Koalagenetic bottleneck

 Koala populations went through an extreme genetic bottleneck in the late 19th and early 20th centuries. Heavy hunting for their fur reduced them to very small populations in parts of Australia. Normally, such a bottleneck leads to very low genetic diversity, which can cause disease vulnerability and reduced fertility. Yet koalas have managed to persist and expand again. Researchers have identified a few key reasons why.

1. Hidden genetic diversity in immune genes

Even though koalas have low overall genome diversity, they retained diversity in certain immune-system genes, especially the MHC (Major Histocompatibility Complex).

• MHC genes help the immune system recognize pathogens.
• Natural selection tends to preserve diversity in these genes even when the rest of the genome loses variation.
• This allowed koalas to still respond to diseases despite the bottleneck.

Result: functional diversity remained where it mattered most for survival.

2. Population recovery from multiple refuges

The bottleneck wasn’t equally severe everywhere.

• Some populations survived in separate refuges (especially on islands and remote forests).
• Later conservation and natural movement allowed mixing between populations.

When different small populations interbred again, their slightly different gene pools combined, restoring some diversity.

3. Rapid population growth after protection

After hunting was restricted in the early 1900s, koalas rebounded.

• Translocation programs moved koalas from surviving populations to new areas.
• Population growth increased the effective population size, slowing further genetic loss.

However, some reintroduced populations still have very low diversity because they came from only a few founders.

4. Tolerance to some diseases

Koalas are commonly infected with Chlamydia (koala disease) and Koala retrovirus.

Some populations appear to have genetic adaptations that help them tolerate or resist these infections. Natural selection likely favored individuals that could survive despite limited genetic diversity.

✅ In short:

Koalas didn’t fully “escape” the bottleneck genetically. Instead, they survived because:

• crucial immune genes kept diversity
• multiple small populations later mixed
• conservation efforts boosted numbers
• natural selection favored disease-resistant individuals

They still have lower genetic diversity than many mammals, which is why habitat loss, climate change, and disease remain major threats today.

If you’d like, I can also explain why koalas are a strange exception compared to species like the Cheetah, which went through a bottleneck but still show extreme genetic uniformity. The comparison is pretty fascinating.