Koalas survived a devastating population crash and their DNA is bouncing back

Population bottlenecks occur when a species experiences a dramatic drop in numbers. These events often reduce genetic diversity and increase inbreeding. Over time, this genetic damage can lower fertility, reduce survival rates, and weaken a population's ability to adapt to environmental changes. These pressures can trigger what scientists call an "extinction vortex," a downward spiral in which shrinking populations and worsening genetic health reinforce each other and push species closer to extinction.

How Population Growth Can Restore Genetic Diversity

Despite these risks, genetic decline is not always permanent. When populations expand rapidly after a crash, growing numbers can sometimes help rebuild genetic diversity. Scientific theory suggests that increasing population size allows more opportunities for genetic reshuffling and the appearance of new mutations. Even when recovery begins with a small group of surviving individuals, continued population growth can help offset the harmful effects of inbreeding and partially restore lost variation.

As a result, rapid demographic expansion may serve as an important buffer against the genetic problems that often follow severe population declines.

Koalas Offer a Natural Test of Genetic Recovery

To examine this possibility, Collin Ahrens and colleagues used the dramatic history of koala population decline and recovery as a natural experiment. Koalas experienced a sharp drop in numbers that produced a severe genetic bottleneck. Later, many populations rebounded.

The researchers analyzed whole genome data from 418 koalas representing 27 populations across Australia. Their analysis revealed that although koalas still have relatively low genetic diversity because of their past decline, signs of genetic recovery are now emerging in several populations.

Recombination May Help Rebuild Koala Genetic Health

The study suggests that recombination may play a key role in this recovery. This process shuffles existing genetic material into new combinations, which can help restore functional diversity as populations expand.

Overall, the findings indicate that rapid population growth can sometimes allow species that have experienced severe genetic bottlenecks to regain part of their lost genetic diversity. These insights could be important for conservation strategies aimed at supporting the long term survival of vulnerable wildlife populations.