Demography
The lower degree of genetic variation in the EF-1ɣ region is expected
due to this being a nuclear gene and having a lower mutational rate than
the two mitochondrial genes . The high haplotype diversity across the
various species and gene regions indicates that enough time has elapsed
for diversification to occur. Majority of the species display high
haplotype diversity and low nucleotide diversity across the gene regions
(except for S. unispinosus which does not have a high haplotype
diversity). This trend is indicative of a rapid expansion from a
relatively small population . In order to gain further insight into the
demographic history of these species for which only a few specimens were
available, more samples are required. The incongruence of the placement
of S. venterstadensis in the haplotype network of CO1 is
potentially due to mitochondrial introgression from an ancient admixture
between the two species (Croucher et al., 2004; Horoiwa et al., 2021;
Kornilios et al., 2016). This could explain why the same pattern is not
observed in the other gene regions, but a larger sample size of S.
venterstadensis is required to fully explore this possibility.
The EF-1ɣ results for S. maraisi indicate that the population has
undergone a recent expansion event based on the negative Tajima’s D and
Fu and Li’s D as well as the shape of the haplotype network This
expansion based on the dated phylogeny likely occurred in the
mid-Miocene, coinciding with the formation of the Benguela current,
leading to the Karoo experiencing a more tropical climate . This may
have given males, which are vulnerable to desiccation, more favourable
conditions to disperse further than is usually possible under arid
conditions. This could have contributed to the wide species range ofS. maraisi seen today. The influence of the Benguela current
formation on accelerated speciation has previously been recorded in
southern Africa. In Namibia and the Northern Cape, the formation of the
Benguela current led to xeric conditions which caused local speciation
events such as the formation of the aeolian sand dune system in Namibia,
which in turn influenced the speciation of the Pachysoma dung
beetles as well as Senecio genus of daisies . The influence of
this current on the succulent Karoo and Fynbos biomes are also well
linked to speciation, such as on the tent tortoise, Psammobates
tentorius as well as numerous plant species . The effect of the
Benguela on the Nama Karoo fauna, where S. maraisi is found is
however largely unexplored, and requires further research.