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The woolly mammoths of Wrangel Island survived. Stuck on a piece of rock in the Arctic Ocean after being cut off from present-day Siberia by rising sea levels, they were the last members of their species to go extinct. Paleontology textbooks have explained their final destruction about 4,000 years ago as a classic case of extinction through inbreeding, in which severely harmful genetic mutations spread to an isolated population and kill it off. New work published June 27 in a journal called Cell shows the textbooks are wrong.
Genetic diversity can be thought of as an insurance policy for a species. If a population contains a sufficient number of unrelated individuals, there is a good chance that one of them will have a genetic trait that will enable it to survive a new threat. As long as that individual is able to pass on that trait to its offspring, the species can avoid extinction. When small isolated populations are forced to breed with each other for many generations, harmful mutations inevitably become concentrated. This phenomenon, known as mutational meltdown, has the power to wipe out entire populations.
Love Dalén and Marianne DeHasquet at the Centre for Palaeogenetics in Sweden were curious to know whether the Wrangel Island mammoths had experienced mutational slowdown. Since Wrangel Island is dry and bitterly cold most of the year, the woolly mammoth remains excavated there are often very well preserved. Even DNA, which degrades quickly in warm climates, can be collected from these samples. This made it possible for Dr. Dalén’s team to study the genetics of the 14 mammoths stranded on the island. They then compared the genetics of these animals with those of seven individuals that were alive in the region before sea levels rose and decimated the population.
Some mammoths in the isolated population died out as early as 4,333 years ago. Others died out as early as 9,219 years ago, about 800 years after Wrangel Island became isolated. Similarities in their DNA led the researchers to conclude that about eight different mammoths founded the population. This probably corresponds to a single herd. Furthermore, as described in textbooks, mammoths initially faced serious genetic challenges. But, remarkably, the most harmful mutations did not overwhelm the population. Instead, individuals with the most severely harmful mutations either died or failed to reproduce, keeping the rest of the population healthy. (Whether this was simply the work of natural selection or the result of mammoths not mating with sick individuals remains to be seen.) For 6,000 years, the population of 200 to 300 mammoths lived in this relative stability. There was not much genetic diversity among them. Very harmful mutations were still causing some creatures to become sick, and mildly harmful mutations were increasing, but mammoths were not being wiped out by mutations.
Then, just as suddenly, they went extinct. Archaeologists have found no evidence of the presence of people on the island at the time, so human hunting seems an unlikely explanation (even though humans are thought to have hunted mammoths in places such as North America). Tundra fires or a few seasons of bad weather may also have wiped them out, but disease seems more likely. One part of the genome of the Wrangel Island mammoths that was greatly affected by isolation was the major histocompatibility complex (MHC). This is a crucial part of the immune system, which gathers material from microbes and trains attacking cells to recognize them. The researchers have calculated that the MHC of the island mammoths lost 49.2% of its genetic diversity, compared with mammoths that were not isolated.
This work has practical implications for conservation efforts. Several species, such as the California condor and the scimitar-horned oryx, have come perilously close to extinction. Efforts to protect their remaining populations from mutational meltdown through selective breeding programs should certainly be applauded, but Dr. Deleon’s work shows that this is only the first challenge. Maintaining a species’ genetic insurance policy over the long term is equally important. But it is not clear that conservationists can do this for species that are threatened by a dramatic loss of genetic diversity.
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© 2024, The Economist Newspaper Limited. All rights reserved. From The Economist, published under licence. Original content can be found at www.economist.com.
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