I’ve been meaning to read Y: The Last Man, about a mysterious disease that has wiped out half of the mammals on earth — the males ones, with one or two survivors. Professor PZ Myers uses the graphic novel as a kickoff point for discussing the evolution of a sex ratio observed:
One thing that isn’t at all implausible, and actually has been observed, is a plague that selectively exterminates males.It’s called Wolbachia. It’s not quite as dramatic as the plague that turns males into hemorrhaging corpses in the graphic novel — it kills developing males as embryos, or more sneakily, disrupts sex determination so that all the embryos develop as females. This is advantageous to the bacterium, because it is transmitted in the cytoplasm of the egg, so males won’t pass it on to their progeny and are useless from the point of view of Wolbachia. It is, like the plague in Y, something that infects a huge range of species, but each species varies in the severity of its response to the bacterium.
My readers with simple camera eyes will be relieved to know that the disease only affects arthropods. Those of you with compound eyes ought to worry: Wolbachia is also being being considered for use as a biological pesticide.
Now here’s a disease that can have dramatic effects on a population — in some cases, the sex ratio can shift from 50:50 females:males to 99:1. That can be devastating to a population, although of course it’s nowhere near as severe as if that ratio were reversed. Here’s where evolution comes into play. What if a mutation for resistance to the sex-distorter effects of Wolbachia arose? What if, say, one of the rare males carried an allele that made his male progeny able to fight off the deleterious effects of infection with Wolbachia?
I think you can guess. That would be a greatly beneficial mutation that would spread with extraordinary rapidity. Since the rare male carriers would face little competition and would be fertilizing many females, they ought to produce lots of progeny and lots more males, who would spread the resistance further.
Now such an example of evolution in action has been directly observed. Butterflies of the species Hypolimnas bolina, the Blue Moon butterfly, in the Polynesian islands have been known for several years to be suffering from an extreme case of the sex-ratio distorter infection, with populations consisting of greater than 99% females. In 2005-2006, males were found to be making a comeback, and a complete shift from a highly skewed sex ratio to the more normal 50:50 proportions was observed to occur in only 10 generations — about a year. It’s a beautiful example of how rapidly natural selection can transform a population when selection pressures are high.