Darwin’s finches continue to inform and confuse
Extinct populations had higher genetic diversity than many survivors. Stephen Fleischfresser considers the implications.
Charles Darwin’s famous Galápagos finches are once again helping us to understand the natural world.
New research on these complex little birds demonstrates that our understanding of the genetic markers used to predict a species’ extinction is overly simplistic, a pressing problem at a time of unprecedented extinction rates around the globe.
While the story of Darwin’s finches is often framed as a “eureka moment” in his understanding of the way organisms transform through natural selection, nothing could be further from the truth.
Darwin arrived at the Galápagos islands in September of 1835. The black lava of the island and the terrible heat caused him some distress, and he was only selectively attentive to the wonders around him.
He was captivated by the local mockingbirds that varied from island to island, but ignored the island-specific tortoises, preferring instead to shoot and eat them in serious numbers. He even rode on the back of one of the larger specimens for fun.
He had noticed the many finches in the dense growth of the islands’ lowlands but the variation in size, colour and habit left him in a state of “inexplicable confusion”. He collected samples of six species from three islands and stored and labelled them rather haphazardly.
On his return to England, he dumped this mess in the lap of the talented artist and ornithologist John Gould of the Zoological Society of London, and it was he that figured out (in only six days) that Darwin’s finches were actually a complexly interrelated group of 12 different species.
Over time, and in tandem with his consideration of the Galápagos mockingbirds, the finches helped Darwin to understand the process that we now call adaptive radiation, the way in which an ancestral species transforms into many new types, dependent on local environments, and thus evolution more generally.
Now it seems the finches, including some of the specimens Darwin himself collected in 1835, are helping science to understand the complexities of the process of extinction.
Heather Farrington, Lucinda Lawson and Kenneth Petren of the Department of Biological Sciences at the University of Cincinnati in the US have used the finches to test the robustness of predictive extinction models and the assumptions that underpin them.
One of the common ways to test whether a population or species is likely to go extinct is to measure its genetic diversity: such testing is quick, easy and cheap. Each gene comes in a number of different forms, called alleles.
Genetic diversity refers to the number of different alleles in a population or species; greater diversity is thought to mean that the population or species will have the capacity to adapt should environmental conditions change.
With more diversity comes a higher likelihood that alleles exist in the population that will provide certain individuals with an advantage in changed conditions. These individuals will then outbreed the rest of the population and over time these alleles will become typical of the population or species. This is basically how evolution works.
"Typically, we would expect populations with high genetic diversity to have a greater potential for long-term survival," says Lawson. "Meanwhile, the low-diversity populations would be more likely to go extinct because that's a common pattern as populations decline to few individuals."
She and her colleagues set out to explore this indicator using Darwin’s finches, which provide a rare opportunity to test whether genetic diversity really is a predictor of extinction.
By looking at the genetic diversity of 212 tissue samples taken from both museum specimens and living birds, they could compare these to the reality that has played out in the islands’ finches, where “many populations went extinct, but far more persisted” over the last 100 years or so. The trio’s hypothesis was “that genetic variation was lower in populations that ultimately went extinct, relative to those that are still extant”.
What they found was the opposite.
Only one of the extinct finch populations, a species called the vegetarian finch, had lower genetic diversity compared to modern survivors. To make matters more confusing, most of the now extinct populations had indications of higher genetic diversity compared to surviving populations that migrated to other islands.
Darwin’s finches continue to confuse.
The scientists, however, think they have an explanation.
The finch populations on the many islands of the Galápagos are not truly isolated from each other. In fact, there is gene flow between the populations on different island via the occasional migration event, producing what is called a metapopulation structure.
The researchers suggest that some of this gene flow, when coming from healthy populations, can mask the lack of genetic diversity in small populations heading toward extinction.
This throws some doubt over the role of genetic diversity in predicting extinction. The authors conclude that “when populations may contain unknown additional dynamics such as ongoing metapopulation gene flow, genetic diversity estimates may be an unreliable signal of decline.”
"The promise of genetics is to sample a few individuals to understand the whole population,” says Petren, “But it's a cautionary note that you might be sampling a fragment. You could be misled."
The findings – published in the journal Conservation Genetics – indicate that for species with complex population structures such as Darwin’s finches, more information on population size and migration potential, as well as larger genetic samples, are needed to more accurately predict the likelihood of a species’ demise.