Exceptions in shellfish prove theory for spread of life on Earth

November 14th, 2007 - 10:16 am ICT by admin  
Writing about their findings in the advanced online edition of the Proceedings of the National Academy of Sciences, the researchers said that their findings confirm a general theory for the spread of life on Earth.

“There’s more of everything in the tropics. More genetic diversity, more diversity in form, more diversity of species,” said David Jablonski, the William R. Kenan Jr. Professor in Geophysical Sciences at Chicago. Biologists call this the “latitudinal diversity gradient.

“They have known about this phenomenon for more than a century but there’s remarkably little agreement on how it’s formed,” Jablonski said.

The researchers insist that their findings highlight the importance of the tropics in maintaining the entire planet’s biodiversity.

During the study, the researchers sifted through a database consisting of 4,600 species of bivalves that occurred in more than 200 locations worldwide. The study was focused on bivalves because of their rich fossil record.

“They’re known from the shallowest intertidal zone to the deepest of the deep sea. They’re known in every latitude, from the north polar ocean to the Antarctic,” Jablonski said of the bivalves, a group that includes clams, scallops and oysters.

“We found one major group that didn’t do that. We call that a contrarian group,” Jablonski added.

The researchers call this group the Anomalodesmata (Anomalos). They revealed that contrary to other marine life, Anomalo diversity peaked in the mid-latitudes of both hemispheres, but dipped in the tropics.

“We knew we had to take a closer look at these guys. We had to see how they fit into the bigger picture, how they got into this strange state. They could’ve shown a whole new evolutionary dynamic. We found out that they do follow the same rules, that they are an exception that proves the rule,” Jablonski said.

“This was really exciting: science is always about the search for rules, generalizations that can explain nature in new ways. The results of the research were a bit surprising, as general rules governing natural systems can be hard to come by,” added Andrew Krug, a Research Associate in Geophysical Sciences at Chicago.

The origin of new Anomalo lineages was concentrated in the temperate zones, coinciding with their peak diversity. The coincidence between peak diversity and prolific evolution was also seen in that group’s relatives, and because both fell in the tropics, a normal diversity resulted.

“You could imagine a situation in which all their evolutionary action was still in the tropics, but they just had so much extinction there that by default their diversity peak was in the temperate zone. But if you know where the diversity peak is, you can predict where evolution is the most prolific,” Jablonski said.

“Thanks to the fossil record, we can show that their weird diversity pattern is because of a failure to diversify in the tropics and not because of supercharged evolution in the temperate zones. Our rule came through with flying colours,” the researcher added.

Jablonski said that the new findings showed how important the tropics were for life on Earth, adding: “The tropics are the engine of biodiversity. As the tropics are undermined or deteriorate for a whole variety of reasons, that actually undercuts evolutionary production on a global scale.” (ANI)

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