Great White Sharks have Races?

ScienceAlert, The DNA of Great White Sharks Defies Explanation. Here's Why.

An animal's genome can be deeply revealing, but ever since researchers started decoding
great white shark (Carcharodon carcharias) DNA more than 20 years ago, their discoveries have raised more questions than answers. In 2024, a study confirmed that, contrary to common thought, this fierce ocean predator does not belong to a single global species. Instead, there appear to be three distinct groups, all descended from a common population that lived 10,000 years ago before the last ice age reduced their numbers. One of the modern groups is in the north Pacific, one in the southern Pacific and Indian Ocean, and one in the north Atlantic and Mediterranean.

No matter how researchers try to explain those groups using evolutionary simulations, they continue to hit dead end after dead end."The honest scientific answer is we have no idea," says study senior author Gavin Naylor, director of the Florida Program for Shark Research at the Florida Museum of Natural History.

Just because they have slightly different genetics doesn't make them separate species. In most animals we would call these subspecies (at best), in humans, races. Fully capable of intermingling, but having enough adaptions to local conditions as to be discernable.

While the nuclear DNA of all three shark groups is mostly the same, their mitochondrial DNA is surprisingly distinct. Nuclear DNA is packaged inside the nucleus of a cell (hence the name), but mitochondrial DNA is packaged inside the mitochondria, which churns out energy for the cell. Unlike nuclear DNA, which is inherited from both parents, mitochondrial DNA (mtDNA) is thought to be inherited from the mother in most multicellular animals – sharks included. Because mtDNA can trace a maternal line, conservation biologists have used it for years to identify population boundaries and migration paths.

When it comes to great white sharks, however, that method isn't working. Even after using one of the largest datasets on great white sharks, globally, researchers came up empty-handed. Previously, scientists suspected the changes in mtDNA were due to female sharks returning to their birthplace to reproduce – a concept known as female philopatry. The hypothesis is even supported by recent observational evidence, which suggests that while both male and female sharks travel vast distances, females return home when it's time to mate.

 

When Naylor and colleagues put that idea to the test, however, it failed to explain the groups of mtDNA. Sequencing the genes of 150 white sharks from around the world, Naylor and his team found no evidence of female philopatry. A small signal would be expected in nuclear DNA if females were only breeding with certain populations. "But that wasn't reflected in the nuclear data at all," says Naylor.

Even when the team ran an evolutionary simulation, showing how sharks might have split off into three groups since their last shared ancestor, the female philopatry hypothesis didn't stand. "I came up with the idea that sex ratios might be different – that just a few females were contributing to the populations from one generation to the next," explains Naylor. That also failed to explain the genetic differences. So did random genetic changes that accumulate over time, called genetic drift.

If the graphic is accurate, the splits between the group happened between 130,000 and 150,000 years ago. 130,000 years ago, the world was near the height of the Eemian Warm period, the previous warm period to our own Holocene. The world was warm, perhaps a bit warmer than our own, but generally similar. White sharks could spread around the globe again. Did the last cold period isolate sharks into three different areas, and encourage the divergence?

The Wombat has Rule 5 Sunday: The Last Champion of Cracker Barrel up at the Other McCain.