Evolution of sexual reproduction

The evolution of sexual reproduction describes how sexually reproducing animals, plants, fungi and protists could have 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 origin of sexual reproduction in prokaryotes is around 2 billion years ago (Gya) when bacteria started exchanging genes via the processes of conjugation, transformation, and transduction. In eukaryotes, it is thought to have arisen in the Last Common Eukaryotic Ancestor (LECA), possibly via several processes of varying success, and then to have persisted. Since hypotheses for the origins of sex are difficult to verify experimentally (outside of evolutionary computation), most current work has focused on the maintenance of sexual reproduction. The maintenance of sexual reproduction - specifically, of its dioecious form - in a highly competitive world had long been one of the major mysteries of biology, as both other known mechanisms of reproduction - asexual reproduction and hermaphroditism - possess apparent advantages over it. Asexual reproduction can proceed by budding, fission, or spore formation not involving union of gametes, which results in a much faster rate compared to sexual reproduction, where 50% of offspring are males, unable to produce offspring themselves. In hermaphoditic reproduction each of the two parent organisms required for the formation of a zygote can provide either the male or the female gamete, which leads to advantages in both size and genetic variance of a population. Sexual reproduction must offer significant fitness advantages to a species because despite the two-fold cost of sex (see below), 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). The issue features in the writings of Aristotle, and modern philosophical-scientific thinking on the problem dates from at least Erasmus Darwin (1731-1802) in the 18th century. August Weismann picked up the thread in 1889, arguing that sex served to generate genetic variation, as detailed in the majority of the explanations below. On the other hand, Charles Darwin (1809-1882) concluded that the effects of hybrid vigor (complementation) 'is amply sufficient to account for the ... genesis of the two sexes'. This is consistent with the repair and complementation hypothesis, described below. Biologists - including W. D. Hamilton, Alexey Kondrashov, George C. Williams, Harris Bernstein, Carol Bernstein, Michael M. Cox, Frederic A. Hopf and Richard E. Michod - have suggested several explanations for how a vast array of different living species maintain sexual reproduction.