Two views of why two sexes

Carl Zimmer describes an experiment supporting the traditional view of the evolution of sex. The theory Why is there sex? To fight the parasite army:
"In the 1970s, several researchers built mathematical models of how parasites influenced the evolution of their hosts and vice versa. Their research suggested that both partners go through cycles of boom and bust. Natural selection favors parasites that can infect the most common strain of host. But as they kill off those hosts, another host strain rises to dominate the population. Then a new parasite strain better adapted to the new host strain begins to thrive, leaving the old parasite strain in the dust.

This model of host-parasite coevolution came to be known as the Red Queen hypothesis, after the Red Queen in Lewis Carroll’s book Through the Looking Glass, who takes Alice on a run that never seems to go anywhere. “Now here, you see, it takes all the running you can do to keep in the same place,” the Red Queen explains.

The Red Queen conundrum, some researchers have argued, may give an evolutionary edge to sex. Asexual strains can never beat out sexual strains, because whenever they get too successful, parasites build up and devastate the strain. Sexual organisms, meanwhile, can avoid these dramatic booms and busts because they can shuffle their genes into new combinations that are harder for parasites to adapt to."

Recent paper supporting the view Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex

The contray view reported here Sex Is Not About Promoting Genetic Variation, Researchers Argue:
"Although many scientists have offered answers to this question, most have focused on the benefit of mixing or purifying genes. But by taking the genome theory into account, Heng's findings may have dethroned the queen of problems in evolutionary biology.

According to Heng, the hidden advantage sex has over asexual reproduction is that it constrains macroevolution -- evolution at the genome level -- to allow a species' identity to survive. In other words, it prevents "Species A" from morphing into "Species B." Meanwhile, it also allows for microevolution -- evolution at the gene level -- to allow members of the species to adapt to the environment.

Considering their observations and those of paleontologists, population geneticists and ecologists cited in the article, Heng and Gorelick argue that new research should focus on the genome, not just the individual genes, because the genome serves as both the genetic information unit and selection package for evolution."

The article available here:

Sex Reduces Genetic Variation: A Multidisciplinary Review by Root Gorelick, Henry H. Q. Heng