CU Boulder researchers challenge long-held assumptions about the relationship between bird migration and the process by which new species arise

Every year, billions of birds take to the skies, riding thermal currents and navigating with an innate sense of direction across distances that would humble even the most accomplished commercial pilots.

“Migration is one of the most spectacular natural phenomena,” says Gina Calabrese, an evolutionary biologist and postdoctoral research fellow in the Safran Lab at the University of Colorado Boulder.

Gina Calabrese, an evolutionary biologist and postdoctoral research fellow in the Safran Lab at CU Boulder, and her research colleagues, tested the theory that bird migration may be a leading force behind the genesis of new species.

Aside from inspiring awe in bird enthusiasts, this ancient ritual has also sparked many scientific theories. One suggests that migration—by way of dividing populations across different routes and destinations—may be a leading force behind the genesis of new species.

“The idea that this behavior could be a major driver of biodiversity has been an attractive one,” Calabrese says.

But does it hold up under evolutionary scrutiny? That’s what she and a team of co-researchers set out to test in a new study published in Systematic Biology.

Rethinking migration and diversity

Calabrese and her colleagues’ research challenges long-held assumptions about the relationship between migration and speciation, or the process by which new species arise. While scientists have documented cases where migratory behavior appears to be splitting populations, her team wanted to know whether this pattern was widespread enough to have shaped bird diversity at a large scale.

“There’s a body of literature that suggests migration could promote the formation of new species, by isolating populations that use different migratory routes or wintering areas,” she explains. “If this were a widespread pattern, we might expect migratory lineages to be more diverse today than other non-migrating birds.”

To test the hypothesis, Calabrese and her collaborators examined evolutionary trees called phylogenies that map out how present-day bird species are related to one another. Drawing from massive data sets of two avian superfamilies, they used statistical models to estimate how quickly different bird lineages have diversified over evolutionary time. They then compared the rates of speciation in migratory birds to those that make a home in one location year-round.

The results weren’t what they had expected.

“We found no consistent evidence that migratory birds speciate faster than non-migratory ones,” Calabrese says. “This was a surprise—especially given how much attention the idea of migration-driven speciation has received.

“There are clear examples where migration is leading to population splits—those are real,” she says. “But those examples are often recent, and they might not always result in fully separate species.”

In other words, migration might occasionally set the stage for speciation, but it’s no guarantee.

“Not every population split leaves a lasting imprint in the fossil record or leads to a new species,” Calabrese adds.

 

“We found no consistent evidence that migratory birds speciate faster than non-migratory ones. This was a surprise—especially given how much attention the idea of migration-driven speciation has received," says CU Boulder researcher Gina Calabrese. (Photo: Todd Trapani/Unsplash)

One reason for this, she suggests, is that many observed migratory divides are evolutionarily young. These populations may just be starting to diverge, and many might merge again over time. Others may remain distinct but not reproductively isolated.

If the goal is to understand how biodiversity has accumulated over millions of years, a short-term snapshot—whether looking at bird lineages today or thousands of years ago—may not tell the full story.

“This is a good example of how something can be true in some cases but not necessarily explain large-scale patterns,” Calabrese says.

Following evidence, not expectations

Calabrese’s recent work is also a case study in scientific humility. When she and her colleagues first set out to test the migration-speciation connection, they weren’t looking to debunk anything. However, when the results started pointing in a different direction than their hypothesis, they remained committed to following the data.

“I think it’s important that we test assumptions—even appealing ones—with data,” Calabrese says.

The process also gave her a new perspective on how the scientific method plays out in real-world applications.

“I was a little anxious at first, until I kind of really felt like I had a handle on what my results were and felt confident in them. And then at that point, your job is just to tell the story of what your data show,” she adds.

While this study might have raised more questions than it answered, that’s part of what keeps Calabrese curious and driven to study the incredible phenomenon that is migration.

“It’s a little disappointing because you want to believe that what you’re studying today is explaining the answers to your bigger questions,” she says. “But it’s also cool because our findings mean that there’s still a lot to understand about how we get the diversity we see today and there’s still some mystery out there to solve, which is cool to me.”

CU Boulder Professor Rebecca Safran contributed to this research, as did Kira Delmore, Jochen Wolf and Daniel Rabosky.

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