The evolution of sex is a major puzzle in modern evolutionary biology. Many groups of organisms, notably the majority of animals and plants, reproduce sexually. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction. Several explanations have been suggested by biologists including W. D. Hamilton, Alexey Kondrashov, and George C. Williams to explain how sexual reproduction is maintained in a vast array of different living organisms.
It seems that a sexual cycle is maintained because it improves the quality of progeny (fitness), despite reducing the overall number of offspring (twofold cost of sex). In order for sex to be evolutionarily advantageous, it must be associated with a significant increase in the fitness of offspring.
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