SCOTT DETROW, HOST:
When a plant or animal population is in serious decline, it can enter what's called a genetic bottleneck. Fewer individuals means less genetic diversity, which in turn makes it harder for the species to recover. But as NPR's Nate Rott reports, a new study of koalas finds it's not a dead end.
NATE ROTT, BYLINE: It's going to take a quick history of koalas in Southern Australia to really understand this new study. So its lead author, evolutionary biologist Collin Ahrens is going to help fill us in.
COLLIN AHRENS: In the early 1900s, they basically almost went extinct in Victoria.
ROTT: They being koalas, as a result of being hunted for fur and food. But Ahrens says the real genetic bottleneck happened just after.
AHRENS: Some people, realizing this in, like, 1920 or so, moved a few individuals - half dozen or less individuals - to an island called French Island.
ROTT: And on that island, the koalas population exploded, to the point where wildlife managers started moving them back to repopulate the mainland. Today, Ahrens says, basically all of Victoria's koalas can be traced back to the few individuals that were brought to the island, so classic genetic bottleneck.
AHRENS: And so the koalas in Victoria should be less healthy, should be more inbred.
ROTT: But a whole genome analysis of more than 400 different koalas spread across Australia tells a different story. Southern koalas are actually seeing their genetic diversity increase as their population expands. And in the North, where genetic diversity has remained high, populations are declining.
ANDREW WEEKS: And that's what the paradox is with koalas right now.
ROTT: Andrew Weeks is the director of Cesar Australia and a coauthor of the new study in the journal Science. And he says Southern koalas are recovering genetically because of something called recombination, which happens anytime any species, us included, reproduce.
WEEKS: You see it in your kids, right? When you've had children, you get differences that occur. You've got a lot of things that are the same, but you get differences that occur.
ROTT: Changes in genetic variation, mutations - some good, some bad.
WEEKS: So the more individuals you have within a population, the more recombination events you have. And so through generations, as you start growing, those recombination events reshuffle the genetic variation.
ROTT: In other words, the koalas expanded so fast on that island creating so much new genetic diversity, they're now getting out of the bottleneck. And Weeks says that could be true for other endangered species too.
WEEKS: You know, what we are now saying is that if you can keep growing those populations, we can see a path to genetic recovery.
ROTT: It's not an ideal path, he says, but one that we did not know was there.
Nate Rott, NPR News.
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