Evolution of male sexual traits that harm females can be due to some reproductive benefit afforded the male by the harm, or to a negative side effect of a gene with some other positive benefit.
Cosima Hotzy and Göran Arnqvist of Uppsala University in Sweden examined the relationship between the vicious spines on the gentialia of male seed beetles (Callosobruchus maculatus), the damage these cause to females during mating, and sperm competition success.
They found a positive correlation between spine size and both harm to females and male fertilization success. However, statistical modelling showed that spine size still correlated with sperm success when harm was kept constant. This suggests that harm to females is simply an unfortunate side effect of the spines.
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Evolution: Painful pairing. Nature 458, 262 (2009). https://doi.org/10.1038/458262b