Explosive Evolutionary Innovation May Not Always Follow Mass Extinctions
Baku, February 15 (AZERTAC). Following one of Earth`s five greatest mass extinctions, tiny marine organisms called graptoloids did not begin to rapidly develop new physical traits until about 2 million years after competing species became extinct.
This discovery, based on new research, challenges the widely held assumption that a period of explosive evolution quickly follows for survivors of mass extinctions.
In the absence of competition, the common theory goes, surviving species hurry to adapt, evolving new physical attributes to take advantage of newly opened niches in the ecosystem. But that`s not what researchers found in graptoloid populations that survived a mass extinction about 445 million years ago.
"What we found is more consistent with a different theory, which says you might expect an evolutionary lag as the ecosystem reforms itself and new interspecies relationships form," said University at Buffalo geology professor Charles E. Mitchell, who led the research.
The research provides insight on how a new mass extinction, possibly one resulting from man-made problems such as deforestation and climate change, might affect life on Earth today.
"How would it affect today`s plankton? How would it affect groups of organisms in general?" asked the paper`s lead author, David W. Bapst, a PhD candidate at the University of Chicago who studied with Mitchell as an undergraduate. "The general motivation behind this work is understanding how extinction and evolution of form relate to each other, and the fossil record is the only place where we can do these sorts of experiments across long spans of time."
The research on graptoloids is scheduled to appear online the week of Feb. 13 in the Proceedings of the National Academy of Sciences.
Other team members included Peter C. Bullock and Michael J. Melchin of St. Francis Xavier University in Nova Scotia, and H. David Sheets of Canisius College in Buffalo. The National Science Foundation and Natural Sciences and Engineering Research Council of Canada supported the study.
Graptoloids are an extinct zooplankton that lived in colonies. Because the animals evolved quickly and had a wide geographic range, their fossil record is rich -- a trove of information on how species diversify.
Bapst, Mitchell and their colleagues examined two different groups of graptoloids in their study: Neograptines and diplograptines. Each kind lived during the Ordovician mass extinction that began about 445 million years ago, but only neograptines survived.
Before the extinction event, diplograptine species were dominant, outnumbering neograptine species. Diplograptines also varied more in their morphology, building colonies of many different shapes.
With diplograptines gone after the Ordovician mass extinction, neograptines had a chance to recover in an environment free of competitors.
According to the ecological release hypothesis, a popular theory, these circumstances should have led to a burst of adaptive radiation. In other words, without competition, the neograptines should have diversified rapidly, developing new physical traits -- new colonial architectures -- to take advantage of ecological niches that the diplograptines once filled.
But that`s not what the researchers found.
To test the adaptive radiation idea, they analyzed the colony forms of 183 neograptine and diplograptine species that lived before, during or after the Ordovician mass extinction -- a total of 9 million years of graptoloid history.
This wealth of data enabled the team to track graptoloid evolution with more precision than past studies could. What the researchers discovered looked nothing like adaptive radiation.
Almost immediately following the Ordovician mass extinction, new neograptine species proliferated, as expected. But according to the study, these new species displayed only small changes in form or morphology -- not the burst of innovation the release hypothesis predicts. In fact, graptoloids had been evolving new physical traits at a more intensive pace prior to the extinction event.
Limited morphological innovation among neograptines continued for approximately 2 million years after the extinction, Bapst said.
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