4.1. Genetic diversity of P. lilfordi: evolution in
islands and population conservation
A fundamental goal in population ecology and evolution is to understand
the processes that maintain genetic diversity, and those that drive
intraspecific/interpopulation divergence across geographic space and
time (Avise, 2000). In small insular populations, free of predators and
with no immigrants, genetic diversity is primarily driven by genetic
drift (due to environmental and demographic stochasticity, both
particularly important in tiny islets) and density-dependent selection
due to competition for the limited local resources (Hoffmann et al.,
2021; Hunt et al., 2022; Travis et al., 2023). Founder effect and high
inbreeding levels are also expected to further reduce the population
genetic variance and accelerate the process of divergence from the
original source population (Keller and Waller, 2002). Each P.
lilfordi island clearly hosted a distinct population (all Fst distances
were significant), with no contemporary gene flow (no recent
translocations detected). Few small islets showed a remarkably high
differentiation from all other islets (i.e. Porros and en Curt, Fst
>0.2 for all pairs, see Figure 4), indicating either a
strong founder effect and/or intense local drift and selection due to
their reduced island size (Sendell‐Price et al., 2021). Despite being
effectively closed, P. lilfordi populations harbored relatively
high genetic diversity (Ho>0.09 and Pi> 0.1
for all populations), even higher than other continental and insular
species of Podarcis (Sabolić and Štambuk, 2021). Diversity was
overall largely comparable among all 16 populations studied and across
their range of distribution (Figure 1 and Figure 3); nonetheless,
association analyses supported the expected pattern of decreasing
genetic diversity with decreasing island size (Furlan et al., 2012).
Smaller islands also presented the highest number of private alleles,
indicative of an ongoing process of differentiation. Mallorca/Cabrera
archipelago typically hosted a higher genetic diversity than Menorca
(Figure 3), with the islands of Cabrera, Guardia and Moltona (South of
Mallorca) currently harboring the largest genetic diversity present in
this species.
As previously reported by Bassitta et al . (2021), we also
observed low levels of inbreeding for most populations (Fis<
0.08), even for tiny islets such as Esclatasang, Foradada, Porros,
Revells and en Curt (all with Area< 0.7 ha, Fis<0.5,
Table 1 and Figure 3). While small populations are expected to show a
reduced heterozygosity (Keller and Waller, 2002), Fis values in P.
lilfordi indicated a slight trend of positive increase with increasing
islet surface, with the highest estimate found for the island of Moltona
(Fis =0.16). A small inbreeding coefficient, negatively associated with
island size, was also found in a previous study based on microsatellites
from the three neighboring populations of Moltona, Na Guardia and en
Curt (Rotger et al., 2021). A potential explanation is the existence of
a population spatial substructure within larger islands, which would
cause a non-random mating of individuals effectively reducing the
observed heterozygosity. Nonetheless, we invite caution in
interpretation of these results as SNP-based estimation of the
inbreeding depression in wild populations is notoriously challenging
(Schmidt et al., 2021) and would require pedigree-studies for a reliable
quantification (Kardos et al., 2016).
Both the relatively high genetic diversity and low Fis values observed
in P. lilfordi would suggest the existence of potential
mechanisms for buffering inbreeding depression in these lizard
populations, increasing their chances of persistence. In line with this,
recent studies support the ability of insular lizards to counteract
genetic depletion even in presence of a strong founder effect (see
Sherpa et al., 2023).