Study Sheds Light on One of Biology’s Most Elusive Mysteries: How New Species Are Created

Almost 150 years ago, Charles Darwin developed the theory of sexual selection, suggesting that when organisms of the same species develop preferences for specific physical traits or behaviors in a mate and no longer breed with those without them, new species are born.  A study published Dec. 12 in the journal Science, however, is the among the first to offer comprehensive documentation of the phenomenon in real time.

Since Darwin proposed the theory in 1875, researchers have primarily studied organisms that have already diverged into distinct species, a process known as speciation. For example, orchids, which now encompass more than 25,000 species, originated from a single common ancestor. The remarkable diversity of the orchid family of plants often leads to the assumption that they evolved different looks to attract different pollinators, said Drew Schield, an assistant professor in UVA’s Department of Biology and lead author of the paper.

“It’s logical to think this way, and it could totally be the case,” said Schield, whose work is focused on the evolutionary processes that drive the origins and maintenance of biological diversity. “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.

In a search for that evidence, Schield collaborated with Rebecca Safran, a professor of ecology and evolutionary biology at the University of Colorado, Boulder, Elizabeth Scordato, associate professor at the California State Polytechnic University, and other researchers to sequence the genomes of 336 barn swallows from around the globe. Their combined effort encompassed every subspecies and three hybrid zones, where subspecies interbreed, in Eurasia. 

“This is one of the very first papers to comprehensively show the role of mate selection decisions in the evolution of new species.” said Safran, senior author of the paper.

The team found a dozen regions in the barn swallow genome associated with the birds’ two most salient 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 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 research team 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.

The findings suggest that among the hybrid individuals with parents from different subspecies, a small number of lucky birds that inherit a favorable combination of ventral color and tail streamer genes are better 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 chest color and tail feather length across subspecies make barn swallows more likely to mate within their own group, Schield explained. If the trend continues, these groups could evolve additional barriers to their ability to interbreed or produce offspring, markers for the formation of separate species.  

“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.”

Yvaine Ye is a science writer with the University of Colorado, Boulder.