Sexual selection is a part of natural selection. Natural selection mainly determines what traits are best for surviving, while sexual selection determines what traits are best for reproductive success.
Sexual selection operates in 2 ways: intrasexual selection and intersexual selection. Intrasexual selection, intra means within, sexual refers to sex, intrasexual selection: members of one sex, usually males, directly compete for the opportunity to mate with individuals of the opposite sex. I will call it male competition. Intersexual selection inter means between, sexual refers to sex, intersexual selection: members of one sex, usually females, choose their mates from individuals of the other sex based on certain desirable characteristics. We also can call it female choice.
Male competition is the reason for the development of some body parts that use for fighting in males. Like antler in male deer, enlarged claw in male crabs, horn in male sheep. Female choice results in showy characteristics in males. For instance, the long and brightly colored tail of male birds, the colorful dot of male fishes. Sometimes, choosy females cause males to initiate courtship displays, that is the reason why male birds always sing more beautifully than female ones.
2 theories for why sexual selection exists are the sexy son hypothesis and the good genes theory. According to the sexy son hypothesis, females choose sexy partners so their sons will be sexy and attract more females when they grow up. About the good genes theory, females choose a mate based on the male’s ability to pass on genes that will increase the survival or reproductive success of her offspring. The stronger deer, the prettier peacock, the brighter fish win the sexual selection game, they can pass their strong genes, pretty genes, bright genes to future generations. The process can result in sexual dimorphism which means a significant difference between the morphologies of the two sexes within a species. And this exists in humans too, men have Adam’s apple, hairy, … women have widened hip, enlarger boob, …
Is that the more beautiful, the better? I don’t think so, with the males, a colorful appearance is fine at attracting females and it is also good at attracting predators. The magnificent appearance brings the males benefit and cost, being brighter, having more girl-fish, but die soon, being uglier, no predation concern, living long but lonely. So, the color of males today is the balance between benefit and cost.
But why should males compete, and why should females be choosy? Some hypotheses are given. One possible reason is related to the different roles that males and females play in the nurturing of offspring. In most animal species, the female is the primary caregiver, whereas the male plays a minor role. But, the roles can be reversed if the male puts the most time and energy to the offspring, like the seahorse.