Sexual selection is a type of natural selection that occurs due to variation in mating success. For example, have you ever wondered why birds-of-paradise (or many other birds) are brightly colored? And why is it typically only the male birds that are so colorful? The male birds court the female birds through color, unique morphological features, songs, and dances. Males that display elaborate versions of these qualities are more desirable to a female, because this informs the female that he is healthy, strong, and intelligent. Essentially, he is persuading the female that his genes are the most desirable. Thus, the males with the most extravagant features are chosen as mates and these traits for bright coloration or elaborate morphological features (or behavior) are passed onto offspring. Consequently, these traits become more frequent within the population. Over time, as these traits are selected for over and over again, the features become more and more exaggerated, even as they begin to interfere with survival (think conspicuous colors that prevent camouflage or large tail feathers that interfere with flight).
Watch the first video below to learn more about sexual selection: