In a dwindling species, gene loss may prove irreversible, study finds
In a remote, mist-wrapped island north of the eastern tip of Siberia, a small group of woolly mammoths were the last survivors of their once thriving species. They fell extinct 4,000 years ago, having endured for about 6,000 years after the mammoths on the mainland had died off. From a message left in the tooth of a mammoth, geneticists have now deciphered the probable reason for the population demise.
The story is relevant to living populations of endangered species, because it supports the idea that as a population dwindles, natural selection becomes less efficient at purging bad mutations, leading to a loss of genes and a slow meltdown of the genome. The implication is that once numbers fall below a certain level, genetic decline is irreversible.
Woolly mammoths once flourished from northern Europe to Siberia. As the last ice age ended, about 10,000 years ago, the mainland population perished, victims of climate change and human hunters. But some populations lived on for thousands of years, notably on two remote islands that had once been part of Beringia, the now-foundered land bridge between Alaska and Siberia.
One of these refuges was Wrangel Island, a mountainous island in polar seas, so inaccessible that Baron von Wrangel, the explorer after whom it is named, never managed to reach it. The other is St. Paul Island, about 280 miles from Alaska and the Aleutians.
The mammoths on St. Paul survived until 5,600 years ago, but the reasons for their extinction have long been a matter of speculation. Last August, a team led by Russell W. Graham of Pennsylvania State University ruled out all the leading candidates, including human predation, polar bears, increased winter snowpack, volcanic activity and changing vegetation.
The real reason, they concluded, after examining lake bed sediments, was simply a lack of fresh water. Elephants are heavy drinkers and mammoths, their close cousins, were probably even more so, because they were adapted to the cold but trying to survive in the post-ice age climate. During dry periods, only one lake on St. Paul was available, and this seems to have failed as thirsty mammoths destroyed the plant cover on its shores.
The mammoths of Wrangel, a much larger island, survived about 1,600 years longer and seem to have met a different fate. A team led by Eleftheria Palkopoulou and Love Dalen of the Swedish Museum of Natural History gained a major insight into the population history of the woolly mammoth by analxzing the whole genomes of two individuals. One was a mammoth from the mainland, from the Oimyakon district of northern Siberia；it died about 45,000 years ago, at a time when the species still flourished. The other was from Wrangel Island and perished about 4,300 years ago, a few hundred years before extinction.
From the amount of genetic variation in each genome, the Swedish team was able to calculate the effective population size - a genetic concept roughly equivalent to the breeding population - of the woolly mammoths in each time period. The Oimyakon mammoth's genome indicated an effective population size of 13,000 individuals, whereas that of the Wrangel mammoth was a mere 300.
