although introduction of lizards from
other islands cannot be ruled out, the islet has reduced chances of
external gene flow, compatible with the retention of ancestral
polymorphisms (Bassitta et al., 2021; Terrasa et al., 2009).
The following process of colonization within the archipelago of
Mallorca/Cabrera did not follow an isolation by distance model (Mantel
test, p>0.1). According to admixture, phylogenetic analyses
and Treemix reconstructions of past migration events, North of Mallorca
was then the source of colonization of main Cabrera Island, with which
they form a unique genetic clade (see Admixture results, Figure 5B).
This closed genetic relationship was also reported by Bassitta et al.,
2021, although it partly differs from mtDNA results (Terrasa et al.,
2009).
Following this scenario, species expansion within the archipelago of
Cabrera proceeded with the colonization of small surrounding islets
(Esclatasang and Foradada), as supported by the observed coancestry with
Cabrera Island (see Figure 5B). This suggests a founder effect in these
small islands, followed by independent evolution leading to the nowadays
well-differentiated genetic clade of Esclatasang and Foradada (Figure 5
and 6). Similarly, admixture and ML tree analyses indicated that islets
on North of Cabrera could have been a source of colonization for
Southern Mallorca islets, specifically the islet of Moltona, with which
they still share a small proportion of coancestry (Figure 5B and 6),
while the geographically closed sister population of Na Guardis has
currently lost all signature of coancestry with Northern Cabrera (Figure
5B). The observed clustering of Cabrera with South Mallorca islets
confirms previous haplotype grouping based on mtDNA data, which also
suggested a directionality of gene flow from Cabrera to South Mallorca
(Network III; Terrasa et al., 2009). Moltona would have then seeded the
small islet of en Curt, with which they form a unique clade (bs=99%,
Figure 6), as evidenced by their shared coancestry (Figure 5B). The tiny
islet of en Curt (0,44 ha) would have followed a process of accelerated
differentiation presumably driven by a founder effect, extensive genetic
drift, and intense density-dependent selection with more than 1500
ind/ha (Ruiz De Infante Anton et al., 2013). As an alternative scenario,
the three islets on the South of Mallorca directly derived from the
ancestral mainland Mallorca population, now extinct, a scenario that we
presently cannot exclude.
According to bathymetric, geological and genetic data subsequent
colonization of Menorca occurred around ~ 2.8 Mya (Brown
et al., 2008; Terrasa et al., 2009). We were not able to assess the root
within this archipelago due to high levels of admixture. Menorcan
populations were highly panmictic during a large period of time and the
colonization and subsequent isolation of most Menorcan islands was
gradual (Pretus et al., 2004), following a vicariance type of
colonization, with an asymmetric flow from south to north of the
distribution of the species (Terrasa et al., 2009).