Final answer:
Evolution of secondary sexual characteristics originates from intersexual selection and intrasexual selection, where the former is about mate choice based on attractive traits, and the latter involves competition within the same sex to secure a mate.
Step-by-step explanation:
The two mechanisms that lead to the evolution of secondary sexual characteristics are intersexual selection and intrasexual selection. Intersexual selection, often referred to as mate choice, occurs when individuals of one sex (typically females) choose mates of the other sex based on certain desirable traits. These traits may include visual, aural, tactile, or chemical cues that lead to traits that are attractive to the opposite sex but not necessarily beneficial to survival.
In contrast, intrasexual selection refers to the competition for mates within the same sex, often resulting in aggressive mating rituals or displays that ultimately benefit the fittest or most dominant participants. Examples of intersexual selection include peahens selecting peacocks with bright plumage, while intrasexual selection may be observed in rams butting heads to establish mating rights.