“The fact that clonal offspring are genetically identical to their parents should make them highly susceptible to infection by coevolved pathogens. The Red Queen hypothesis predicts that clones are prevented from replacing sexual populations because infection by debilitating parasites erodes their intrinsic twofold reproductive advantage” (www.sciencedirect.com, 2000)
This suggests that asexual reproduction is continually kept in check by the action of parasites, and than sexually reproducing individuals, although also susceptible to parasitic infections, are less susceptible as a species to parasites.
One of the main proponents of the Red Queen hypothesis was Hamilton. In the late 1970s, with the help of two colleagues from the University of Michigan, Hamilton built a computer model of sex and disease, a slice of artificial life. It began with an imaginary population of 200 creatures, some sexual and some asexual. Death was random. As expected, the sexual race quickly died out. In a game between sex and "asexual reproduction, asexual always wins if everything is equal. This is because asexual reproduction is easier, and it's guaranteed to pass genes on to the offspring exactly the same as the parental.
Next they introduced several species of parasite, 200 of each, whose power depended on "virulence genes" matched by "resistance genes" in the hosts. The least resistant hosts and the least virulent parasites were killed in each generation. Now the asexual population no longer had an automatic advantage. It was predominant most often if there were lots of genes that determined resistance and virulence in each creature. In the model, as resistance genes that worked would become more common, then so too would the virulence genes. Then those resistance genes would grow rare again, followed by the virulence genes. As Hamilton (1978) put it, "antiparasite adaptations are in constant obsolescence." But in contrast to asexual species, the sexual species retain unfavored genes for future use. "The essence of sex in our theory, is that it stores genes that are currently bad but have promise for reuse. It continually tries them in combination, waiting for the time when the focus of disadvantage has moved elsewhere." (Hamilton, 1978)
Another theory suggests, “The mutation accumulation hypothesis predicts that sex functions to reduce the population mutational load” (Evolution: Vol. 52, No. 2, pp. 604–610.)
According to this idea, sexual lineages persist because they are more efficient than asexual's at clearing their genomes of mutations. With the onset of time, asexual lineages are expected to be eliminated due to the build up of a large mutation load. Potentially, this idea is sound but would require there to be extensive mutation of the order of one per genome, and the mutations would have to affect fitness in a very specific way.
Another theory is the DNA repair hypothesis, “Crossing over does not occur in Saccharomyces cerevisiae...evidence that the mechanisms of crossing over are shared with those of the DNA repair system”(Botany, Moore, Clark, Vodopich 1998). If a chromosome remains haploid throughout it’s life, then it has no template against which to guide the repair of any damaged or mutated DNA. Therefore the main purpose of meiosis I is DNA repair through the process of crossing over. In asexual reproduction there is of course, no meiosis, only mitosis. Asexually reproducing individuals are therefore more likely to transfer mutated alleles to the next generation rather than sexually reproducing individuals.
The transposon hypothesis states, “The spread of transposons is enhanced by sexual reproduction....” (Botany, Moore, Clark, Vodopich. 1998). Transposons are sequences of DNA that can act as inhibitors, modifiers, or mutators. They move, apparently at random, throughout the genome affecting the expression of genes. Transposons are also known as selfish DNA since they exploit sexual reproduction to enable the production of more transposons. Transposons are important pieces of DNA since they serve as inhibitors to the expression of certain cancer causing genes. Therefore it is thought that sexual reproduction is a means of maintaining transposons within the population, as they can have very positive effects.
The next part of the question concerns why species, in the majority, organise themselves into two sexes; male and females. In a sexually reproducing species, males and females are defined by the kind of gamete they produce. The individuals who produce lots of small, mobile gametes are called males. Those who invest everything in a relatively few large gametes are called females.
“Because of all the resources a female will put into each egg, it makes sense, in most cases, for her to be choosy about whose genes she allows to combine with it, and to continue to invest in its growth and survival after fertilization. For the male, it usually pays best to compete with other males for access to as many eggs as possible. This tends to give rise to the more traditional male/female sex roles.” (http://www.pbs.org/wgbh/evolution/library/01/6/quicktime/l_016_04.html)
This suggests that the role of the male/female system is to allow the female to choose a mate. The idea is that the males will all compete for the females, and so the female has the best opportunity to choose a suitable mate, i.e., the fittest male. In many animal species there are strict sexual rituals that the males must go through to be able to pass its genes onto the next generation, by impressing the female. In some species this involves a fight between the males whereby the strongest will mate with the female. This is so the female will combine with genes from the male who is bigger, stronger and faster. In other species, the ritual may involve elaborate displays, such as the giant peacock, which displays its wings to the female. All in all these rituals serve to maintain a strong gene pool in a population, which can only be maintained by there being two sexes.
Overall, the favour of evolution towards sexual reproduction and the sexes that make it possible, seems to give the species a greater chance of survival by maintaining a rich and diverse gene pool. The evolutionary pressure and species interactions that are forced onto species have resulted in these processes, in each species drive to remain alive and abundant.
Bibliography
Hartl, D.L., Jones, E.W., (1999). The chromosomal basis of heredity. Essential Genetics, 2nd edition, 92-96. pp552
Moore, Clark, vodopich., (1998). Why sex? Botany, 2nd edition, 226-227. pp919
http://www.pbs.org/wgbh/evolution/library/01/6/quicktime/l_016_04.html, 2000
Evolution: Vol. 52, No. 2, pp. 604–610. (1999)
www.sciencedirect.com, 2000
Oxford English dictionary, 1981