Escaping bottlenecks: The demographic path to genetic recovery in koalas (Phascolarctos cinereus).

Population bottlenecks can lead to evolutionary dead ends by eroding genetic diversity and intensifying inbreeding. Although theory predicts possible escape routes, direct observations of this process are rare. Using whole-genome data from 418 koalas, we found that populations with higher genetic diversity harbored the greatest mutational loads and had declining effective population sizes (Ne), whereas historically bottlenecked but recovering populations displayed reduced mutational load, exhibited increasing Ne, and regenerated new, rare variants. We concluded that this pattern was due to rapid demographic expansion, which reshuffled genetic variation through recombination and affected Ne more quickly than it did conventional diversity metrics. Our findings suggest that recovery of bottlenecked populations can occur through rapid demographic growth and that this can reestablish evolutionary potential in threatened populations. Editor's summary: Population genetics theory predicts that bottlenecks, or times when abundance in a population crashes to a small number of individuals, can lead to an extinction vortex due to loss of genetic variation. Nonetheless, there are cases in nature where this process has occurred but the species has recovered. Ahrens et al. looked at populations of once widespread koalas across regions of Australia to determine how bottlenecks affected their diversity. They found that populations that had experienced reductions showed signs of recovery, such as increasing effective population size and reduced mutational load due to rapid demographic expansion. —Sacha Vignieri [ABSTRACT FROM AUTHOR]