Gebruiker:Martix/Kladblok/Evolutie van seksuele voortplanting
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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.[1][2][3] 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[4]. 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[5].
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 in a highly competitive world had long been one of the major mysteries of biology given that asexual reproduction can reproduce by budding, fission, or spore formation not involving union of gametes, which reproduce at a much faster rate compared to sexual reproduction. 50% of offspring from sexual reproduction are males, unable to produce offspring themselves.
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).