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The [[evolution]] of [[sexual reproduction]] is a major puzzle. The first [[fossil]]ized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1 to 1.2 billion years ago. [http://proxy.arts.uci.edu/~nideffer/Hawking/early_proto/orgel.html] Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all [[animal]]s and [[plant]]s. Bacterial conjugation, the transfer of [[DNA]] between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.

The [[evolution]] of [http://www.biolreprod.org/ sexual reproduction] is a major puzzle. The first [[fossil]]ized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1 to 1.2 billion years ago. [http://proxy.arts.uci.edu/~nideffer/Hawking/early_proto/orgel.html] Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all [[animal]]s and [[plant]]s. 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 in response 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.

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 in response 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.