Endangered songbird challenging assumptions about evolution

This new research, out now in Science, looked at a newly discovered, endangered songbird located only in South America -- the Iberá Seedeater -- and found that this bird followed a very rare evolutionary path to come into existence at a much faster pace than the grand majority of species.

By comparing this bird to a closely related neighbor (the Tawny-Bellied Seedeater) in the same group (the southern capuchino seedeaters), the researchers determined that genetic shuffling of existing variations, rather than new random mutations, brought this species into existence -- and their own behaviors are keeping them apart.

This species is one of only two known examples across the globe to have traveled this path, challenging the typical assumptions of how new species form.

"One of the aspects of this paper that makes it so cool is that we were able to address this question of how the Iberá Seedeaters formed from multiple different perspectives," said Sheela Turbek, a graduate student in ecology and evolutionary biology (EBIO) at University of Colorado Boulder and the study's lead author.

"Not only did we collect on-the-ground data on who mated with one another and the identity of their offspring, but we also generated genomic data to examine how similar these two species are on a genetic level. We then zoomed out further to look at where the Iberá Seedeater fits in the context of the broader capuchino group."

"Many studies will address one of these aspects or questions but not combine all of these different pieces of information into a single study."

The southern capuchino seedeaters are a group of recently evolved songbirds found throughout South America that is branching rapidly, with many of its species in the early stages of evolution. This family is best known for the dramatic variation with the males in terms of songs and plumage color, while the females are largely indistinguishable even to the most familiar researchers.

The Iberá Seedeater, the most recent member of this family, was first discovered in the remote, swampy grasslands of Iberá National Park in northern Argentina by study co-authors Adrián S. Di Giacomo and Cecilia Kopuchian from Centro de Ecología Aplicada del Litoral, Argentina, in 2001, and then described in scientific literature in 2016.

In that national park, though, are six other closely related species of capuchinos, including the Tawny-Bellied Seedeater, that breed closely beside each other. These species, despite occupying the same environment and eating the same food, rarely interbreed.

And so, researchers wondered why -- and how -- the Iberá Seedeater even came to be.

They explored these questions in two ways: First, they looked at how this new species may have formed by examining the ways in which its DNA differs from the Tawny-Bellied Seedeater, and second, looking at what mechanisms might be preventing it from interbreeding with the other species that occur in the park.

To do that, Turbek went down to Argentina for the breeding season for three years, staying two and a half to three months at a time, searching for and monitoring nests, collecting blood samples from adults and nestlings, and then, in the final year, performed a behavioral experiment to see whether plumage or song played a roll in terms of species recognition.

"The field work involved in collecting the assortative mating and behavioral data is extraordinarily hard, which is why these kinds of datasets rarely exist. This study and publication are a testament to Sheela's skill and hard work in the field," said Scott Taylor, an assistant professor in EBIO at University of Colorado Boulder, an author on the paper and Turbek's advisor.

What they found is that the two birds are closely related genetically, only distinguishable by the genes involved in plumage coloration. As well, they found that the males responded most aggressively to songs and plumage variations aligning with their own species.

This all means that the species could very well reproduce and hybridize -- they just choose not to, therefore reinforcing their own reproductive barriers.

On a broader level, though, when comparing the Iberá Seedeater to other capuchino species, the researchers found that the Iberá Seedeater shares genomic variants with other capuchinos in these regions, but the variants have been shuffled to form a unique combination, which, the researchers argue, could be an evolutionary shortcut that most likely underlies much of the diversity among the different subspecies of this family.

"This is a really beautiful story about a process that we have never seen in quite this way before," says co-author Irby Lovette, director of the Fuller Evolutionary Biology Program at the Cornell Lab of Ornithology.

"The classic and most common evolutionary model for new species is the accumulation of genetic mutations when those species are separated by a geographic barrier over perhaps millions of years. But here we found that genetic shuffling can happen quickly and without geographical isolation. It's almost like 'instant speciation.'"

Leonardo Campagna, a research associate at the Cornell Lab of Ornithology and the senior author on the paper, agrees:

"This is the clearest example in birds of how reshuffling of genetic variation can generate a brand-new species."

The only other organism where this type of evolution has been seen, according to Turbek, is a group of fish found in Africa called the Lake Victoria cichlids.

"It's interesting to see this mechanism operating in something as different as birds," Turbek commented.

While this study focused in part on the role of male behaviors, the researchers are very interested in taking it one step further, examining the role that female choice may also play in reproduction.

"There are many more questions that we have to address," Turbek said.