Mechanical isolation emerges from individual-level morphological covariation under reinforcement

Mechanical isolation is often viewed as a discrete species-level premating barrier arising from divergence in reproductive morphology. However, whether such isolation reflects discrete species-level incompatibility or emerges from continuous variation in morphological matching among interacting individuals remains unclear. We investigated how reinforcement and sexual selection shape morphological compatibility between male grasping appendages and the corresponding female contact structures in the damselflies Ischnura elegans and Ischnura graellsii, and how this compatibility constrains heterospecific tandem formation. Using geometric morphometrics across allopatric and hybrid-zone populations, we quantified sex-specific evolutionary trajectories of reproductive traits. Females of I. elegans exhibited greater phenotypic change consistent with asymmetric reproductive character displacement. Female trajectories converged in morphospace during late secondary contact, whereas male trajectories showed weaker and less directional phenotypic change. Partial least squares analyses revealed asymmetric covariance between heterospecific male and female reproductive traits during late secondary contact, indicating that, despite overall divergence, some male-female trait combinations allow functional matching. Linking the major axis of heterospecific covariance to tandem success showed that the probability of tandem formation success depends asymmetrically on the degree of size mismatch between partners, and that this relationship differs between reciprocal crosses. Our results suggest that reinforcement reshapes species-level trait distributions, but the outcome of heterospecific interaction depends on morphological matching rather than between individual males and females.