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Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the offspring. It is characterized by two processes: 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 cross over to achieve genetic recombination.
The evolution of sex is a major puzzle in modern evolutionary biology. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years ago with DNA forming 3.5 - 4.6 billion years.Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.
A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the 'ratcheting' of evolutionary speed as one clade competes with another for a limited resource.
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