Worms forced to reproduce asexually succumbed to a nasty bacterial infection and died. The researchers say the results are the most convincing evidence to date for a key theory in evolutionary biology. The theory holds that sex evolved because it lets organisms reshuffle their genes into new combinations to stay a step ahead of parasites. Sex has long troubled evolutionary biologists. Okay, this explains sex in a whole new perspective. Parasites create a situation where, in spite of the disadvantages of sex, it is good for an organism to reshuffle its genome with that of another.