Itâ€™s easy to imagine how a physical barrier, like a river or a mountain range, could create new species. If two populations of the same creature get stuck on either side of the divide, they wonâ€™t be able to breed. Theyâ€™ll evolve along their own separate paths, until the differences between them become so big that breeding would be impossible, even if they were to meet. One species becomes two.
There are also many examples where new species arise without any barrierâ€”where one population splits in two even though all of its members share the same space. This is called sympatric speciation (from the Greek for â€œsame fatherlandâ€). It was proposed more than a century ago, and has been controversial for much of that time. But scientists have found more and more examples that support the concept, where new wasps, flies, fish and trees evolve side by side. Just last week, news broke about a deadbeat ant that branched off from its parent species, while living in the same colony.
The latest intriguing example comes from James Van Leuven and John McCutcheon at the University of Montana. It involves a bacterium called Hodgkinia that split into two distinct species, while living in the cells of an insect. There is no barrier. Sardines in a can have nothing on Hodgkinia. These bacteria are crammed into the same tightly packed microscopic structures, but somehow, theyâ€™ve managed to become two distinct species.
The two daughter species are like two halves of their ancestor. Theyâ€™ve each lost genes that the original Hodgkinia had, but theyâ€™ve jettisoned different genes. Each compensates for the losses of its sister species. They complement each other perfectlyâ€”put them together, and youâ€™d (almost) the complete genome of the ancestor.
Van Leuven and McCutcheon made their discovery by studying cicadasâ€”insects known for their ear-splitting songs. About five years ago, they showed that one species of cicada has two bacteria living inside its cellsâ€”Sulcia and Hodgkinia. This is pretty normal. Many insects have helpful internal bacteria or â€œendosymbiontsâ€. In sap-sucking groups like cicadas, these microbes act like dietary supplements, making nutrients that are missing from their diet.
Things got strange when Van Leuven and McCutcheon analysed DNA from a South American cicada called Tettigades undata. They found many fragments of Hodgkinia DNA but, try as they might, they couldnâ€™t unite those pieces into a single genome. They always assembled into two separate ones. For simplicity, Iâ€™m going to call these H1 and H2.