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Description
It is an astonishing finding--derived from more than a century of painstaking research into the cellular basis of reproduction in a huge variety of organisms--that sex is the most prevalent mode of reproduction among the great division of life (the eukaryotes), which includes animals, green plants, algae, fungi, and protozoa. (1)
To geneticists, sexual reproduction is the formation of a new individual from a cell (zygote) produced by the union of two different cells (gametes). In the case of animals, the gametes are an egg and a sperm. When the resulting individual reproduces, its gametes contain a patchwork of genetic information derived from each of the two gametes that generated it (a process called recombination). Recombination happens regardless of whether the zygote divides to form many separate single-celled individuals (as in simple organisms, like yeast), or whether the daughter cells remain associated to produce a complex multicellular organism, like an oak tree or a person. In contrast, with asexual reproduction, a single parent produces offspring that are usually exact genetic replicates of itself.
We have good grounds for believing that regular sexual reproduction evolved very early in the history of the eukaryotes, and that most instances of asexual reproduction among them are the result of subsequent evolution. All mammals and all birds reproduce sexually, but only a few dozen species of reptiles, amphibia, and fish reproduce asexually. (2) Similarly, only about 0.1 percent of the over three hundred thousand species of flowering plants are thought to reproduce asexually. (3)
Most asexual species seem to be of recent evolutionary origin, since they have close sexual relatives and evidently have not had time to proliferate into diverse forms. (4) There are only one or two cases where an asexual group of multicellular organisms seems to have been around long enough to diversify, most notably the Bdelloid rotifers. These minute animals, which live in transient freshwater habitats (such as drops of water on mosses), have been classified into several hundred species on the basis of anatomical and molecular differences among them. No males have ever been found--and study of their genomic makeup supports the view that they represent an ancient asexual group, many millions of years old. (5) Nonetheless, the Bdelloid rotifers represent the exception, and not the rule.
Asexuality seems to be more common among single-celled eukaryotes, like protozoa, but the difficulty of studying their life cycles in nature makes it hard to exclude the cryptic occurrence of sex. And even so, regular sexual reproduction is widely distributed among single-celled eukaryotes. The common features of the cellular and molecular mechanisms involved in sexual reproduction in these and multicellular eukaryotes show that the cellular machinery involved in sexual reproduction probably had a single origin around the time of the evolution of the first eukaryotes, about two billion years or so ago.
The big question about sex is: why bother? It seems much simpler for organisms to produce offspring without going to the trouble of making gametes, which in the case of animals like ourselves can only meet each other as a result of elaborate behavioral and anatomical adaptations. Why should there be males? Why don't women simply produce babies in the same way as Bdelloid rotifers: an egg is generated by the same process of cell division that makes the cells of the rest of the body; it then develops into an offspring. Indeed, why not just split in half and regenerate the missing half, as some flatworms do?
These questions are not new: as Edward Gibbon maliciously pointed out, the early fathers of the Christian church were sorely troubled by the question of why God had not provided human beings with "some harmless mode of vegetation" with which to propagate themselves. Their objections to sex were, of course, purely moral. But even the amoral intellectual framework of neo-Darwinian evolutionary biology has raised a searching question concerning the prevalence of sex--or, more specifically, about its so-called twofold cost, which John Maynard Smith brought to the attention of biologists in 1971. (6) One can understand this cost by considering a sexual population with an equal number of males and females in each generation. Now imagine that within this population a mutation arises that causes females to reproduce asexually by means of all-female offspring. If the mutation has no other effect, the average number of offspring per mother will be unchanged. The mutant females will thus produce twice as many daughters as their sexual competitors. A simple calculation shows that the frequency of the mutants within the female population will double each generation while they are still rare, and that they will spread rapidly through the population, replacing the sexual females and causing the extinction of males.
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