Olivia Judson in the NYT:
[I]f you look at the great tree of life, asexual groups tend to be out on the twigs: there are no great branches of the tree that contain only asexuals. In other words, no one can point to a big group, such as birds or fish, or even snails, and say, “That’s a group composed only of asexuals.” What this means is that asexuality evolves often, but rarely persists for long: asexuals typically go extinct soon after they appear.
The swift extinction of asexuals, and the absence of big asexual groups, suggests that sex is essential for long-term evolutionary success: giving up sex is a Bad Idea, a kind of evolutionary suicide.
Exactly why this is so remains unclear. But it’s thought to have something to do with the fact that sex generates new gene combinations. Whereas a sexual creature like you or me inherits a unique mix of genes from our parents, asexuals are lumbered with the same set of genes their mother had. For an asexual, then, the only source of genetic novelty is mutation.(Mutations and sex are both sources of genetic variation, but they work differently. Mutations — accidental changes to DNA — are the ultimate source of genetic novelty. However, mutations tend to be harmful more often than they are helpful: they tend to disrupt genes that are already working. Sex, in contrast, takes pre-existing genetic variation and shuffles it, generating new gene combinations.)
Which brings me to the bdelloids. These animals are the great exception: a group of more than 450 species from which sex is entirely absent. How are they managing to flourish despite this epic period of abstinence?