From MSNBC:
Scientists have long wondered why organisms bother with sexual reproduction. It makes a whole lot more sense to just have a bunch of females that can clone themselves, which is how asexual reproduction works. Turns out sex might have evolved as a way to concentrate lots of harmful mutations into individual organisms so they could be easily weeded out by natural selection, a new computer model suggests.
It provides a way for organisms to swap and shuffle genes and to create offspring with new gene combinations that might survive if the environment suddenly changes.
More here.