Great white sharks survived the Ice Age - but it changed them forever

Great white sharks almost didn’t make it. Around 25,000 years ago, with sea levels 130 feet lower than they are today, the oceans shrank.

Much of the oceanic habitat white sharks relied on disappeared under massive glaciers. What was left became a tight squeeze for one of the world’s top predators.

White sharks survived – but just barely. And how they bounced back still puzzles scientists today.

What happened to great white sharks?

A new genetic study sheds light on that history. The work reveals that great whites were once reduced to a single, mixed population in the southern Indo-Pacific. Then, around 7,000 years ago, that group began splitting into smaller, isolated populations.

Today, there are three genetically distinct populations of white sharks: one around Australia and South Africa, one in the North Atlantic, and one in the North Pacific. They are widespread, but still scarce.

Study co-author Gavin Naylor is director of the Florida Program for Shark Research at the Florida Museum of Natural History.

“There are probably about 20,000 individuals globally,” said Naylor. “There are more fruit flies in any given city than there are great white sharks in the entire world.”

Genetic mystery of white sharks

Back when glaciers covered huge parts of the planet, sea levels dropped drastically. This left white sharks boxed into limited areas with shrinking access to food and breeding grounds.

As apex predators, they were already few in number. When space got tight, their numbers plummeted even further.

Something odd happened when the climate warmed. Sea levels rose, and white sharks rebounded. Yet when scientists tried to understand the genetic changes during this rebound, they found something that didn’t add up.

The researchers expected to find evidence of philopatry – a breeding pattern in which females return to the same location to reproduce.

This behavior had previously helped explain why nuclear DNA (inherited from both parents) appears nearly identical across white shark populations, while mitochondrial DNA (passed down only from mothers) shows striking variation.

But when Naylor and his team searched for signs of philopatry using the largest dataset of white shark DNA to date, it didn’t hold up.

White shark DNA doesn’t make sense

The story starts in 2001. A research team analyzed DNA from white sharks in Australia, New Zealand, and South Africa. They saw that while nuclear DNA was similar, mitochondrial DNA – passed down only from mothers – showed clear differences between regions.

The results suggested that male sharks roamed freely and spread their genes, while females stayed close to home when it came time to breed. That would explain why mitochondrial DNA looked more isolated and unique, while nuclear DNA looked more mixed.

The theory made sense and held up for years. But no one had done a full test on great whitesharks – until now.

New data, same genetic mystery

In 2012, the Florida team began sequencing the white shark genome using samples collected from across the Atlantic, Pacific, and Indian oceans. They analyzed both mitochondrial and nuclear DNA.

What they found matched older patterns: nuclear DNA was consistent across regions, but mitochondrial DNA showed sharp regional differences.

To test if philopatry was causing this, the team looked at the nuclear DNA from female sharks. If females returned to the same location to breed, a subtle signal should show up in that data too.

“But that wasn’t reflected in the nuclear data at all,” Naylor said. So, the experts ran simulations to see if 10,000 years – roughly the time since the last Ice Age – was enough for philopatry to drive the changes seen in the mitochondrial DNA. Turns out, it wasn’t.

Next, the team considered reproductive skew – the possibility that only a few female white sharks were producing most of the next generation. This has been seen in species like meerkats and some fish, but the explanation didn’t fit for great white sharks.

Natural selection is an unlikely cause

The team also considered natural selection. Maybe specific versions of mitochondrial DNA helped some sharks survive better than others.

That would mean any slight difference in a mitochondrial sequence could be deadly, weeding out variation and locking each population into its own genetic path. It’s possible, but unlikely.

For selection to be that powerful in such small populations, the pressure would have to be overwhelming. If it is the causative agent, the selective force would have to be brutally lethal, explained Naylor.

“If you collect enough mass in a concentrated space, say on the order of a black hole, the otherwise benign force of gravity becomes powerful enough to devour light.”

Natural selection, in this case, would need to act like that – destroying any slight deviation from the favored mitochondrial type before it had a chance to spread.

The mystery remains unsolved

White sharks still keep much of their history hidden beneath the surface. The mitochondrial mystery remains.

None of the known explanations – philopatry, reproductive skew, or natural selection – fully accounts for the genetic disconnect. “The honest scientific answer is we have no idea,” Naylor said.

There’s a lot to learn. But with more data, scientists hope to finally piece together how these rare and powerful animals survived near-extinction and bounced back.

“These white sharks come along, get a nice blubbery sausage. They fatten up, they breed, and then they move off around the ocean,” Naylor said. That part, at least, is clear. The rest is still out there – waiting to be found.

The full study was published in the journal Proceedings of the National Academy of Sciences.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–