Integrating phylogeography and morphology in Pachygrontha antennata (Heteroptera: Pachygronthidae): toward a new subspecies concept in the genomic era

Given the continuous nature of speciation across temporal and spatial dimensions, the precise delineation of taxonomic units has long posed a persistent challenge. Subspecies-level taxonomic units, which reflect the evolutionary potential within species, may represent the incipient stages of speciation. Here, we assessed the limitations of purely morphological subspecies delineation and inappropriate application of analytical methods in Pachygrontha antennata, and redefined subspecies with evolutionary models and phylogeographic scenarios. The results showed the limited role of mitochondrial genes in identifying subspecific structures, as well as the over-interpretation of statistical results on morphological characteristics. The 2 major clades identified through SNP matched with the redefined subspecies and were supported by multiple evidence. Following their initial divergence approximately 1.089 million years ago, these lineages maintained consistent but asymmetric bidirectional gene flow until their decline began around 31.7 thousand years ago, when measurable morphological and niche differentiation emerged. By integrating phylogeography with species delimitation, our study provides a framework for defining subspecies in the genomic era and an example for studying the early gray zone of the speciation continuum. Such incipient lineages help us better understand speciation processes and their underlying drivers, and make recommendations for an integrative taxonomic approach.