Tuesday, June 23, 2009

The Evolution of Sex

On the Origin of Sexual Reproduction
Sexual reproduction is costly, so what are the benefits? One of the hypotheses has a catchy name: the Red Queen theory - from the Lewis Carroll character who tells Alice she must keep running faster just to stay in the same place. In biology it's the theory that organisms have to keep changing to keep ahead of their parasites. The recombination of characteristics in sexual reproduction may help.
The widespread system of sexual reproduction found in humans and many other eukaryotes alternates haploid and diploid stages. In humans the gametes (egg and sperm) are hapolid, they only have one copy of each chromosome, while embryos and adult forms are diploid, they have two copies of each chromosome. There's an elaborate mechanism, meiosis, which converts the diploid form into the haploid form. There's another mechanism, ferilizaton, which goes from two haploid forms (the egg and sperm) to the diploid form.

The Evolution of Meiosis From Mitosis
... if there is one event in the whole evolutionary sequence at which my own mind lets my awe still overcome my instinct to analyse, and where I might concede that there may be a difficulty in seeing a Darwinian gradualism hold sway throughout almost all, it is this event—the initiation of meiosis. W. J. HAMILTON (999, p. 419)
THE origins of meiosis in early eukaryotic history have never been satisfactorily explained. Since the reduction-division process in meiosis is essential for sexual life cycles, discussion of the origins of meiosis has been closely tied to debates about the evolutionary value of sex itself and the selective pressures for its maintenance. Yet the cytological events involved in the origins of meiosis are as puzzling as the question of selective pressures. While meiosis almost certainly evolved from mitosis, it has not one but four novel steps: the pairing of homologous chromosomes, the occurrence of extensive recombination between non-sister chromatids during pairing, the suppression of sister-chromatid separation during the first meiotic division, and the absence of chromosome replication during the second meiotic division. This complexity presents a challenge to any Darwinian explanation of meiotic origins. While the simultaneous creation of these new features in one step seems impossible, their step-by-step acquisition via selection of separate mutations seems highly problematic, given that the entire sequence is required for reliable production of haploid chromosome sets. Both Maynard Smith (1978) and Hamilton (1999) regarded the origins of meiosis as one of the most difficult evolutionary problems.

This article discusses something which always bothered me. In meiosis, the chromosomes are replicated first, resulting in four copies of each homolog chromosome. But each gamete requires only one copy. Why not seperate into two gametes directly, without the initial replication - it seems simpler. Apparently, since meiosis evolved from mitosis, and the initial replication is very similiar to what happens in mitosis, it is a relatively simple evolutionary change.
Thus, and perhaps counterintuitively, the evolution of two-step meiosis requires fewer new events than the seemingly simpler one-step process

This article proposes that the main benefit of meiosis is prevention of damage during recombination. Recombination itself is helpful for repairing damaged chromosomes.

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