Final answer:
Sexual selection is the concept that explains differences in the mating success of individuals within a species, leading to sexual dimorphism where males and females have distinctly different traits related to reproduction.
Step-by-step explanation:
The concept that explains differences in the number of mates or in the reproductive capacity of individuals of one sex in a population is sexual selection. Sexual selection leads to sexual dimorphism, where males and females of a species exhibit different traits that improve their chances of reproduction. Such traits may include body size, colors, ornaments, or behaviors that signal attractiveness or dominance to potential mates or competitors. For example, male peacocks have bright feathers, which they display to attract females. This is an example of intersexual selection. On the other hand, when males compete with each other for access to females, as seen in rams that butt heads, this is known as intrasexual selection. Both mechanisms of sexual selection can lead to increased reproductive success for individuals that possess favorable traits.