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A small group of woolly mammoths became trapped on Wrangel Island about 10,000 years ago when rising sea levels separated the island from mainland Siberia. Small, isolated populations of animals lead to inbreeding and genetic defects, and it has long been thought that the Wrangel Island mammoths eventually succumbed to this problem around 4,000 years ago.

However, a paper in Cell on Thursday compared 50,000 years of genomes from continental and isolated mammoths from Wrangel Island and found that this was not the case. What the paper’s authors found not only challenges our understanding of this isolated group of mammoths and the evolution of small populations, but also has important implications for conservation efforts today.

Very narrow place

It is the culmination of years of genetic sequencing by members of the international team behind this new paper. They examined 21 mammoth genomes—13 of which were recently sequenced by lead author Marian Dehask; others were sequenced years before by co-authors Patrícia Pečnerová, Foteini Kanellidou and Héloïse Muller. Genomes were obtained from Siberian woolly mammoths (Mammuthus primigenius), from both the mainland and the island before and after it was isolated. The oldest genome is from a female Siberian mammoth that died about 52,300 years ago. The youngest were among the male mammoths on Wrangel Island, who died at the exact time when the last of these mammoths died (one of them died only 4333 years ago).

This is a remarkable and telling time period: the sample included mammoths from a population that started out large and genetically healthy, went through isolation, and eventually went extinct.

The mammoths, the team notes in their paper, experienced a “climatically turbulent period,” particularly during an episode of rapid warming called the Bølling-Allerød interstadial (approximately 14,700 to 12,900 years ago)—a time that others suggest may have has led to the local extinction of the woolly mammoth. However, mammoth genomes studied during this time period do not show that warming had any adverse effects.

The adverse effects did not appear – and drastically so – only after the population was isolated on this island.

The team’s simulations show that at its smallest population, the total population of Wrangel Island mammoths was fewer than 10 individuals. This represents a serious difficulty in the population. This is observed genetically by increased runs of homozygosity in the genome, caused when both parents contribute nearly identical chromosomes, both derived from a recent ancestor. Homozygosity runs among isolated mammoths on Wrangel Island were four times greater than before sea level rose.

Despite this dangerously low number of mammoths, they have recovered. The population size, as well as the level of inbreeding and genetic diversity, remained stable for the next 6,000 years until their extinction. In contrast to the initial population bottleneck, genomic signatures over time seem to indicate that inbreeding eventually shifted to more distantly related pairs, suggesting either a larger mammoth population or a behavioral change.

Within 20 generations, their simulations show that the population size will have increased to about 200-300 mammoths. This is consistent with the slower decline in heterozygosity they found across the genome.

Long-term negative effects

Wrangel Island mammoths may have survived against the odds, and harmful genetic defects may not have caused their extinction, but the study suggests their history is complex.

At about 7,608 square kilometers today, slightly larger than the island of Crete, Wrangel Island would offer plenty of space and resources, even though these are big animals. For 6,000 years after their isolation, for example, they suffered from inbreeding depression, which refers to increased mortality as a result of inbreeding and the resulting defects.

This inbreeding also promotes the clearance of deleterious mutations. This might sound like a good thing—and it can be—but it usually happens because individuals carrying two copies of harmful mutations die or fail to reproduce. So it’s only good if the population survives it.

The team’s results show that clearing genetic mutations can be a long evolutionary process. Lead author Marian Dehask is a paleogeneticist who completed her PhD at the Center for Paleogenetics. She explained to Ars that, “Clearing out deleterious mutations for over 6,000 years basically shows long-lasting negative effects caused by these extremely deleterious mutations. Since the purge of the Wrangel Island population lasted for such a long time, this indicates that the population experienced negative effects from these mutations until their extinction.