The Iberá Seedeater (right) evolved in just a few thousand years through changes in the male’s plumage and song, and in part by shuffling genes found in several nearby species, including the Tawny-bellied Seedeater (left). Larger pictures show men; smaller images are female (not to scale). Illustration by Jillian Ditner.
Speciation – or the emergence of a new species – is a slow and steady process that takes place over millions of years among a stranded group of creatures that are isolated from the rest of their species by geographical barriers and must evolve on their own.
At least that was generally thought.
New research published in the journal Science challenges the typical model of speciation by documenting how a songbird recently discovered in South America made a very rare evolutionary path. The study looks at the origins of the Iberá pea eater in northern Argentina, which appears to have evolved from a unique mix and match of existing genetic traits between 10 or more other species of pea feeders in the same area.
The Iberá seed eater appears to have evolved from a unique mix and match of existing genetic traits between 10 or more other species of seed eater in the same area of northern Argentina. This short video compares the similar patterns and colors of males of different species and ends with Iberá Seedeater. Women, shown on a smaller scale for comparison, are essentially identical among the species. Illustrations by Jillian Ditner.
“These different genes were converted into a unique combination that created the Iberá Seedeater,” said lead author of the study, Leonardo Campagna, a research fellow at the Cornell Lab of Ornithology. “This species has a mosaic of plumage genes derived from existing genetic variations that have already been found in other seed-eaters.”
According to Campagna, the study shows that mating behavior alone can be a powerful evolutionary force that prevents a recently divergent species from re-crossing with other closely related species – even though they live in the same places, eat the same food, nest and at the same time are genetically almost identical. In other words, speciation does not always require genetic mutations that occur in an isolated population.
And it doesn’t take millions of years. Campagna estimates that it took on the order of thousands of years for the evolutionary process to take place and the Iberá Seedeater to be born as a distinct species that would only mate with its own species.
“It’s very fast in evolutionary terms!” exclaims Campagna. “This is the clearest example in birds of how remodeling genetic variation can produce a brand new species.”
“This is a beautifully thorough and comprehensive study of the roles of genes, plumage and behavior in creating a new species,” said Rosemary Grant, an eminent scientist in the field of evolutionary biology who was not involved in the study. Grant and her husband Peter spent decades studying the species group of Darwin’s finches in the Galapagos Islands and are known to discover how natural selection influenced the shape of beaks. “This paper contributes significantly to our increasing understanding of speciation.”
The researchers used a classic experimental setup to test whether different-looking seed-eaters could be separate species. They made a model of a male Iberá sea theater and played his song over hidden speakers. Nearby Iberá Seedeaters believed their territory was under attack and attacked the bait, while Tawny-bellied Seedeaters ignored the model. Pictures by Sheela Turbek. Then the researchers reversed the test. They set up a model Tawny-bellied Seedeater and played its song. This time the nearby Tawny-bellied Seedeaters attacked the model as if they were defending their territory. But Iberá Seedeaters did not recognize the tawny belly as a threat and did not respond to the bait. One reason male birds sing is to warn other males about their own species. So the lack of response is evidence that the two forms are indeed separate species.
Carlos Daniel Cadena, evolutionary biologist at the Universidad de los Andes, Colombia, who was also not involved in the research, agreed: “The idea that new species of Capuchin are emerging [seedeaters] created by reshaping existing genetic variations to create new phenotypes where selection drives divergence is mind-boggling. “
The Iberá Seedeater was first discovered in 2001 by study co-authors Adrián S. Di Giacomo and Cecilia Kopuchian from the Centro de Ecología Aplicada del Litoral, Argentina, in the remote, marshy grasslands of the Iberá National Park in northern Argentina. The lead author of the new study, Sheela Turbek, a graduate student at the University of Colorado Boulder, spent two seasons in the field locating nests and conducting behavioral experiments in Iberá National Park, where the Iberá pachyderm lives among six other closely related species of peed.
The different species vary in color. For example, the Iberá Seedeater has a black neck and sand-colored belly, while the Tawny-bellied Seedeater has reddish cheeks, neck, chest and belly. The researchers compared the entire genome of these two species and found only three narrow regions that differed. These regions contain only 12 genes, 3 of which are involved in plumage coloring.
The study’s authors believe that the same remixing process identified in this study is likely to account for a large part of the diversity among the dozen seed-eating species in this region of South America – genetic combinations likely to arise when seed-eaters occasionally crossbreed and form hybrids .
Study co-author Irby Lovette, director of the Fuller Evolutionary Biology Program at Cornell Lab of Ornithology, summarized the study results in the next chapter on understanding speciation: “This is a really nice story about a process we are in that process have never seen it much earlier. “