A male Blue Moon Butterfly. From Wikipedia.
Ecological catastrophes don’t always have to involve a meteor, intense volcanism, or global warming; disease can wipe out a population so quickly and efficiently that the concept of widespread epidemic is absolutely terrifying. Killers aren’t indiscriminate, however, and often times the opposite is true, especially as evidenced in the new Science paper “Extraordinary Flux in Sex Ratio.”
Being a male Blue Moon Butterfly (or Great Eggfly, Hypolimnas bolina) living in the Samoan islands wasn’t such a good deal in recent years, a bacteria of the genus Wolbachia (known for its devastating effects on arthropods) killing off 99% of the male butterflies by 2001. Evolution, however, provided some males with a natural resistance to the bacterial attack, however, and since the spring of 2005 male butterflies have been making a comeback.
Such is natural selection at work, and if this was all there was to the story it would be an open-and-shut case, but there is a very intriguing twist to the story. In 2001, only 3 out of 64 females produced any male offspring at all, but in the new study all 14 of the captured females produced male offspring. In about the time of 10 generations, male populations that were practically decimated have returned to a near 1:1 male/female ratio, an absolutely amazing change of fortune for the males and the species. The disease remains in the population, both males and females carrying the disease, but the disease was perhaps too good, leaving only those males that would not fall victim to it which then gave rise to a resistant population.
But what of the future of these insects? When you have a reported 100:1 female/male ratio, then genetic diversity is going to be greatly reduced, perhaps opening the door to a future disease (even a strain of Wolbachia that can overcome the suppressor genes that give the male butterflies resistance), at which time the butterflies may not have enough variation to survive. If we’re to look at this as an evolutionary arms race between butterfly and bacteria, whomever is the quickest draw will ultimately win, and the butterflies will have to restore the amount of variation amongst their populations if they are to survive the next epidemic, Wolbachia or otherwise.