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.