The Devonian extinction event was unique in that it was afailure to produce new species. It appears that rising sea levels or subsidingland masses led to a broad over running of local biomes by invasivespecies. This suppressed the speciationprocess.
Maybe it is that simple. Or perhaps conditions on earth became more hostile and many populationswere driven into refugia. An exampletoday is the refugia surrounding the seas off Northern Australia . Remnantpopulations are hanging on while geology does its thing.
The assumption has always been that speciation occurs whenan isolated group diverges. Yet maximumspeciation appears to occur in hugely productive biomes. Perhaps speciation actually needs a lot ofopportunity. Simply assuming breedingchoice matters, then a large population allows choice.
If red hair is chosen as a tribal characteristic, thensimple breeding choice will slowly breed out that characteristic. After all we do that with animals all thetime and it is sped up by availability.
We have returned again the power of choice in arranging ourgenetic inheritance.
What triggers mass extinctions? Study shows how invasive species stopnew life
Jan 7, 2011
An influx of invasive species can stop the dominant natural process ofnew species formation and trigger mass extinction events, according to researchresults published today in the journal PLoS ONE.
The study of the collapse of Earth's marine life 378 to 375 millionyears ago suggests that the planet's current ecosystems, which are strugglingwith biodiversity loss, could meet a similar fate.
Although Earth has experienced five major mass extinction events, theenvironmental crash during the Late Devonian was unlike any other in theplanet's history.
The actual number of extinctions wasn't higher than the natural rate ofspecies loss, but very few new species arose.
"We refer to the Late Devonian as a mass extinction, but it wasactually a biodiversity crisis," said Alycia Stigall, a scientist at Ohio University
"This research significantly contributes to our understanding ofspecies invasions from a deep-time perspective," said Lisa Boush, programdirector in the National Science Foundation (NSF)'s Division of Earth Sciences,which funded the research.
"The knowledge is critical to determining the cause and extent ofmass extinctions through time, especially the five biggest biodiversity crisesin the history of life on Earth. It provides an important perspective on ourcurrent biodiversity crises."
The research suggests that the typical method by which new speciesoriginate--vicariance--was absent during this ancient phase of Earth's history,and could be to blame for the mass extinction.
Vicariance occurs when a population becomes geographically divided by anatural, long-term event, such as the formation of a mountain range or a newriver channel, and evolves into different species.
New species also can originate through dispersal, which occurs when asubset of a population moves to a new location.
In a departure from previous studies, Stigall used phylogeneticanalysis, which draws on an understanding of the tree of evolutionaryrelationships to examine how individual speciation events occurred.
She focused on one bivalve, Leptodesma (Leiopteria), and twobrachiopods, Floweria and Schizophoria (Schizophoria), as well as a predatory crustacean,Archaeostraca.
These small, shelled marine animals were some of the most commoninhabitants of the Late Devonian oceans, which had the most extensive reefsystem in Earth's history.
The seas teemed with huge predatory fish such as Dunkleosteus, andsmaller life forms such as trilobites and crinoids (sea lilies).
The first forests and terrestrial ecosystems appeared during this time;amphibians began to walk on land.
As sea levels rose and the continents closed in to form connected landmasses, however, some species gained access to environments they hadn'tinhabited before.
The hardiest of these invasive species that could thrive on a varietyof food sources and in new climates became dominant, wiping out more locallyadapted species.
The invasive species were so prolific at this time that it becamedifficult for many new species to arise.
"The main mode of speciation that occurs in the geological recordis shut down during the Devonian," said Stigall. "It just stops inits tracks."
Of the species Stigall studied, most lost substantial diversity duringthe Late Devonian, and one, Floweria, became extinct.
The entire marine ecosystem suffered a major collapse. Reef-formingcorals were decimated and reefs did not appear on Earth again for 100 million years.
The giant fishes, trilobites, sponges and brachiopods also declineddramatically, while organisms on land had much higher survival rates.
The study is relevant for the current biodiversity crisis, Stigallsaid, as human activity has introduced a high number of invasive species intonew ecosystems.
In addition, the modern extinction rate exceeds the rate of ancientextinction events, including the event that wiped out the dinosaurs 65 millionyears ago.
"Even if you can stop habitat loss, the fact that we've moved allthese invasive species around the planet will take a long time to recover frombecause the high level of invasions has suppressed the speciation ratesubstantially," Stigall said.
Maintaining Earth's ecosystems, she suggests, would be helped byfocusing efforts and resources on protection of new species generation.
"The more we know about this process," Stigall said,"the more we will understand how to best preserve biodiversity."
The research was also funded by the American Chemical Society and Ohio University
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