Final answer:
Darwin called the process where differential reproductive success is influenced by variations in individuals' success at obtaining mates 'sexual selection'. This can lead to sexual dimorphism, where males and females of a species show distinct physical characteristics or behaviors due to different selection pressures.
Step-by-step explanation:
Darwin called differential reproductive success due to variation among individuals in their success at getting mates sexual selection. This type of selection is one aspect of natural selection and can lead to the evolution of traits that improve an individual's chances of mating but might not necessarily increase their chances of survival. Sexual selection occurs in two forms: through intrasexual selection, involving competition amongst the same sex, typically seen as male-male conflicts over territory or mates, and intersexual selection, where one sex chooses a mate based on desirable characteristics, like feather colors or mating dances.
Furthermore, when males and females of a species exhibit different physical characteristics or behaviors, the term sexual dimorphism is used to describe this difference. These phenotypic distinctions are often the result of sexual selection pressures that favor traits in one sex that are attractive to the other, even if they are costly in terms of survival.