An interesting by Goddard et al (2005) using genetically modified yeasts could explain the importance of sex in evolution. This supports Weismann's claims that sex improves genetic variation and thereby promote evolutionary adaptation. Goddard's team created a asexual strain of yeast and subjected it together with a sexual strain to 2 novel environments, harsh and benign. Both strains did equally well in the benign environment. However the sexual strain showed a higher growth in the harsh environment (with elevated temperature). This could be the first experimental evidence to the age old theory that sex promotes the fitness of the species.
See - Goddard, M.R., Charles, N. and Burt A 2005 Sex increases efficacy of natural selection in experimental yeast populations. Nature 434 pp. 636 - 640