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So how does sexual selection explain ornamentation? Darwin's problem was the ubiquity of large, costly, complex traits like peacock's tails that seem to contribute nothing to an animal's survival ability. Natural selection, as Darwin defined it, arises from individual differences in survival ability. It cannot favor traits opposed to survival. Since most ornaments decrease an individual's survival ability, they presumably could not have evolved by natural selection for survival.

This means that evolution must include some form of creative, trait-shaping selection other than natural selection. Darwin reasoned that in a sexually reproducing species, any traits that help in competing for sexual mates will tend to spread through the species. These traits may evolve even if they reduce survival ability. While natural selection adapts species to their environments, sexual selection shapes each sex in relation to the other sex. In The Origin, Darwin argued that sexual selection depends "not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring."

Darwin didn't know about genes or DNA. But he understood that in a sexually reproducing species, the only way to pass a trait from one generation to the next was, by definition, through sexual reproduction. If an animal doesn't have sex, its heritable traits will die with it, and it will leave no hereditary trace in the next generation. As far as evolution is concerned, the animal may as well have died in infancy. Survival without reproduction means evolutionary oblivion. On the other hand, reproduction followed by death can still translate into evolutionary success. Sexual inheritance puts sexual reproduction at the heart of evolution. The concept of sexual selection is simply a way of describing how differences in reproductive success lead to evolutionary change.

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