It would seem obvious that rising sea levels that cut off the last population of woolly mammoths on Wrangel Island from the Siberian coast 10,000 years ago caused in-breeding, leading to their eventual extinction. But a new genetic analysis, reported in Cell, counters that claim.

That result was unexpected, since an earlier report indicated that the mammoth population likely built up harmful genetic mutations, according to Love Dalén, a scientist with the Centre for Palaeogenetics of Sweden, and one of the paper’s authors.

“I was fairly surprised both at how few the mammoths on Wrangel Island were in the beginning (less than 10 individuals), and that we did not find any evidence for an accumulation of highly deleterious mutations,” Dalén says. He calls the relative lack of harmful mutations surprising, because an earlier paper published in 2017 in PLOS Genetics predicted they may have played a factor.

Natural Selection and Genetic Drift

The genetic story of this population — which likely started from 8 individuals and grew to 200 to 300 mammoths over 6,000 years within 20 generations — is a bit more complicated. Two sometimes competing genetic factors were at play.

Natural selection tends to purge harmful variants, because offspring carrying them would be less likely to survive. But genetic drift works to preserve mutations across generations.

This effect tends to be stronger in smaller populations, so it would make sense that it would impact the Wrangel Island mammoths.

Read More: Rapid Evolution Changes Species in Real Time

What Made the Island Mammoths go Extinct?

Although it appears that these two genetic forces competed, it wasn’t necessarily a fair fight.

“Whereas genetic drift affects all deleterious mutations in the same way, natural selection will be stronger the more deleterious a mutation is,” says Dalén. “That way, natural selection is expected to remove the very deleterious mutations from the population, while genetic drift leads to an increase in slightly deleterious ones.”

Therefore, over time, the most harmful mutations were purged, while a bunch of relatively innocuous ones remained. So, if inbreeding didn’t do in the mammoths, what did?

“This leaves us with some other possibilities, such as a disease outbreak, some sort of tundra fire, or very short-term climatic perturbation, where the latter two could have ruined the plant growth for one or a few seasons, which could have had disastrous consequences for such a small population,” Dalén says.

Read More: How Do We Know When a Species Is Extinct?

Conservation of Today

Although the interplay between natural selection and genetic drift didn’t appear to wipe out the mammoths of yore, it still has implications for conservation efforts of today.

For example, conservationists wishing to preserve a species that has almost gone extinct, but has partially recovered, could look to the Wrangel Island mammoths. The implication is that moving a few individuals between two isolated populations could slow genetic drift.

The study’s authors have yet to analyze DNA from the most recent Wrangel Island mammoths. However, researchers have found fossils from the last 300 years of the mammoths’ existence and plan to analyze their genomes.

Read More: Scientists Are Trying to Save These Animals From Extinction

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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.