Sexual selection is the selection of characters that enhance reproductive success rather than (even at the expense of) survival. The individuals in which the characteristic is well developed will reproduce more often, and leave more offspring, than individuals in which the characteristic is absent or less well developed.

Sexual selection is similar to natural selection, in that a characteristic evolves because it causes the individuals that possess the trait, or that possess it in a more highly developed state, to leave more offspring than individuals that do not possess it. The difference is that sexual selection favors characteristics that directly enhance reproductive success rather than characteristics that enhance it indirectly through survival or obtaining resources. Charles Darwin proposed natural selection as the major mechanism of evolution in his world-changing 1859 book On the Origin of Species By Means of Natural Selection. He also proposed sexual selection in his 1871 book The Descent of Man and Selection in Relation to Sex.

In most animal species, males can produce more offspring than can females. This is partly because sperm cells are small and cheap, while egg cells are large and expensive. In animals with external fertilization, such as most fishes, this may be almost the entire difference in reproductive cost between males and females. In species with internal fertilization (such as most terrestrial vertebrates), at least part of the embryonic development occurs within the body of the female. Bird and reptile eggs are relatively large by the time the female lays them. In terrestrial species with live birth (vivipary), it is the female in which embryonic development occurs. In most animal species in which newborns receive parental care, it is the female that provides a disproportionately large amount of that care. . .

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