Reproductive systems are adaptations of organisms that promote crossbreeding with other organisms in the population. Reproductive systems are part of the life history of an organism, which is the pattern of growth and reproduction, from birth to death. Sexual reproduction allows new gene combinations to be produced. Many adaptations have evolved that enhance reproductive success, differently for males and females.

Across the animal kingdom, individuals typically seek mates that are genetically unrelated to them. Humans do this not only through social convention (the "taboo" against incest is one of the few nearly universal human morals) but also subconsciously. Experiments by Swiss evolutionary biologist Claus Wedekind, in which women were asked to rate the attractiveness of the scent of shirts worn by men (whose identity was concealed), showed that the women chose men who had immune system proteins most different from theirs.

Evolution depends on the availability of genetic variation, therefore genetic recombination, in populations; thus the importance of reproductive systems can hardly be overemphasized. Since natural selection favors only those characteristics that benefit individuals, the genetic variation promoted by reproductive systems must confer advantages on individuals, not just on populations. This entry considers seed plant and animal examples, but other kinds of organisms (e.g., fungi) also have adaptations that promote crossbreeding. . .

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