Koala populations once written off as genetically doomed are bouncing back through rapid growth

Science, March 2026, AAAS

When a population crashes, the genetic math is usually grim. Fewer individuals means less genetic diversity, more inbreeding, and a growing burden of harmful mutations. This is the extinction vortex, and it has been the dominant framework for assessing genetic risk in conservation biology for decades.

But populations sometimes bounce back. When they do, does the genetic damage heal? A genomic study of 418 koalas across 27 Australian populations, published in Science, provides evidence that it can.

Testing the theory with koalas

Australia's koalas offer a natural experiment in population genetics. In the 19th and early 20th centuries, koala populations in parts of southern Australia were decimated by hunting, disease, and habitat destruction. Small founding groups were relocated to islands and isolated habitats. These populations passed through severe genetic bottlenecks.

Theory predicts that such populations should carry the scars of that bottleneck indefinitely: low diversity, high inbreeding, reduced evolutionary potential. But theory also predicts that if a bottlenecked population rebounds rapidly enough, growth can promote genetic reshuffling and the introduction of new mutations, potentially counteracting the damage.

Collin Ahrens and colleagues at Cesar Australia used whole-genome sequencing to test which prediction held true.

Recovery in progress

The data showed that koala populations with severe historical bottlenecks retained the genetic signature of that event, including low overall diversity. But many of these populations were expanding, and that expansion was producing measurable genetic consequences.

Through recombination, the shuffling of existing genetic material into new combinations during reproduction, these growing populations were generating new genetic diversity from old raw material. Harmful mutations were being reduced. Adaptive potential was increasing.

Counterintuitively, populations with higher overall genetic diversity, such as those in northern Australia, carried more harmful mutations and showed declining effective population sizes. The static diversity metric pointed one way; the dynamic trajectory pointed another.

Rethinking how conservation measures genetic health

The implications reach beyond koalas. Many threatened species worldwide have experienced bottlenecks, habitat loss, and rapid environmental change. Assessing their genetic risk using a single snapshot of diversity could lead to misclassification of both danger and recovery.

The study suggests that conservation genetics needs to account for population trajectory, not just current diversity levels. A population that looks genetically impoverished today may be on a path toward recovery if it is expanding. A population with high diversity may be accumulating genetic damage if it is declining.

This has direct management implications. Translocation, moving individuals between populations to boost diversity, has been widely used for koalas but is now considered high-risk, complex, and expensive. If some bottlenecked populations are recovering genetically on their own, the case for costly intervention weakens.

What the study cannot promise

The recovery observed in southern koala populations depends on rapid population growth, a condition not all bottlenecked species will meet. Species with slow reproductive rates, continued habitat loss, or ongoing threats may not rebound quickly enough for recombination and new mutations to outpace genetic deterioration.

Northern koala populations remain in serious decline despite higher genetic diversity. The study does not suggest that all bottlenecked populations will recover. It demonstrates that under favorable conditions, recovery is possible, and that static genetic metrics alone cannot predict outcomes.

The study's 418 genomes represent a snapshot in time. Whether the recovery trends will continue under future pressures, including climate change, chlamydia infection, and ongoing habitat fragmentation, is an open question that will require continued monitoring.