Evolution of sex

The evolution of sexual reproduction describes how sexually reproducing animals, plants, fungi and protists evolved from a common ancestor that was a single celled eukaryotic species. There are a few species which have secondarily lost the ability to reproduce sexually, such as Bdelloidea, and some plants and animals that routinely reproduce asexually (by apomixis and parthenogenesis) without entirely losing sex. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance.

The maintenance of sexual reproduction in a highly competitive world has long been one of the major mysteries of biology given that asexual reproduction can reproduce much more quickly because 50% of offspring from sexual reproduction are males, unable to produce offspring themselves. However, research published in 2015 indicates that sexual selection can explain the persistence of sexual reproduction in animals.

Since hypotheses for the origins of sex are difficult to test experimentally (outside of Evolutionary computation), most current work has focused on the maintenance of sexual reproduction. Sexual reproduction must offer significant fitness advantages to a species because despite the two-fold cost of sex, it dominates among multicellular forms of life, implying that the fitness of offspring produced outweighs the costs. Sexual reproduction derives from recombination, where parent genotypes are reorganized and shared with the offspring. This stands in contrast to single-parent asexual replication, where the offspring is identical to the parents. Recombination supplies two fault-tolerance mechanisms at the molecular level: recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time) and complementation (also known as heterosis, hybrid vigor or masking of mutations).

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