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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Sing Like Nobody’s Listening

Typically to get something to speciate, you have to isolate the animal, say, on an island. Over time, an animal species will diverge genetically from its compatriots on the mainland, since it can’t breed with them and mix genes. You can also get this phenomenon on the mainland, if, for example, a new river cuts through an ecosystem, dividing a population in two.

But other crossbill species infiltrate the South Hills variety’s territory all the time. “We catch them every year, hundreds move through,” says University of Wyoming ecologist Craig Benkman, an author of the study. “Some breed, some stay, but most move on. So it’s not like there was a geographic isolation.”

How on Earth, then, did the South Hill crossbill end up diverging genetically and largely stop breeding with other crossbills in the absence of isolation? It wasn’t a new river that transformed this ecosystem in southern Idaho: It may have been a bigger phenomenon.

“It turns out in the South Hills, based on the pollen fossil record, based on our forest reconstructions, there was probably very little lodgepole pine forest about 5,000 to 7,000 years ago,” says Benkman. That changed when the climate began to warm and the conifers moved in. Yet squirrels–a crossbill’s major competitor for pine seeds–never followed. In the absence of this competition, it seems the South Hills crossbill initiated a coevolutionary arms race with the pines. Natural selection would favor pines with thicker scales to keep the birds out, but would also favor birds with bigger bills that would get the birds in.

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But why wouldn’t those other crossbill species get into the population and muddle the genes, canceling the speciation? The answer might be a certain avian quirk: Birds are totally cliquey. Crossbills flock together year-round. They watch each other and talk to each other to assess the quality of their food. And a flock has a characteristic tone that changes subtly over the generations. It’s a kind of secret code that individuals use to find food and mates.

So say a population settles into the lodgepole pine forest and develops a variant call. “Basically, that call will just take off and be favored,” Benkman says. “And given that they flock year-round and they choose mates in flocks, you’re gonna get reproductive isolation, and that appears to be what’s key.” Other crossbills arriving from time to time wouldn’t know the secret code, and therefore wouldn’t settle into the flock. Gene flow, or the exchange of genes between populations, will still be possible, though significantly limited. Even though the South Hills crossbill wasn’t physically isolated, it was reproductively isolated–just what you need to get a new species.

The Origin of Species

More and more, scientists are turning up cases of such divergence without geographic isolation, and turning up evolutionary dogma in the process. “Fish are another example, where you have fish that are adapted to the bottom of the lake, or deep parts of the lake, or shallow parts of the lake, or different light environments,” says evolutionary biologist

  • A situation like that still allows for gene flow, which from a classical evolutionary perspective seems like an unlikely way to end up with divergence. “It used to be more of a black or white, you can’t diverge with gene flow or you can diverge with gene flow, and I think that’s been loosened,” says Nosil. These days, Nosil adds, the big question is this: “Most people are accepting to some extent you can diverge with gene flow, but can you do that to the point where we actually get different species?”

    What Benkman and his colleagues say is that yes, the South Hills crossbill is genetically distinct enough to earn it the honor of being a distinct species. It’s genetic work like this that has transformed the debate over speciation without geographic isolation. Technology means scientists can look at morphology and prove there’s an underlying genetic shift.

    But what’s to say the South Hills crossbill wasn’t at one point geographically isolated in some way, then moved into Idaho and mixed with other species? Well, remember that Benkman actually modeled the development of the bird’s habitat. “I think that strengthened this specific paper, but it also highlights a larger issue with this debate,” says Nosil, “which is basically if you go out in nature and you see two birds or two fish or two insects or whatever in the same area and they kind of look different, maybe they’re different species, you don’t know if they’ve always been in the same area.” Benkman has good historical evidence that not only did the South Hills crossbill diverge without geographic isolation, but it did so as a consequence of its coevolution with the lodgepole pine.

    What’s so bizarre about this bird’s evolution is that it’s, well, a bird–arguably the most mobile kind of critter out there. Crossbills can fly wherever they please, so isolating them is tough. “I think that these studies are interesting in part because it’s a taxonomic group where we expected this not to really take place all that often,” says biologist Katie Langin, whose own work has shown that jays in California have also been able to diverge without geographic isolation. “And we’re increasingly seeing that it can.”

    If it worked for the South Hills crossbill, who knows how many other less mobile creatures have done the same? That’s pretty damn exciting for evolutionary biology, not to mention for a little bird in Idaho with one hell of a monopoly.