Genetic study suggests that “there are no sharks” as a distinct biological group.
New research from Yale University is questioning something we’ve all always taken for granted: that sharks exist. In fact, researchers now say, sharks do not form a natural, clearly defined biological group.
DNA analysis of dozens of species indicates that the traditional classification of these marine predators may be wrong or, at least, oversimplified.
The, yet to be published, compared the genetic material of different sharks with each other and also with that of other related cartilaginous fish, such as rays, rays and chimaeras. In total, biologists analyzed the complete genomes of 48 species to understand more precisely how these ancient animals relate to each other.
The findings raise enormous doubts by suggesting that sharks may not share a unique common ancestor, separate from rays and rays. This could mean two things, he explains. Chase Brownsteina doctoral student at Yale and first author of the work, at: either sharks do not constitute, after all, a natural group from an evolutionary point of view, or else rays and rays may have to be seen as a type of shark.
The researchers resorted to comparing entire genomes, rather than analyzing just a few isolated genes. Thomas Nearprofessor of ecology and evolutionary biology at Yale and co-author of the study, explains that this method allows for a more robust view of the evolutionary relationships between species.
The results, however, are not linear. Some parts of the DNA — namely the exons, segments of genes involved in the production of proteins — point to the possibility that all sharks belong to the same group. But other genetic regions tell a different story: Certain species, such as basking sharks and cow sharks, may be closer to rays than to other sharks.
This hypothesis suggests that some of these animals with a typical shark appearance may have separated very early in the evolutionary tree, becoming distant relatives from the rest of the group.
Researchers estimate that if sharks are indeed a distinct group, they emerged about 300 million years ago. This period roughly coincides with the time when the ancestors of these cartilaginous fish shared a common ancestor with modern-day amphibians.
For the Yale team, the importance of the study goes beyond classifying sharks. Sharks, rays and rays belong to one of the oldest lineages of jawed vertebrates, making them crucial for understanding the early evolution of animals with jaws and bony skeletons.
