When a species experiences a population crash—or bottleneck—its genetic diversity takes a hit. The lower diversity triggers a cascade of problems, from inbreeding to combinations of bad alleles, and ultimately reductions in health and survival. For example, northern elephant seals, which were hunted nearly to extinction during the 19th century, continued to suffer lower reproductive and feeding success despite population recovery to more than 200,000 individuals.
However, a new study published today in Science shows that genetic recovery from bottlenecks is possible. Koalas (Phascolarctos cinereus) used to be common across Australia, but suffered a bottleneck from the loss of habitat, hunting, and disease, such that they became endangered in portions of their range. Using whole-genome data from 418 koalas drawn from 27 populations across Australia, researchers from several Australian universities examined the current genetic diversity of koalas.
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Their population modeling showed that the koala population in Victoria in the southeast region of the country had experienced particularly dramatic changes, bottlenecking at just 102 individuals, followed by an expansion to 494 individuals in 35 generations. Prior studies had deemed the Victoria population less diverse as a result, but this new whole-genome examination, by including rare alleles, painted a more nuanced picture. Despite enduring the diminished diversity of an extreme bottleneck, Victoria koalas did show signs of genetic recovery.
Rare alleles reappeared in the population, which mirrors the pattern seen when a small population of invasive species colonizes a new place and then expands. As populations grow, genes get reshuffled through recombination and changed through new mutations, yielding genetic diversity. The study authors suggest that recombination—a powerful force for disrupting bad allele combinations and boosting adaptive potential—has been underappreciated in conservation biology.
“Recovery of bottlenecked populations can occur through rapid demographic growth,” concluded the study authors. “This can reestablish evolutionary potential in threatened populations.”
Essentially, a quick rebound of a bottlenecked population may reestablish lost genetic variation, setting the stage for recovery to a healthy amount of evolutionary potential. Or more simply put, a bottleneck doesn’t always mean a slow march to extinction. 
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Lead image: slowmotiongli / Shutterstock
