Weak premating isolation between Clitarchus stick insect species despite divergent male and female genital morphology

Documenting natural hybrid systems builds our understanding of mate choice, reproductive isolation, and speciation. The stick insect species Clitarchus hookeri and C. tepaki differ in their genital morphology and hybridize along a narrow peninsula in northern New Zealand. We utilize three lines of evidence to understand the role of premating isolation and species boundaries: (1) genetic differentiation using microsatellites and mitochondrial DNA; (2) variation in 3D surface topology of male claspers and 2D morphometrics of female opercular organs; and (3) behavioral reproductive isolation among parental and hybrid populations through mating crosses. The genetic data show introgression between the parental species and formation of a genetically variable hybrid swarm. Similarly, the male and female morphometric data show genital divergence between the parental species as well as increased variation within the hybrid populations. This genital divergence has not resulted in reproductive iso...