Tropical diversity: evolutionary cradle.
To probe the origination question, Jablonski turned to a vast data set, compiled by himself and a colleague, concerning the 42 marine invertebrate orders known to have appeared in the fossil record sometime in the Mesozoic, the most recent 250 million years of Earth's history. Factoring in the movement of Earth's continents over that time, he analyzed the location of the oldest known examples of each order.
Of the 26 orders known by well-preserved fossils, the number that originated in the tropics roughly equaled the number that originated elsewhere, a finding that might seem to suggest the tropics were no different from other regions. But that conclusion fails to take into account one important fact: Paleontologists have spent much more time collecting fossils in the middle latitudes than they have working in the tropics. Evidence for that collection bias comes from a study of 16 orders that do not form well-preserved fossils: Jablonski finds that these orders appear overwhelmingly for the first time in areas outside the tropics. Pulling the data together, orders appear to originate more frequently in the tropics, he says.
Jablonski suggests three possible explanations. The tropics may have served as an evolutionary cradle because they had a greater area of shallow sea than other areas, providing a place for new organisms to evolve. The sheer number of species present in the tropics could have allowed for prodigious evolution. Or each tropical species may have more evolutionary potential than species in less favorable environments, he says.
|Printer friendly Cite/link Email Feedback|
|Article Type:||Brief Article|
|Date:||Jul 11, 1992|
|Previous Article:||The neglected sex gland: a protein in the blood forecasts cancer of the prostate.|
|Next Article:||Trilobites: not forced off the block.|