Victorian koalas descend from fewer than a thousand individuals. Hunted nearly to extinction in the early twentieth century, the surviving populations carry the genetic signature of a severe bottleneck — long stretches of homozygous DNA, low heterozygosity, the markers conservation genetics uses to diagnose terminal decline.
Weeks, Miller, and Ahrens sequenced 418 koalas across 27 populations in Queensland, New South Wales, and Victoria. The southern populations — the bottlenecked ones — are expanding. As they grow, recombination is breaking up the inherited blocks of identical DNA and generating new combinations. Genetic diversity is increasing. Not back to pre-crash levels, but the trajectory is upward.
The northern populations tell the opposite story. They carry higher diversity — more heterozygosity, fewer homozygous runs — but their numbers are declining. By every static measure, the northern koalas are healthier. By every dynamic measure, they're at greater risk.
The conventional framework treats genetic diversity as a diagnostic: low diversity means high risk, high diversity means safe. The koala data inverts this. Low diversity plus population growth is a recovery. High diversity plus population decline is a crisis. The snapshot misreads the trajectory. The photograph says one thing; the movie says the opposite.
The through-claim: genetic health is a derivative, not a value. What matters is not how much diversity a population has but which direction it's moving. A population at the bottom of a crash that's expanding is rebuilding. A population at the top of its diversity that's shrinking is spending down. The measurement that ignores direction misdiagnoses both.