Final answer:
Sexual selection occurs when different reproductive pressures lead to the evolution of traits in organisms that enhance reproductive success, often at the cost of survival. It is evident in the striking differences between males and females, or sexual dimorphism, and includes forms of intersexual and intrasexual selection.
Step-by-step explanation:
Sexual selection is a special case of natural selection, focusing on an individual's ability to mate and thus produce offspring. It typically leads to the evolution of traits that may seem contradictory to survival but are advantageous for reproductive success. A classic example is how male birds often display more vibrant colors than females, a form of sexual dimorphism arising from different reproductive pressures.
However, a more nuanced understanding reveals that sexual selection can manifest as either intersexual selection, where individuals of one sex choose mates of the other based on specific traits, or intrasexual selection, where individuals of the same sex compete for mates. These modes of selection can lead to secondary sexual characteristics, which might include, for example, the peacock's elaborate tail that, while it may hinder survival by attracting predators, signals to potential mates that the peacock is fit enough to survive with such a handicap.
Negative frequency-dependent selection and positive frequency-dependent selection involve choosing individuals based on the rarity or commonality of phenotypes, respectively, influencing genetic variance in populations differently than sexual selection.