Discussion
Our study provides valuable insights into the population genetics of theC . variipennis species complex and highlights the presence of potential cryptic species. For most of the species examined, minimal genetic divergence was observed across populations, suggesting the maintenance of gene flow even over large geographic distances. The only exception was C. occidentalis , which showed a high level of geographic isolation, as well as two distinct genetic clusters. We confirmed that mitochondrial data is not reliable to properly differentiate three out of five species, due to the lack of segregation between the mitochondrial haplotypes of C. albertensis , C. sonorensis , and C. variipennis . This stands in stark contrast to their clear differentiation and high level of divergence inferred from the SNP data. Though a substantial amount of divergence exists between all five species, hybridization and introgression are present at low levels in sympatry suggesting that post-zygotic isolation barriers have not evolved in this group. Thus, pre-zygotic isolation through either ecological or behavioral segregation is likely responsible for divergence within this complex. With a considerable amount of overlap between some species (Fig. 1), each sympatric population is potentially experiencing a set of unique selective pressures to maintain species boundaries.