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Tropical diversity: evolutionary cradle.

Compared to the cold polar regions and the temperate middle latitudes, Earth's tropics teem with life. Zoologists who catalogue the millions of organisms living today know that the richest assortment of plants and animals resides along the planet's warm-weather waistband. But those who study the record of past life have long wondered how the tropics grew so diverse. Does that region naturally favor the evolution of new organisms, or have plants and animals accumulated in the tropics after originating elsewhere? In short, are the tropics an evolutionary cradle or a museum? A new study by David Jablonski of the University of Chicago favors the cradle analogy.

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.
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Author:Monastersky, Richard
Publication:Science News
Article Type:Brief Article
Date:Jul 11, 1992
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