The South Hills Crossbill Is Evolving in a Seriously Bizarre Way

In the pine forests of Idaho, a bird called the South Hills crossbill is waging one seriously bizarre evolutionary war.

Over the last 5,000 years or so, the crossbill—so named because the two halves of its bill cross over each other instead of aligning—has menaced the lodgepole pine, developing an ever-bigger beak to break into the tree’s cones and steal its seeds. In response, the tree has evolved ever-thicker cone scales. And the South Hills crossbill evolves a bigger bill. And the tree responds. And on and on through the millennia.

That’s not the weird bit. Species evolving together like this is known as coevolution. Happens all the time. The weird bit is that the South Hills crossbill may have speciated without geographic isolation—which is sort of problematic for traditional evolutionary theory. Because while the South Hills crossbill was diverging from other crossbills, it did so while those other crossbills were freely flying through its territory, according to a study published today in Molecular Ecology. That adds to a growing body of evidence that in certain fascinating cases, you may not need geographic isolation to get a new species, challenging what was long gospel among many evolutionary biologists. Gasp—I know.

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