New discovery reveals mystery of the woolly mammoth
A small breeding population may have doomed the last mammoths, a confirmation of gene and population theory that could change how we think about conservation efforts today.
—The shaggy megafauna that roamed Siberia and North America together with our ancestors captivate the imagination, but now it looks like they’re giving us a practical lesson in genetics that could help inform conservation efforts.
Scientists compared the DNA of two mammoths: a member of a dwindling island population with an individual from the booming herds of the more distant past. Their findings, published Thursday in the journal PLOS Genetics, provided some of the first concrete proof of the genetic theories describing how population size affects genetic fitness. “Genomic meltdown” may have doomed the last herd of mammoths, a conclusion that on its face suggests dire consequences for modern endangered species, but that could also offer valuable insight into how to best keep today's rarest creatures from crossing the threshold into extinction.
The furry beasts ruled the tundra for over a million years until climate change turned grasslands into forests and hungry humans arrived on the scene. These pressures caused the mainland population to go extinct about 10,000 years ago, but two pockets managed to survive millenniums longer.
Two arctic islands became their last refuge, with populations surviving on St. Paul island until a lack of fresh water did them in 5,600 years ago, leaving the species to make their final stand on the remote Wrangel Island, where they stuck it out for another 1,600 years.
Researchers compared the DNA of a 4,300-year-old Wrangel Island specimen with that of a 45,000-year-old mainland mammoth. Genomic diversity measures suggest that the mainland individual was part of a breeding population 43 times larger than the 300 remaining island mammoths.
They found that the island genome was damaged compared to that of the mainland mammoth, suggesting that the lack of diversity in the breeding pool may have led to a breakdown in the integrity of the gene pool. As a result, many island mammoths may have had poor senses of smell, and a new coat as the stiff hairs that protected individuals from the cold became soft and shiny. The mighty woolly mammoth became a “satin” mammoth.
Experts can’t be sure that these genetic changes caused the Wrangel population to die out, but lead author Rebekah Rogers of the University of California, Berkeley, finds the timing highly suspicious. "We found these bad mutations were accumulating in the mammoth genome right before they went extinct," she told the BBC.
This result contradicts a 2012 paper, which found that while the genetic diversity did indeed drop after the shrinking population became isolated, it continued at a reduced but stable level for thousands of years, until some other cause drove the final nail into the coffin. "I'm personally leaning towards environmental change," co-author Love Dalen, of the Swedish Museum of Natural History, told the BBC at the time.
Regardless of what ended the Wrangel Island mammoths, the study has great significance in the field of genetics, where genome evolution theory has long predicted that damaging mutations should pile up in small populations of organisms.
"The mathematical theories that have been developed said that [individuals in small populations] should accumulate bad mutations because natural selection should become very inefficient," Dr. Rogers explained to the BBC.
The problem was that this accumulation takes a long time, making it difficult to confirm the theory by observing the change as it happens within a single species.
But the mammoth made just such an empirical observation possible.
“This is probably the best evidence I can think of for the rapid genomic decay of island populations,” Hendrik Poinar, an evolutionary geneticist at McMaster University who was not involved in the study, told The New York Times.
The confirmation may have serious consequences for efforts to prevent modern species from going the way of the mammoth.
“This is a very novel result," Dr. Dalen, who published the DNA sequences this study was based on, told the BBC. "If this holds up when more mammoth genomes, as well as genomes from other species, are analysed, it will have very important implications for conservation biology."
The paper identifies Asiatic cheetahs (fewer than 100 individuals), pandas (1600 individuals living in highly fragmented territories), and mountain gorillas (300 individuals) as examples of small populations in danger of suffering the same “genomic meltdown” as the mammoths.
Saving such species may be challenging, because once genes get deleted, it’s “difficult to see how genomes could recover quickly,” the authors write. “With small effective population sizes, adaptation through both new mutation and standing variation may be severely limited.”
Their work suggests the existence of a population point of no return, after which a species may never recover, no matter what careful protections are afforded to the endangered individuals.
But there's a silver lining. A better understanding of the challenges facing small populations can help focus conservation efforts, and direct where limited funds should be best spent. Concentrating resources on preserving vulnerable species before their numbers dwindle could be a more cost-effective strategy than large expenditures on groups that have already suffered a great loss of genetic diversity.
"So if you can prevent these organisms ever being threatened or endangered then that will do a lot more to help prevent this type of genomic meltdown compared to if you have a small population and then bring it back up to larger numbers, because it will still bear those signatures of this genomic meltdown," Rogers explained to the BBC.
Simply put, an ounce of prevention may be worth a pound of cure.
[Editor's note: The original version of this article omitted the full attribution for lead author Rebekah Rogers of the University of California, Berkeley.]