Evolutionary isolation and ecological stasis in the endangered Comal springs Dryopid beetle (Stygoparnus comalensis)

The Comal Springs dryopid beetle (Stygoparnus comalensis) is an
endangered, subterranean-obligate, long-toed water beetle known only from
three spring complexes in the Edwards Aquifer region of Texas. Due to
their small size and reliance on groundwater karst habitats, little is
known of the biology of these beetles. The aim of this study was to
investigate evolutionary, ecological and morphological divergence among
known S. comalensis localities in order to inform conservation management
plans for this species. We used genotyping-by-sequencing (n = 85), stable
isotope ($\delta^{15}$N and $\delta^{13}$C) analysis (n = 104), and
morphometric analysis (n = 80), to describe variation among the three
known localities and to ask whether ecological or morphological
differentiation is associated with population genomic
divergence.  S. comalensis populations are highly structured with
no evidence of gene flow. Stable isotope ratios and morphology did not
significantly differ among populations, suggesting that isolation is the
driver of genetic diversification in S. comalensis. These results indicate
that isolated populations show great levels of overall genomic divergence,
while stasis or stabilizing selection has constrained ecological or
morphological divergence. The three localities where S. comalensis has
been found should be considered as evolutionarily distinct units based on
levels of genomic differentiation. Conservation efforts should prioritize
maintenance of the known localities and continued surveys to discover
other localities for this endangered beetle.