In a recent study, researchers explain how varying preferences for certain traits in mate selection among barn swallow populations are causing these birds to diverge, potentially leading to the creation of new species.
Beauty truly depends on perspective—even for barn swallows.
The birds’ mate preferences differ based on their geographic locations—some may prefer mates with paler chest feathers, while others are attracted to a richer red chest hue. This variation in mate choice is helping scientists tackle a significant question in biology: How do new species come into existence?
In a study orchestrated by the University of Colorado Boulder, biologists conducted genetic analyses of barn swallows worldwide, revealing compelling evidence that sexual selection—the process where organisms choose mates based on preferred characteristics—plays a crucial role in the formation of new species.
The findings were published on December 12 in the journal Science.
“This research is among the first to thoroughly demonstrate how mate selection influences the evolution of new species,” stated Rebecca Safran, the paper’s senior author and a professor in the Department of Ecology and Evolutionary Biology. These results enhance our understanding of how new species develop, a fundamental yet often perplexing aspect of life on Earth.
Validating Darwin’s Theory
Charles Darwin introduced the concept of sexual selection in 1875, suggesting that certain traits, like vibrant plumage or unique mating displays, evolve to attract partners. Over time, if members of the same species develop distinct preferences for different characteristics and stop interbreeding, this could lead to the emergence of new species—a process known as speciation.
For 150 years, researchers have largely examined species that have already undergone divergence. For instance, the nearly 25,000 species of orchids evolved from a single ancestor, and their incredible diversity has led to the assumption that they developed different appearances to attract various pollinators, according to Drew Schield, the study’s first author and an assistant professor at the University of Virginia.
“It makes sense to think this way, and it may well be true,” Schield mentioned, noting his research was conducted while he was a postdoctoral researcher at CU Boulder. “However, since speciation has already happened, it’s challenging to ascertain with certainty.” This limitation has made it difficult to find direct evidence linking sexual selection to the emergence of new species.
Barn swallows provide a unique instance to study the speciation process as it occurs.
These birds are among the most common and widely distributed species across the globe. Presently, there are six subspecies of barn swallow, each exhibiting slightly different traits that influence mate selection based on their habitat.
For example, the East Asian subspecies, Hirundorustica gutturalis, features a pale chest and shorter tail streamers—long outer tail feathers. In contrast, Hirundo rustica tytleri, located in Siberia, displays longer tail streamers and a red chest. The European and western Asian subspecies, Hirundo rustica rustica, has a pale chest and elongated tail streamers.
Reconnecting After Isolation
Research indicates that the ancestors of barn swallows departed from the Nile River valley in northern Africa about 11,000 years ago, spreading throughout the Northern Hemisphere. For thousands of years, various populations remained largely isolated, cultivating distinct traits and giving rise to subspecies.
Approximately 800 to 2000 years ago, certain subspecies expanded their territories, resulting in overlapping habitats. In some areas, subspecies now interact and can produce hybrid offspring.
Safran and her research team aimed to determine whether sexual selection was indeed influencing the speciation process among these birds.
The team, including Elizabeth Scordato, an associate professor at California State Polytechnic University, examined the genomes of 336 barn swallows from different regions, covering all subspecies and three hybrid zones in Eurasia where subspecies interbreed.
They identified several genomic regions linked to two crucial sexually selected traits: ventral coloration (the color of the chest and belly feathers) and tail streamer length.
During reproduction, genes from both parents combine and form the genetics of their offspring. When two populations come into contact, the exchange of genetic material serves as an indicator of their genetic similarity. A low rate of gene flow suggests that the populations are not interbreeding very often, indicating divergence.
The study indicated that while genetic material flowed freely among barn swallow hybrid zones, the segments of the genome related to chest color and tail streamer length were less likely to be transferred between populations.
This implies that among hybrid birds, only a limited number, who inherit particularly favorable combinations of these traits, are more successful at attracting mates. Those with less advantageous trait combinations tend to have lower reproductive success.
“The genes are encountering a barrier due to differing sexual selection pressures, preventing them from moving freely between populations,” Schield explained.
The varying preferences for tail feather length and chest color among the subspecies may lead barn swallows to preferentially mate within their own groups, Schield added. If this trend persists, these groups may eventually stop interbreeding altogether, marking a step towards the establishment of distinct species.
Looking ahead, the team intends to gather more samples and examine how being a hybrid affects reproductive success.
“It’s exciting that we’ve captured a real-time view of the evolutionary process in this common species and gained insights into how and why their populations are moving apart,” Safran remarked. “Understanding this process is essential for exploring important questions related to biodiversity, evolution, and conservation.”