Bridge species drive tropical engine of biodiversity
New research sheds light on how that pattern came about. Furthermore, it confirms that the tropics have been and continue to be Earth's engine of biodiversity.
By examining marine bivalves (two-shelled mollusks including scallops, cockles and oysters), a model system for large-scale ecological and evolutionary analysis, the study shows that most evolutionary lineages started in the tropics and expanded outward.
"This 'out of the tropics' dynamic is the major process that shapes the latitudinal pattern of biodiversity that we see today on land and sea," said lead author David Jablonski, the William R. Kenan Jr. Professor in Geophysical Sciences at the University of Chicago. His team focuses on marine bivalves because they combine a wealth of important biological patterns with a large but manageable number of living species (about 8,000) and a rich fossil record.
The new research will be published this week in the online Early Edition of the Proceedings of the National Academy of Sciences presents evidence that the "out of the tropics" process is driven mainly by bridge species, a new term referring to evolutionary lineages that straddle the boundary between the tropics and cooler neighboring regions.
"We thought the 'out of the tropics' process would be driven by the formation of new species at the edge of the tropics, but that doesn't seem to be true," Jablonski said. "Whether bridge species really are the conduit, 'out of the tropics' for all those lineages still needs to be confirmed. We'll tackle that next, by examining molecular data on species within these lineages, inside and outside the tropics, to see how they're related."
As with the PNAS study, this follow-up research would require examining data on both fossils and living organisms. "Alas, it's still rare for paleontologists to integrate the fossil record with data on present-day organisms, but for large-scale biodiversity studies like this, it's a very powerful approach, often an essential one," Jablonski said. "Biodiversity is a product of origination, extinction and immigration, and when the fossil record is adequate, as it is with bivalves, it provides the most robust window into the dynamics that produced present-day patterns."
Surprising findings
The new research corroborates the "out of the tropics" model that Jablonski and others introduced in a 2006 publication. In fact, the new research documents this dynamic over the past 12 million years -- even during the Ice Ages, when the temperature differences between the equator and the poles became more severe. That runs counter to the broadly accepted principle of "niche conservatism," which suggests that related species tend to retain the ecological limits of their ancestors, Jablonski said. "Most species we studied do adhere to that principle, but the ones that do not are crucial to the deployment of biodiversity on Earth."
There are many such bridge species but each evolutionary lineage generally has only one or two. Therefore, bridge species are widespread in an evolutionary sense but rare in terms of overall biodiversity, according to the research. And the fossil record shows that most of today's bridge species started as tropical species. "Somehow they left their tropical cradle, adapting to the colder temperatures and more variable climates of the temperate zones," said Jablonski. "It's impressive that they apparently expanded their ability to tolerate these harsher conditions."
Another surprise is that the most widespread species of bivalves are not the ones with the broadest temperature tolerances. Rather, they are often the ones with limited temperature tolerances that have specialized on temperatures that are the most widespread. "This is important because broad geographic range is one of the most reliable estimates of the extinction-resistance of a species," Jablonski said. "What surprised us is that the most widespread temperatures -- and therefore many of the most widespread bivalve species -- occur in the tropics."
Homo sapiens is even more impressive than the bridge species in this study, he added. Humans have not only immigrated out of the tropics but throughout Earth. Only a handful of marine species range all the way from equator to poles, Jablonski has found, but humans are as widespread as any species known today. "As a species we're pretty extinction-resistant, but even widespread tropical species may be at risk today," said Jablonski.
As research on both living and fossil species has shown, broad geographic ranges buffer species from extinction when pressures are local, such as from hurricanes, pollution or overexploitation. But when -- for marine species -- those pressures alter global ocean temperatures, circulation patterns or chemistry, a broad geographic range is less likely to help, Jablonski said.
"That's what happened at the end-Cretaceous mass extinction 65 million years ago, and that could be looming today."
Other co-authors from UChicago are Christina L. Belanger (now at South Dakota School of Mines and Technology), Sarah K. Berke (now at Siena College), Shan Huang, Andrew Z. Krug (now at Flint Hill School), and Adam Tomasovych (now at Slovak Academy of Sciences). Kaustuv Roy, University of California, San Diego; and James W. Valentine, University of California, Berkeley, are also co-authors.
Funding: The National Science Foundation's Sedimentary Geology & Paleobiology and Systematic Biology Programs; NASA's Exobiology Program; the John Simon Guggenheim Foundation; and University of California Faculty Research Awards.
Welcome to SUV System Ltd!
SUV System Ltd is ISO 90012008 Certified electronics distributor with 10 years of experiences.
We have built up long term business relationship with about many companies which are stockers and authorized agents. we have a steady and reliable supply to meet customer's demands to the greatest extent .Confidently, we are able to lower your cost and support your business with our years of professional service.
SUV System Ltd is Electronic Components Distributor Supplies,Find Quality Electronic Components Supplies Products IC(Integrated Circuits),Connectors,Capacitor,Resistors,Diodes,Transistors,LED at Suvsystem.com. Sourcing Other Energy, Environment, Excess Inventory Products from Manufacturers and Suppliers at Suvsystem.com
Electronic Components distributor:http://www.suvsystem.com
Connectors Distributor:http://www.suvsystem.com/l/Connectors-1.html
IC Distributor:http://www.suvsystem.com/l/IC(Integrated-Circuits)-1.html
LED Distributor:http://www.suvsystem.com/l/LED-1.html
Capacitor Distributor:http://www.suvsystem.com/l/Capacitor-1.html
Transistor Distributor:http://www.suvsystem.com/l/Transistors-1.html
Resistor Distributor:http://www.suvsystem.com/l/Resistors-1.html
Diode Distributor:http://www.suvsystem.com/l/Diodes-1.html
SUV System Ltd insists on the managing faith ofsincereness,speciality,foresight, win-win,so we build up stable-relationship customers located all over the world, including the States, Europe, Argentina, UAE, Malaysia, Australia,and India etc
we are focus on the following fields,and hope we can help you.
CMF5557K600FKEK CMF5557K600FKEB CMF5557K600FKEA CMF5557K600FKBF CMF5557K600FHRE CMF5557K600FHEK CMF5557K600FHEB CMF5557K600DHWF CMF5557K600DHRE CMF5557K600DHR6 CMF5557K600DHEB CMF5557K600DHEA CMF5557K600BHRE CMF5557K600BHR6 CMF5557K600BHEA CMF5557K600BEEK CMF5557K600BEEA CMF5557K600BEBF CMF55576R00FKRE70 CMF55576R00FKR870 CMF55576R00FKR770 CMF55576R00FKR670 CMF55576R00FKEB70 CMF55590K00BHWF CMF55590K00BHR8 CMF55590K00BHR7 CMF55590K00BHR6 CMF55590K00BHEK CMF55590K00BHEB CMF55590K00BHBF CMF5558R300DHEK CMF5558R300DHBF CMF5558K300DEEK CMF5558K300DEBF CMF5558K300BHWF CMF5558K300BHR8 CMF5558K300BHR7 CMF5558K300BHR6 CMF5558K300BHEB CMF5558K300BHEA
http://www.suvsystem.com/a/2502.aspx
