Final answer:
Intrasexual selection is when members of the same sex compete for mates, a process which, along with intersexual selection, can lead to sexual dimorphism.
Step-by-step explanation:
The blank in the given student's question should be filled with the term intrasexual selection, which occurs when members of the same sex compete for access to mates of the opposite sex. This is a type of sexual selection, where significant energy is expended in the activities of locating, attracting, and mating with a partner. Examples of intrasexual selection include rams butting heads or males engaging in displays to fend off competitors, often to mate with females in a given territory.
In contrast, intersexual selection, also a form of sexual selection, involves choosing a mate based on desirable characteristics, such as the bright plumage of a peacock, which females may find attractive. Both intrasexual and intersexual selections can result in the development of secondary sexual characteristics that are crucial for successful reproduction but may not necessarily enhance survival.
Sexual dimorphism is the phenotypic difference between males and females of the same species, resulting from these differing selective pressures.