Final answer:
The situation with male water striders with exaggerated grasping structures and females with obstructive structures is an example of antagonistic coevolution, where both sexes evolve in response to each other's adaptations.
Step-by-step explanation:
The situation described where male water striders forcibly copulate with females and exhibit exaggerated grasping structures, while females have developed structures to obstruct the male's grip, can be described as a result of antagonistic coevolution. This evolutionary process occurs when two species influence each other's evolution in response to selective pressures exerted by one on the other, often resulting in an evolutionary arms race. For example, as the male water striders develop more efficient grasping structures to mate with females, the females correspondingly develop structures to counteract this grip, facilitating or thwarting mating attempts.
In the context of sexual selection, two main types are identified by Darwin: intrasexual selection, involving competition among individuals of the same sex, typically males, for mates; and intersexual selection, involving mate choice by one sex, typically females. The situation described in the question does not align with the Zahavi principle, sensory bias, or intrasexual selection but is a clear example of antagonistic coevolution where both sexes have adapted characteristics to control mating outcomes in response to each other's evolutionary changes.