http://en.wikipedia.org/wiki/Evolution_of_sex
The Evolution of Sex
The evolution of sex is a major puzzle in modern evolutionary biology. Many groups of organisms, notably the majority of animals and plants, reproduce sexually. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction. Several explanations have been suggested by biologists including W. D. Hamilton, Alexey Kondrashov, and George C. Williams to explain how sexual reproduction is maintained in a vast array of different living organisms.
The two-fold cost of sex
This diagram illustrates the two-fold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation. In all sexual species, the population consists of two sexes, only one of which is capable of bearing young (with the exception of simultaneous hermaphrodites). In an asexual species, each member of the population is capable of bearing young. This implies that an asexual population has an intrinsic capacity to grow more rapidly each generation. The cost was first described in mathematical terms by John Maynard Smith [1]. He imagined an asexual mutant arising in a sexual population, half of which comprises males that cannot themselves produce offspring. With female-only offspring, the asexual lineage doubles its representation in the population each generation, all else being equal. Often all else is not equal, however, in which case the realized fitness cost to sex may be much less than this intrinsic two-fold cost of producing males. For example, an asexual mutant arising in a sexual population occupies a niche frozen to that of its parental genotype because the asexual descendants are genetically self-identical. Analysis of competitive Lotka-Volterra equations suggests that the asexual lineage may never realize its full two-fold advantage in population growth capacity, if the broader niche of the sexual population confers even a small competitive edge [2].
An additional cost is that males and females must search for each other in order to mate, and sexual selection often favours traits that reduce the survival of individuals [1].
Evidence that the cost is not insurmountable comes from George C. Williams, who noted the existence of species which are capable of both asexual and sexual reproduction. These species time their sexual reproduction with periods of environmental uncertainty, and reproduce asexually when conditions are more favourable. The important point is that these species are observed to reproduce sexually when they could choose not to, implying that there is a selective advantage to sexual reproduction [3].
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Note: sexual reproduction has advantages once it has been established. The dilemma is how it could ever have evolved in the first place.
The assumption of multiple advanced types at the beginning of course solves this dilemma.
It is interesting to note that the Wikipedia article labeled
The Evolution of Sex doesn't discuss the evolution of sex.
