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.
Related: Surprise Discovery Reveals There Are 3 Different Kinds of Great White Shark
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.
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.
The team of scientists argues that "an alternative evolutionary mechanism must necessarily be operating".
But the only other known explanation is that natural selection may have honed each group's mtDNA, and that seems far-fetched. There are only 20,000 great white sharks in the world, which is a very small population, relatively speaking. If there's something beneficial in the evolution of some forms of mtDNA, then it would have to save the sharks from something "brutally lethal", says Naylor.
He has doubts that that is the case. Some piece of the puzzle is clearly missing.
"The mitochondrial variability observed in natural populations was never reproduced in any of the simulations – even under extreme female philopatry, suggesting that other forces have contributed to the discordance," the authors conclude.
"The same approach would benefit other species of shark where female philopatry has previously been assumed based on genetic data."
The study was published in PNAS.
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