Biologist Lixing Sun authored the book "On the Origin of Sex: The Weird and Wonderful Science of Reproduction," exploring the evolutionary puzzles of sexual reproduction and mating systems across species. Naturalist Charles Darwin stated, "We do not even in the least know the final cause of sexuality." He added that "The whole subject is as yet hidden in darkness."

Darwin, who had ten children with his wife, experimented with plants during the latter part of his life. He studied cowslips, which have flowers in two different forms, and published a paper on them in 1862. More recently, evolutionary biologist John Maynard Smith wrote a book about the puzzle of sex in the 1970s. Smith noted, "On the most fundamental questions—the nature of the forces responsible for the maintenance of sexual reproduction and genetic recombination—my mind is not made up."

Smith proposed a thought experiment involving a mutation that allows a female to reproduce through parthenogenesis, labeling the reproductive benefit the "twofold advantage of parthenogenesis," a concept also known as the "twofold cost of sex." Other theories attempt to explain sexual reproduction, such as Muller's ratchet, named for Nobel Prize-winning geneticist Hermann Muller, which posits that sexual reproduction counters the buildup of damaging mutations. The Red Queen hypothesis argues that sexual reproduction helps species stay ahead of parasites, while the tangled-bank hypothesis holds that it allows for more efficient resource use through genetically diverse offspring. The Vicar of Bray hypothesis suggests genetic mixing from sexual reproduction enables species to adapt more quickly to environmental changes.

Mating systems vary across species. The protozoan Tetrahymena thermophila, approximately one-twentieth of a millimeter long, can reproduce by splitting in half or by conjugation. Conjugation involves two cells fusing and swapping genetic material. This protozoan has seven distinct mating types, leading to twenty-one possible pairings. Sun commented, "That's one wildly complicated dating pool for a single-celled organism!" Other organisms also display complex mating types; the common white-button mushroom has eighteen, the fairy inkcap mushroom has one hundred and forty-three, and the split-gill mushroom has twenty-three thousand three hundred and twenty-eight.

In species such as elephant seals, red deer, and rattlesnakes, mating privilege is determined by fighting. In lions, only one male and his associates sire offspring, and a new male taking charge of a pride typically kills the previous male's cubs. Mountain gorillas exhibit similar behavior, with new males frequently killing babies fathered by the previous silverback. Sun noted, "Infanticide is surprisingly common, especially among carnivores and primates, often driven by the brutal logic of mate competition."

Jacanas, which are waders found in the tropics, represent a different mating system. In most of the eight jacana species, males incubate the eggs. Female jacanas frequently kill the chicks of other birds, mate with the male tending them, and leave him to raise a new clutch. Neotrogla, a genus of bark lice native to Brazil, live in dry caves and feed predominantly on bat guano. Male Neotrogla produce spermatophores, which are capsules of sperm containing protein-rich fluid. Female Neotrogla have evolved a penis-like appendage used to extract spermatophores from mates.