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.