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  (Source: biology.unm.edu)
Researchers investigate if the diversity of species is in equilibrium or expanding

University of Pennsylvania computational biologists are claiming that species on Earth are accumulating much more slowly now than they did in the past. Hélène Morlon and Joshua Plotkin, both of the Department of Biology in the School of Arts and Sciences at the University of Pennsylvania, conducted this study using the family trees of today's species and found that speciation rates have declined over time. 

These researchers were curious as to whether the diversity of species on Earth is in equilibrium or continually expanding. In addition, they wondered if Earth has "an invisible stop sign" that would limit species' diversity. 

To answer these questions, Morlon and Plotkin used an updated computational approach to indicate the dynamics of species diversification. Nine patterns of diversification were used as alternative models where a total of 289 phylogenies (evolutionary trees) were examined, representing arthropods, mammals, flowering plants, birds, mollusks and amphibians. The conclusion was that "diversity is generally not at equilibrium," but speciation rates have fallen over time nonetheless. This might mean that the diversification of species are somewhat restrained, and may eventually reach equilibrium. 

"What we see is diversification rates that are declining but not yet to zero," said Plotkin. "We are not yet in equilibrium. Either there is a limit to the total species number and we haven't reached it yet, or there is no such limit. But the rates of diversification are typically falling; when we will hit zero is not yet obvious."

What is obvious is that there have been recent losses of certain species because of human impact, but Morlon and Plotkin's study involves geologic time scales that are much longer, which helps them to better understand today's species. 

Despite these recent losses, researchers were surprised at the lack of extinction they observed in the evolutionary trees of species because fossil records show that several species have become extinct "over geologic time." Morlon and Plotkin found this when looking specifically at the diversity of whales. The diversity of whales has declined during the last 12 million years, but in the analyses of evolutionary trees, extinction was not obvious.  

"By taking advantage of existing data from the flood of genomic research, we hope to combine efforts with paleontologists gathering fossil data," said Plotkin. 

This study was published in PLoS One this month.





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