According to a new genomic study of Australia’s koala populations, rapid demographic rebound may be able to restore once-lost genetic variation and drive recombination in ways that re-establish long-term evolutionary potential in previously bottlenecked populations. Population bottlenecks can lead to evolutionary dead ends by eroding genetic diversity and intensifying inbreeding. Over time, these genetic repercussions can reduce fertility, survival, and resilience to environmental change, driving a so-called “extinction vortex” in which dwindling numbers and declining genetic health greatly amplify extinction risk. However, this genetic decline is not always irreversible. When populations rebound rapidly, their growth can sometimes restore genetic diversity. Theory suggests that ever-growing numbers – even in small founding groups – can promote genetic reshuffling and the introduction of new mutations, counteracting the loss of variation and easing the harmful effects of inbreeding. Thus, in theory, rapid demographic expansion may be a key mechanism for buffering against the genetic pitfalls typically associated with severe population declines. Here, Collin Ahrens and colleagues leveraged the precipitous decline – and the severe genetic bottleneck that resulted – and rapid rebound of koalas as a natural experiment to test whether population recovery was able to facilitate genetic recovery. Ahrens et al. used whole-genome data from 418 koalas drawn from 27 populations across Australia and found that, although koalas endured severe population decline and genetic bottleneck that left them with low genetic diversity, populations now show signs of genetic recovery. The findings suggest that this rebound may be driven in part by recombination, which shuffles existing genetic material into new combinations, helping to restore functional diversity as populations expand. Overall, the findings indicate that recovery of bottlenecked populations can occasionally occur through rapid population growth, which has important implications for conservation management strategies.
Science
Escaping bottlenecks: The demographic path to genetic recovery in koalas (Phascolarctos cinereus)
5-Mar-2026