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Genetic diversity couldn't save Darwin's finches

August 22, 2019

Researchers at the University of Cincinnati found that Charles Darwin's famous finches defy what has long been considered a key to evolutionary success: genetic diversity.

The study of the finches of the Galapagos Islands could change the way conservation biologists think about species with naturally fragmented populations to understand their potential for extinction. 

UC graduate Heather Farrington and UC biologists Kenneth Petren and Lucinda Lawson found that genetic diversity was not a good predictor of whether populations of finches would survive. A UC lab analysis of century-old museum specimens found that six of eight extinct populations had more genetic diversity than similar museum specimens from which descendants survive today. In most other species, low genetic diversity is a signal of a population in decline.

Researchers examined 212 tissue samples from museum specimens and living birds. Some of the museum specimens in the study were collected by Darwin himself in 1835. Only one of the extinct populations, a species called the vegetarian finch, had lower genetic diversity compared to modern survivors.

Lawson said the findings are explained by the fact that these birds can migrate in between populations.

Specifically, researchers believe a biological phenomenon called sink-source dynamics is at play in which larger populations of birds from other islands act as a "source" of immigrants to the island population that is naturally shrinking, the "sink." Without these immigrant individuals, the natural population on the island likely would continue to dwindle to local extinction. The immigrants have diverse genetics because they are coming from a variety of healthier islands, giving this struggling "sink" population inflated genetic diversity.

Petren said the findings serve as a warning that the genetics of individuals in fragmented populations might not tell the whole story about a species. And that is important for scientists who increasingly use genetics to account for the flow of genes between populations when determining a threatened species' likelihood of extinction.

"The promise of genetics is to sample a few individuals to understand the whole population. But it's a cautionary note that you might be sampling a fragment. You could be misled," he said.

Petren has been studying the birds for 25 years at UC's McMicken College of Arts and Sciences. He said the island's 18 recognized species of finches are unusual for other reasons. Some finches that look most different are actually closely related, he said. And similar-looking finches that birders might have trouble telling apart are actually far apart on the evolutionary family tree.

"It's a paradox. If Darwin fully understood what was going on, it might have blown his mind," Petren said. "These finches are not the first case you would pick to formulate the notion that species can change over time because the patterns of change are so complicated."

The UC study was published in August in the journal Conservation Genetics after first appearing online in April. It suggests that genetic diversity may not be the best predictor of extinction risk for mobile species like the island-hopping finches. That's because healthier populations may contribute individuals to declining ones.  

Lawson said factors such as historical diversity or the possibility of gene flow between populations should be considered in addition to the snapshot view provided by a genetic analysis for a fuller understanding of a species' potential for extinction.

"Typically, we would expect populations with high genetic diversity to have a greater potential for long-term survival," she said. "Meanwhile, the low-diversity populations would be more likely to go extinct because that's a common pattern as populations decline to few individuals. Surprisingly, we found that most of the extinct populations had higher genetic diversity."

The study was sponsored in part by the National Science Foundation, Sigma Xi, the American Ornithologists' Union and UC's Office of Research.

Darwin's "On the Origin of Species" was groundbreaking in our understanding of evolution through natural selection. "Survival of the fittest" is a household phrase and a shorthand description of any competition.

While scientists today know more about how new species are formed, the principles Darwin developed remain the foundation of evolutionary biology, Petren said.

"Certainly, genetics is new. But the fundamental principles still stand. It's amazing how much of his work remains fundamentally true today," Petren said.
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University of Cincinnati

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