A genetic mystery in white sharks (formerly great white sharks) has stumped scientists for over 20 years. Between three major populations of the sharks, differences in mitochondrial DNA have raised eyebrows. While migration patterns initially emerged as a suitable answer, a new study has overturned this common theory.
The study, published in Proceedings of the National Academy of Sciences, has revealed that this long-running mystery originated following the end of the last Ice Age 10,000 years ago. Over time, one population of white sharks split into three distinct populations that each displayed genetic variation. Multiple theories to explain the DNA differences have missed the mark so far, yet scientists aren’t finished looking for answers.
The White Shark Mystery
Before the end of the last glacial period 10,000 years ago, white sharks lived in a single population in the Indo-Pacific Ocean due to low sea levels. However, when Earth began to warm up again, they migrated to other parts of the world, setting the stage for them to genetically diverge around 7,000 years ago.
Today, three populations with distinct genetic traits exist: one in the southern hemisphere near Australia and South Africa, one in the northern Atlantic, and one in the northern Pacific. White sharks' overall numbers, however, are very low.
“There are probably about 20,000 individuals globally,” said study co-author Gavin Naylor, director of the Florida Program for Shark Research at the Florida Museum of Natural History, in a statement. “There are more fruit flies in any given city than there are great white sharks in the entire world.”
In 2001, a study came upon something unexpected in the DNA of white sharks. Based on genetic samples of sharks in Australia, New Zealand, and South Africa, researchers found that the DNA in the nuclei of their cells was mostly the same between individuals, but the DNA in the mitochondria of sharks from South Africa was different from those in Australia and New Zealand.
Researchers originally believed that the isolation of the groups would have allowed each one to develop its own differences in mitochondrial DNA. But this didn’t explain why their nuclear DNA was all nearly identical.
Read More: Great White Sharks Band Together to Prey on Whales
A Common Theory Debunked
An emerging theory stated that differences in mitochondrial DNA arose from the tendency for female sharks to not travel far and always return home for breeding season (a behavior called philopatry), while males traveled vast distances. It’s important to note that the female sharks are also usually the ones that give their mitochondria to the next generation.
According to this theory, the nuclear DNA stayed similar among populations because males would go out and mingle with sharks in other populations. The mitochondrial DNA differences, meanwhile, were attributed to females typically returning to the same place to breed.
The new study, however, discovered that migration couldn’t possibly be the answer. The researchers conducted a similar study to the one in 2001, analyzing DNA from varying shark populations. They ran tests to determine the validity of the philopatry theory, but DNA results showed that it wasn’t actually responsible for the mitochondrial schism between populations.
An Unsolved Case
Right now, the researchers are left with one potential explanation: natural selection. But even this seems like an unlikely answer. That’s because natural selection favors larger populations to pass down favorable traits to offspring, while genetic drift often leaves smaller populations with an increased chance of passing down random traits that can sometimes be unfavorable.
The study’s results confirmed that genetic drift cannot explain mitochondrial DNA differences since it is a random process that doesn’t target one particular type of DNA while sparing another.
The only remaining explanation is natural selection, although Naylor claims that because of small population sizes in white sharks, this process “would have to be brutally lethal.” By this, he means that any “deviation from the mitochondrial DNA sequence most common in a given population would likely be fatal, thus ensuring that it was not passed on to the next generation,” according to the statement.
The researchers note that this is far from a definitive conclusion, and that further studies are necessary to validate natural selection as a possible explanation. For now, the mystery of great white sharks’ DNA lives on.
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Article Sources
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Jack Knudson is an assistant editor at Discover with a strong interest in environmental science and history. Before joining Discover in 2023, he studied journalism at the Scripps College of Communication at Ohio University and previously interned at Recycling Today magazine
