We thought inbred koalas were at risk of extinction. But what we discovered upends genetic conventions
- Written by Andrew Weeks, Associate Senior Research Scientist, The University of Melbourne
If you follow media coverage of koalas, you could be forgiven for feeling confused.
Recent stories describe a “koala paradox”: endangered in the north of Australia, abundant in the south; genetically diverse in some regions, genetically depleted in others.
Koalas populations are often described simultaneously as being in crisis, or overabundant. These accounts attempt to capture the complexity of this species across different histories and geographic locations. But they also reveal a deeper problem with how we assess genetic risk in wildlife (the likelihood a population will go extinct because it has lost too much genetic diversity).
Our new research shows relying too heavily on genetic indicators – how genetic diversity and inbreeding are measured – can be misleading. And we found the koala to be a powerful case study for a much broader lesson in conservation.
The assumption we rarely question
Conservation often rests on a simple logic: a population crash – a rapid and steep decline in population size – reduces genetic diversity and increases inbreeding. Genetically diverse populations, meanwhile, are believed to be more resilient and less susceptible to decline.
This logic is not wrong, but it is incomplete.
It treats genetic health as static, rather than a dynamic outcome shaped by how populations grow or shrink over time. Koalas provide a useful test. Different populations have experienced very different histories; from extreme collapse followed by rapid recovery, to slower but ongoing decline.
What we found and what their DNA reveals
We analysed DNA from 418 koalas sampled across 27 populations in Queensland, New South Wales and Victoria. This allowed us to reconstruct their population history and size over time. We also examined how different genetic variants respond to population decline and recovery. What emerged was surprising.
Koala populations with higher genetic diversity, particularly those in northern Australia, tended to carry more harmful variants. They also showed declining population sizes. In contrast, populations that had passed through severe historical crashes but were now expanding, showed signs of genetic recovery.
This does not mean population crashes are harmless. They are dangerous and can be irreversible. But it does mean they are not always evolutionary dead ends.
Why recovery can start before diversity rebounds
The key lies in how DNA responds when populations grow rapidly. Generally speaking, when populations expand, recombination (the reshuffling of genetic material each generation) spreads new genetic combinations through the population. This breaks up inherited blocks of DNA and generates new genetic variation. In turn, this can increase a population’s ability to adapt, allowing numbers to grow faster than traditional genetic indicators might suggest.
In koalas, this process is clearly visible. As populations expand, genes are mixed and matched in new ways, creating new genetic variation. Many traditional genetic indicators fail to detect these changes. However, our analyses can reveal them.
This suggests genetic indicators of diversity can lag behind the true health of a population, and sometimes mislead conservation assessments. A population may appear genetically depleted if we rely only on these indicators, even while its diversity is quietly being rebuilt. Conversely, a population can look genetically healthy while its population size is actually becoming unstable, putting that diversity at risk over time.
Correcting a common misconception
Victorian koala populations are often portrayed as genetically compromised because they experienced an extreme population crash in the past. Our results show a more nuanced picture.
Victorian populations still carry the genetic signature of this extreme crash, when fewer than 1,000 koalas remained in the wild. However, many are now on a path to genetic recovery. At the DNA level, their genes are being reshuffled and new genetic variation is appearing. This represents the early stages of genetic recovery, not genetic collapse.
The greater long-term concern is for populations that are rapidly declining but still appear genetically healthy. If population size collapses, genetic diversity can be lost very quickly.
Why this matters beyond koalas
Our results suggest the picture for koalas is more nuanced than previously thought. Southern “inbred” populations are growing again and gaining genetic diversity, whereas northern populations are shrinking, regardless of how genetically diverse they appear today.
This matters far beyond koalas. Many threatened species have experienced population crashes, translocations or reintroductions (such as on French Island and Kangaroo Island) and rapid environmental change. If we judge their future using static genetic indicators, we risk getting the picture wrong, both about their risk of genetic decline and their chance of recovery.
What matters just as much is the direction a population is heading. Is population size rising or falling? Are new genetic variants appearing or disappearing? Is recombination boosting their evolutionary potential, or being choked by small population size?
Rethinking genetic risk
One of the most important messages from our study is this: low genetic diversity does not automatically imply high extinction risk. And high genetic diversity does not guarantee safety. Genetic indicators only make sense when we consider the population’s history, and whether its numbers are rising or falling. Without that context, even well-intentioned conservation decisions can miss the mark.
Koalas, so often used as symbols of the conservation crisis, offer something rare: direct evidence that genetic recovery is possible, and insight into how to detect it early.
If conservation genomics is to guide policy effectively, it must move beyond static genetic indicators. We need to start tracking where populations came from and where they are headed, not just where they are now.
Authors: Andrew Weeks, Associate Senior Research Scientist, The University of Melbourne
