Coercive sexual behaviour evolutionary arms race

[Abab]

Was just flicking through wiki and saw an interesting page on animal sexual behaviour and I came across this

'In 2007, research suggested that in the Acilius genus of water beetles (also known as "diving beetles"), an "evolutionary arms race" between the genders means that there is no courtship system for these beetles. "It's a system of rape. But the females don't take things quietly. They evolve counter-weapons." Cited mating behaviours include males suffocating females underwater till exhausted, and allowing only occasional access to the surface to breathe for up to six hours (to prevent them breeding with other males), and females which have a variety of body shapings (to prevent males from gaining a grip). Foreplay is "limited to the female desperately trying to dislodge the male by swimming frantically around.'

Why would sexual contact coercive or not lead to adaption through natural or sexual selection through random mutation? Surely evolution from natural or sexual selection will make the female EASIER to coerce, if that makes the task of copulating with her easier for the survival of their genes.

[canalon]

No because women are picky. It is not in their interest to waste their energy costly eggs on th efirst male that happent to be around. They want a strong guy to have strong offspring. If they resist, the male must be stronger, the stronger they are, the stronger the male need to be and they will have strong offspring.

[Abab]

Why would there be a natural selection arms race between genders tho? Those beetles have no mating ritual, so if a randomly mutated female beetle's back made her harder to copulate with, surely she'd have less offspring than one that was easier.

[canalon]

Yes but since only the strong males could mate with her, her offspring would be stronger than average, giving them an edge in the mating game. Either by allowing them to mate with any female they want (strong males are more likely to coerce any female) or by allowing to refuse the weaklings (stronger female are harder to coerce into mating).

[merv]

maybe these females just want a guy from across the pond instead of their brother? he just hasnt worked it out yet.