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Sexual reproduction - WiKi

Sexual reproduction is a process that creates a new organism by combining the genetic material of two organisms. It occurs both in eukaryotes[SUP][1][/SUP][SUP][2][/SUP] and prokaryotes:[SUP][3][/SUP] in multicellular eukaryote organisms, an individual is created anew; in prokaryotes, the initial cell has additional or transformed genetic material. In a process called genetic recombination, genetic material (DNA) originating from two different individuals (parents) join up so that homologous sequences are aligned with each other, and this is followed by exchange of genetic information. After the new recombinant chromosome is formed, it is passed on to progeny.

Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. The evolution of sexual reproduction is a major puzzle. The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1 to 1.2 billion years ago.[SUP][4][/SUP] There are two main processes during sexual reproduction in eukaryotes: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually exchange genetic information to achieve homologous recombination. Evolutionary thought proposes several explanations for why sexual reproduction developed and why it is maintained. These reasons include fighting the accumulation of deleterious mutations, increasing rate of adaptation to changing environments[SUP][5][/SUP] (see the red queen hypothesis), dealing with competition (see the tangled bank hypothesis) or as an adaptation for repairing DNA damage and masking deleterious mutations.[SUP][3][/SUP][SUP][6][/SUP][SUP][7][/SUP] The maintenance of sexual reproduction has been explained by theories that work at several different levels of selection, though some of these models remain controversial. New models presented in recent years, however, suggest a basic advantage for sexual reproduction in slowly reproducing, complex organisms, exhibiting characteristics that depend on the specific environment that the given species inhabit, and the particular survival strategies that they employ.[SUP][8][/SUP]