Sexual selection describes the acquisition or selection of a mate based on a specific heritable trait; therefore, it is special case of natural selection because the mate or a rival for mates, not the environment, is the selective agent. Specifically, sexual selection is based on secondary sexual characteristics (e.g., coloration or behavior). This type of selection usually leads to the enhancement of sexual dimorphism (differences in the secondary sex characteristics) between males and females. For example, think of the peacock and its feathers; female peahens are more likely to mate with males bearing long, brightly colored tail feathers with a larger number of eye spots (although these feathers are not directly related to reproduction in any way). Also, the peacock's tail feathers are not a survival advantage; rather, they probably make the peacock more vulnerable to predation and they take a lot of energy to maintain. So there is a balance between sexual and natural selection. If female preference pushes a trait further in one direction, natural selection will push back if the exaggerated trait reduces the fitness of the male because he cannot survive to reproduce.
It is important to point out that sexual selection and natural selection are different in one important aspect. Natural selection increases the adaptation of a population to a specific set of environmental conditions, whereas sexual selection does not. However, both are forms of selection because they act to change the allele frequencies a population.
Other examples of secondary sexual characteristics and sexual dimorphism include the male lion's mane, the bright colors of the male cardinal and male blue jay, and the striking sexual differences in the gigantic beetles Macrodontia cervicornis and Dynastes hercules. Note: the males of both species are over 12 inches long!
Another example of sexual selection is observed in fantail darter fish (Etheostoma flabellare). Females show a mating preference toward males that are already guarding eggs. This behavior is presumably a result of the increased fitness of males that are able to protect eggs to maturity. As an effect of this behavior, some males appear with modified epidermal cells that look like fleshy knobs on their dorsal fins. These fleshy knobs mimic eggs in shape and color. Females have shown mating preference for males displaying these egg mimics, increasing their fitness.
Genetics Selection and Speciation Part 2 VoiceThread Transcript