The koalas everyone gave up on are making a genetic comeback

Scientists have long believed that when an animal population crashes, it is on a one-way road to genetic ruin. Fewer animals, less genetic diversity, more inbreeding, weaker offspring, and then extinction. This is a framework that has guided wildlife conservation policy for decades. But a new study published in Science is turning that assumption on its head, and koalas are to blame.

A landmark genomic study led by researchers from the University of Sydney and Cesar Australia has found that koala populations once considered the most genetically vulnerable are now showing clear early signs of recovery. The team sequenced 418 whole genomes from koalas from 27 populations across Australia, one of the largest genomic studies ever undertaken on a single threatened species. What they found challenges some of the most fundamental assumptions in conservation biology.

Being almost extinct actually helps
The surprising result at the core of this work is that Victoria’s koala populations, which plummeted to perilously low levels in the late 1800s and early 1900s, are now recovering in both abundance and genetic health. These southern populations, for decades, were thought to be genetically compromised, victims of a severe “bottleneck” that decimated their numbers and wiped out much of their genetic diversity.


But according to ScienceDaily's coverage of the AAAS research findings, rapid population growth seems to actively encourage a biological process called recombination, the natural reshuffling of DNA into new combinations. This genetic mixing can help previously bottlenecked populations regain diversity and, importantly, lose the harmful mutations that have accumulated over time.

“It's the populations that had passed through severe historical bottlenecks that are expanding, resulting in the accumulation of new mutations and greater genetic combinations,” said Dr. Collin Ahrens, Principal Research Scientist at Cesar Australia. “This growth, like we see in Victoria, comes with real genetic advantages: fewer harmful genetic mutations, increased adaptive capacity and early signs of genetic regeneration.”

In other words, the near-death experience may have prepped these populations for a stronger future.

The old rulebook is broken
For decades, the metric of choice in conservation genetics has been genetic diversity; a higher number generally indicates a healthier population with better odds of long-term survival. This thinking has influenced everything from zoo breeding programs to decisions about moving wildlife across continents.

The problem, as this research makes clear, is that genetic diversity is a snapshot in time. It tells you where a population has been, not where it is heading. A widely cited 2021 paper published in PNAS on conservation genetics as a management tool notes the importance of knowing a population’s trajectory, not just its current state, in order to make effective conservation decisions. That insight feels more urgent than ever.

“For decades, we've treated genetic diversity as a simple scorecard of extinction risk,” said Dr. Andrew Weeks, Director of Cesar Australia. “But evolution is dynamic. Understanding whether a population is expanding, stabilizing, or declining over time may be just as important as measuring how much diversity it holds today.”

Why this matters beyond Australia
Koalas have had a tough few decades. The 2019–2020 Black Summer bushfires alone burned an estimated 25% of the koalas’ habitat in New South Wales, impacting an estimated 61,000 koalas. Koalas were officially listed as endangered in Queensland, New South Wales and the Australian Capital Territory in 2022.

Meanwhile, northern koala populations are seriously declining and southern ones are actually overabundant. This split makes wildlife management a constant juggling act. Translocation, the movement of animals from one region to another to bolster struggling populations, was once a common tool but is now viewed as risky, costly and ecologically complex.

The new genomic data can alter the way those decisions are made by managers. But the implications go well beyond Australia. Many of the world's most critically endangered animals have undergone severe bottlenecks. If the genetic recovery dynamics observed in koalas are not unique to the species, it may change the way conservationists evaluate and prioritize threatened populations worldwide.

Revisiting the equation of extinction
The main message of the study is time and direction. Recombination plays a role here, and if the population is increasing, what looks genetically depleted today might look quite different in a generation or two. Static diversity measures estimate extinction risk in recovering populations but may overestimate it. In diverse-looking but quietly declining populations, static diversity measures may underestimate the risk.

If you have ever wondered whether conservation efforts actually work or whether it is already too late for some species, this study offers something rare: a reason for cautious optimism, based on hard data.