Beauty is in the eye of the beholder—even if that beholder is a barn swallow.
Depending on where the birds live, some of them may favor mates with a paler chest color while others find a redder chest more attractive. The difference in what these birds prefer when it comes to choosing a mate is helping scientists unlock one of biology's greatest mysteries: How do new species originate?
In a new study led by the University of Colorado Boulder, biologists used genetic sequencing from barn swallows around the globe to provide real-time evidence that sexual selection, in which organisms choose mates based on traits they find attractive, drives the emergence of new species.
The study was published Dec 12 in the journal Science.
"This is one of the very first papers to comprehensively show the role of mate selection decisions in the evolution of new species," said Rebecca Safran , the paper's senior author and professor in the Department of Ecology and Evolutionary Biology. The new findings shed light on how new species form, a fundamental but elusive process for all life on Earth.
Proving Darwin right
Charles Darwin proposed the theory of sexual selection in 1875. It suggests that organisms evolve showy traits, like extravagant plumage or eye-catching dance moves, to attract mates. When organisms of the same species develop preferences for different traits and no longer breed with each other, new species could emerge over time, a process known as speciation.
For the past 150 years, researchers of sexual selection have primarily studied organisms that already diverged into distinct species. For example, orchids, which now encompass more than 25,000 species, originated from a common ancestor. Their remarkable diversity often leads to the assumption that they evolved different looks to attract different pollinators, said Drew Schield , the paper's first author and assistant professor at the University of Virginia.
"It's logical to think this way and it could totally be the case," said Schield, who did the research while a postdoctoral researcher at CU Boulder. "But with speciation already having occurred, it's impossible to know for certain." As a result, it has been difficult to find direct evidence that sexual selection drives the emergence of new species.
Barn swallows provide a unique opportunity to explore the speciation process as it unfolds.
These birds are one of the most common and widespread species on our planet. Currently, there are six subspecies of barn swallow each looking slightly different in some traits critical to the mate choice decisions depending on where they are.
For example, the East Asian group, Hirundo rustica gutturalis, has a pale chest and shorter tail streamers—the elongated outer tail feathers. Hirundo rustica tytleri, found in Siberia, has long tail streamers and red chest feathers. The subspecies in Europe and western Asia, Hirundo rustica rustica, has a pale chest and long tail streamers.
Reuniting after isolation
Evidence suggests that the bird's ancestors left the Nile River valley in northern Africa about 11,000 years ago and spread out across the Northern Hemisphere. For thousands of years, different populations barely interacted and developed diverse traits, forming subspecies.
Some 800 to 2000 years ago, certain subspecies expanded their territories, and habitats began to overlap. In some parts of the world, subspecies now interact with each other, producing hybrid offspring.
Safran and her team set out to investigate whether sexual selection in these birds was driving the speciation process.
The team, including Elizabeth Scordato, associate professor at the California State Polytechnic University, sequenced the genomes of 336 barn swallows from around the globe, encompassing all subspecies and three hybrid zones, where subspecies interbreed, in Eurasia.
The researchers found a dozen regions in the barn swallow genome associated with the birds' two sexually selected traits: Ventral coloration—the plumage color of their chest and belly— and tail streamer length.
When individuals reproduce, the genes from both parents reshuffle and combine to form the genes of their offspring. When two populations encounter one another, the flow of genetic material from one to another is a marker of how similar the populations are. If the rate of gene flow is low, it means the two populations are breeding with each other at a lower rate than they would if they are the same species.
The study found that in barn swallow hybrid zones much of their genes flows freely across groups. But the genetic regions coding for ventral coloration and tail streamer length hardly transfer to other populations.
It suggests that among the hybrid individuals with parents from different subspecies, a small number of lucky birds that inherit a favorable combination of tail streamer and ventral color genes are able to attract mates. Hybrids that receive less favorable combinations tend to be less successful in reproduction.
"These genes are hitting a boundary due to divergent sexual selection, and they stop moving from one population to the other," Schield said.
The different preferences for tail feather length and chest color across subspecies make barn swallows more likely to mate within their own group, Schield added. If the trend continues, these groups could no longer interbreed or produce offspring, markers for the formation of separate species.
Next, the team plans to sample more birds and study whether being a hybrid affects reproductive success.
"It's very cool that we could capture a real-time evolutionary portrait of this common animal and understand how and why the populations are diverging," Safran said. "Our understanding of the process is fundamentally important for addressing a wide range of questions related to biodiversity, evolution and conservation."
