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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. One of the most widely accepted explanations for the advantage of sex lies in the creation of genetic variation. There are three possible reasons this might happen. First, sexual reproduction can bring together mutations that are beneficial into the same individual (sex aids in the spread of advantageous traits). Second, sex acts to bring together currently deleterious mutations to create severely unfit individuals that are then eliminated from the population (sex aids in the removal of deleterious genes). Last, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among relatives.

These classes of hypotheses are further broken down below. It is important to realise that any number of these hypotheses may be true in any given species (they are not mutually exclusive), and that different hypotheses may apply in different species. However, a research framework has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanism is acting in each species.

Source from : http://en.wikipedia.org/wiki