When an invasive bacteria ravaged the male embryos of the Blue Moon butterfly, it left populations that were nearly entirely female. But in an extraordinary example of natural selection at work, the males made a comeback, going from 1% of the population to 39% in the span of one year, or 10 generations. Researchers at UC Berkeley and elsewhere witnessed this evolutionary event and credited it to the rise of a suppressor gene that stopped the male-killing bacteria in its tracks.