The Economist has a story about a mutant asexual crayfish lineage. It points out how this is a good test of the sex-is-good-against-disease thesis. The basic logic is that an allele which causes asexuality has a greater short term natural increase (because it has a 100% as opposed to 50% chance of being passed to the next generation). Nevertheless, because the conventional reshuffling that occurs during sexual reproduction through the synthesis of two haploid gametes into a diploid (as well as recombination) is not operative in a the clonal lineage, they are vulnerable to fast adapting pathogens which might wipe them out. One could conceptualize the modal genetic profile of the lineage as the selective pressure exerted upon the pathogens. In the case of an asexual lineage the the pathogen has a straight shot at a stationary adaptive peak (mutation of course will introduce variation over time). In contrast, sexual lineages with their wide distribution of of genetic architectures are like moving targets, as a population of pathogens shifts toward one peak, selection within the host population reshapes the adaptive landscape (remember the power of selection is proportional to extant heritable variation, which sexual lineages have in abundance vis-a-vis clonal ones). Imagine, if you will, a rugged adaptive landscape with many peaks always bubbling and morphing, as the pathogens race up the slopes the ground underneath them shifts and gives away and soon they find themselves having to traverse a radically altered surface. In contrast, an asexual population would exhibit one sharp peak of relative stability, and its lack of genetic variation means that the surface itself is relatively rigid and stable.
Note: Some researchers have proposed that the persistence of sex in complex organisms is a function of phylogenetic constraint. That is, once the lineages flipped from asexual to sexual they couldn’t flip back, even if it was adaptive. This sort of view is rather diminishes the power of selection to overcome phylogenetic hurdles. To my mind crayfish are a rather complex organism. But to those of you who know organismic biology: what is the most complex animal which can reproduce asexually? (I’m assuming it is a reptile?) It is supposed that imprinting prevents the emergence of asexuality in mammals.