March 5, 2026
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Koalas show how species can bounce back from genetic bottlenecks
Scientists have discovered a potential way out of devastating genetic bottlenecks that could help these Australian animals, as well as many other vulnerable and endangered species
Without interrupting their busy sleep schedule, Australia’s cute, if ferocious, koalas have turned a truism of genetics on its head.
In short, the spread of certain koala populations shows how bottlenecks – which occur when a species’ numbers suddenly shrink, reducing genetic diversity – do not necessarily condemn an animal to inbreeding and eventually extinction. Former bottleneck species can bounce back and regain a surprising amount of diversity.
“The assumption that a bottleneck leads to eventual extinction is not set in stone,” says Rachel O’Neill, a genome biologist at the University of Connecticut, who was not involved in the new research.
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For the study, published March 5 in Science, researchers dug into the entire genomes of 418 koalas from different populations to understand how the effective population sizes of different groups had changed over time. While a population size corresponds to the number of animals in a group, an efficient population size measures how these animals’ genetic diversity is distributed between individuals after sexual reproduction.
The researchers found a pattern they did not expect among koala populations in the Australian state of Victoria, where the animals have high rates of inbreeding and genetic deformation. Their effective population sizes cratered in the late 19th century due to the fur trade, creating an expected bottleneck. But surprisingly, the researchers found that the effective population sizes of koalas in Victoria have increased over the past 40 generations, while koala populations in Queensland and New South Wales – which conservationists have generally considered genetically healthier – have shown sharp declines.
“It still looks like they’re in bad shape, but if you dig further, we actually find that there’s recovery from the bottleneck,” says study co-author Collin Ahrens, an evolutionary biologist at the independent research company Cesar Australia.
The genetic recovery has come thanks to explosive population growth — in Victoria, koalas are now so numerous that their management consists of trying to limit their numbers rather than increase them, Ahrens notes. What is happening is that these populations have increased so dramatically that there has been a lot of opportunity for mutations and even for the limited genes that were retained during the bottleneck to cluster in different ways.
“Recombination reshuffles the genetic variation,” says Ahrens. “It’s very important and something that has been very difficult to measure.”
What happened to the Victorian koalas has an interesting parallel in invasive species. Scientists have long known that invasive organisms can quickly spread after just a few individuals are introduced to a new ecosystem that they find to their liking. Instead of being plagued by inbreeding, they sometimes thrive genetically, much to the detriment of the species around them.
The implications of the new research could reach far beyond koalas, given the number of species threatened by climate change and other human-driven pressures. “Right now we’re seeing a lot of human-driven declines, and I think their findings kind of give these populations hope,” said Caitlin Curry, a population geneticist at the San Diego Zoo Wildlife Alliance, who was not involved in the new research. “If we give them the right resources and tools to have some kind of rapid expansion, that might also restore their evolutionary potential.”
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