In the animal kingdom, much of the evolutionary process boils down to one thing: sex. In mating and passing on their genes, animals shape how future generations will look and behave. A steady stream of successful reproduction shapes a species. So what happens when that stream gets stopped up? Weird things. Scientists in Kenya have found two closely related butterfly populations living in close proximity yet unable to interbreed, thanks to a microbe that keeps killing off all the males. The researchers published their findings in the Proceedings of the Royal Society B.

The richly colored African Queen butterfly (Danaus chrysippus, also known as the "plain tiger") can be found across Africa, Europe, and Asia. There are three known subspecies: D. chrysippus chrysippus; D. chrysippus alcippus; and D. chrysippus orientis. Many of these butterflies carry a companion: the parasitic bacterium Spiroplasma ixodeti. Throughout most of the Queen’s realm, infection with the parasite is harmless. There’s just one place where Spiroplasma starts acting up: Nairobi, Kenya. Two subspecies—D. chrysippus chrysippus and D. chrysippus alcippus—call this area home, and both undergo some pretty intense changes when infected with Spiroplasma.

The bacteria effectively kills their children. But not all their children—just the males. It works like this: a female infected with Spiroplasma will lay both male and female eggs, but the male offspring will fail to hatch, and may be eaten by their sisters.

As a result, there are very few male African Queens of either subspecies in this region. The two mostly-female populations coexist but never interbreed, since, well, they couldn’t. Most males in the area are travelers from other, more normal regions. They, too, may carry the parasite or get it from their mates, but it doesn’t hurt them.

Elsewhere, African Queen subspecies intermingle freely, creating new wing color patterns through hybrid generations. Here though, a tiny bacterium has driven a genetic wedge between the two lady-led societies. In time, the subspecies could diverge so drastically that they become two different species altogether.

It’s all very strange, the authors say. Typically, speciation is driven by big changes in the environment, not tiny microbes.

Co-author Walther Traut is a biologist at the University of Lübbek. "This is like a smoking gun for the way in which species become distinct,” he said in a press statement. “It is rare that we can find the molecular basis for how species develop."

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