- A team of scientists found multiple relatively old mitochondrial lineages of Spectacled Tetrakas living in the same location, making them suspect multiple species had merged back into one.
- Scientists with the Field Museum in Chicago sequenced the birds’ DNA and the mtDNA of their lice to determine that what they call “despeciation” had occurred.
- According to John Bates, associate curator of birds at the Field Museum, there might be some important lessons for conservationists in the team’s discovery.
We’re used to thinking of evolution as leading to greater diversity — one species evolving into multiple new, distinct species. But a new find in Madagascar shows evolution can work in the other direction, as well.
Researchers with Chicago’s Field Museum say they’ve discovered a case in which multiple bird species merged back into one — what they’ve called “despeciation,” or, evolution working “backward.” They summarized their findings in a paper published in the journal Ecology and Evolution.
The phenomena of despeciation was observed in Spectacled Tetrakas, a small songbird from Madagascar, by Dr. Nicholas Block while he was with the Field Museum and a PhD candidate at the University of Chicago (he’s now a professor at Stonehill College in Easton, MA). Block and his team determined the birds’ biological history by examining their DNA and the lice that co-evolved with them.
The team found that Spectacled Tetrakas’ mitochondrial DNA (mtDNA) can vary as much as 7.6 percent from one bird to the next — a huge difference in genetic makeup for animals of the same species living alongside each other as these birds do. For reference, the researchers note in their report that human mtDNA varies by no more than 1.5 percent from individual to individual, no matter where in the world they’re from.
The mtDNA showed very deep divergences within the tetrakas, Block told Mongabay. Some lineages had been separate for as much as 3.6 million years.
“This would not be all too surprising if the lineages were geographically isolated, but it turns out that up to three very divergent lineages can be found in the exact same location,” Block added. “That appears to be unique among all birds and potentially all vertebrates.”
Finding multiple relatively old mitochondrial lineages in the same location made Block suspect despeciation had taken place, so he and his team sequenced the birds’ nuclear DNA and the mtDNA of their lice to see if that hypothesis was correct.
The birds’ nuclear DNA — essentially, the genetic instructions for how to grow a Spectacled Tetraka — turned out to be extremely similar. The fact that their mtDNA is so different suggests that the birds had different ancestors, Block says, while their similar nuclear DNA means the separate species must have merged once again.
So, in a sense, evolution worked backwards to bring these birds back together after they had diverged into separate species — a phenomenon the authors chose to call “despeciation.”
But that DNA evidence on its own wasn’t proof enough that despeciation had occurred, so the scientists next turned to the birds’ lice, which often evolve along with a specific host, thus giving each bird species its own unique louse.
Block and team found that the Spectacled Tetrakas from different areas in Madagascar were hosting different kinds of lice, which further strengthened their case that the birds had undergone despeciation, as the different louse species must have evolved when the bird lineages were separate, and maintained that separation when the birds merged back together.
It’s almost impossible to determine precise evolutionary history in a scenario like this, Block says, but he theorizes that climate fluctuations and forest fragmentation are what caused the peculiar development of the Spectacled Tetraka.
In warmer, wetter climates of the past, the humid forests of Madagascar were very extensive and were connected throughout the eastern part of the island. In cooler, drier periods, on the other hand, the forests occupied a smaller area and likely became fragmented.
Block thinks that the tetrakas may have evolved their genetic differences during a period when their range was broken into isolated regions, and now they’ve come back together as the regional climate has warmed, leading to more extensive forested areas. (Block points out that the warming climate that he thinks may have reconnected the tetrakas’ ranges predates recent anthropogenic climate change.)
Despeciation has been observed before, Block told Mongabay, but mostly in relatively young lineages. “In those examples, which mostly come from lake fish, it seems that human disturbances to the lakes broke down ecological barriers between the young species, leading to the species breeding together again and despeciating,” he added.
Block doesn’t think we can say there’s any evolutionary advantage to merging lineages, but he says there isn’t a disadvantage, either. And according to co-author John Bates, associate curator of birds at the Field Museum and one of the paper’s co-authors, there might be some lessons for conservationists in the team’s discovery.
“I think the point is that our data set is the result of a natural phenomenon,” Bates told Mongabay. “Conservation prioritization perhaps should involve not spending limited conservation money on situations where this is what is happening naturally.”
Bates says there are two good examples in the U.S., interactions between Blue-winged and Gold-winged Warblers and Mallards and American Black Ducks, where this lesson might apply. “In both cases the first species hybridizes with the second species and females of the second species appear to prefer males of the other species,” Bates said.
“In the future, both Golden-winged Warblers and American Black Ducks may become genetically swamped by the species that hybridizes with them. While both situations may have been exacerbated by human alteration of the habitat, I think one could argue that these are situations where we may want to let nature takes its course.”
Bates said that as climate change and habitat alteration are increasingly happening around the globe, scientists are becoming aware of more situations where species are intermingling and even merging in ways we’ve never observed before.
“As managers, we will be responsible for deciding what happens in these situations in the short term (100s of years), and that has long-term implications (100,000 of years and beyond).”
CITATION
- Block, N. L., Goodman, S. M., Hackett, S. J., Bates, J. M., & Raherilalao, M. J. (2015). Potential merger of ancient lineages in a passerine bird discovered based on evidence from host‐specific ectoparasites. Ecology and evolution,5(17), 3743-3755.
