Can sexual selection promote within-species divergence of male genitalia? A study case with a male-dimorphic arachnid.

Sexual selection can drive divergence in male genital morphology between species, but its role in within-species divergence remains underexplored. Male-dimorphic species offer an opportunity to investigate this, as different morphs often employ distinct reproductive tactics and face different challenges in sperm competition and female stimulation during copulation. We tested the hypothesis that sexual selection promotes within-species genital divergence using the harvestman Poecilaemula lavarrei as a model. This arachnid has two male morphs: larger, territory-holding majors, and smaller, sneaker minors. We first described copulatory interactions, finding no differences in female acceptance or copulation duration between morphs. Morphological comparisons showed that major males have longer pars basalis and ventral plate area, although no differences in ventral plate width, shape, or relative area of the mat of microsetae were detected. Allometric analyses revealed similar hypoallometric patterns for pars basalis length and ventral plate width in both morphs. However, ventral plate length was isometric in minors and hypoallometric in majors, which is the first report of allometric differences in genital traits between male morphs of a species. Our results suggest that despite differing reproductive tactics, the form and intensity of sexual selection on genital morphology is similar between morphs, potentially constrained by stabilizing selection.