The Devonian extinction event was unique in that it was a
failure to produce new species. It appears that rising sea levels or subsiding
land masses led to a broad over running of local biomes by invasive
species. This suppressed the speciation
process.
Maybe it is that simple.
Or perhaps conditions on earth became more hostile and many populations
were driven into refugia. An example
today is the refugia surrounding the seas off Northern
Australia . Remnant
populations are hanging on while geology does its thing.
The assumption has always been that speciation occurs when
an isolated group diverges. Yet maximum
speciation appears to occur in hugely productive biomes. Perhaps speciation actually needs a lot of
opportunity. Simply assuming breeding
choice matters, then a large population allows choice.
If red hair is chosen as a tribal characteristic, then
simple breeding choice will slowly breed out that characteristic. After all we do that with animals all the
time and it is sped up by availability.
We have returned again the power of choice in arranging our
genetic inheritance.
What triggers mass extinctions? Study shows how invasive species stop
new life
Jan 7, 2011
An influx of invasive species can stop the dominant natural process of
new species formation and trigger mass extinction events, according to research
results published today in the journal PLoS ONE.
The study of the collapse of Earth's marine life 378 to 375 million
years ago suggests that the planet's current ecosystems, which are struggling
with biodiversity loss, could meet a similar fate.
Although Earth has experienced five major mass extinction events, the
environmental crash during the Late Devonian was unlike any other in the
planet's history.
The actual number of extinctions wasn't higher than the natural rate of
species loss, but very few new species arose.
"We refer to the Late Devonian as a mass extinction, but it was
actually a biodiversity crisis," said Alycia Stigall, a scientist at Ohio University
"This research significantly contributes to our understanding of
species invasions from a deep-time perspective," said Lisa Boush, program
director 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 of
mass extinctions through time, especially the five biggest biodiversity crises
in the history of life on Earth. It provides an important perspective on our
current biodiversity crises."
The research suggests that the typical method by which new species
originate--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 a
natural, long-term event, such as the formation of a mountain range or a new
river channel, and evolves into different species.
New species also can originate through dispersal, which occurs when a
subset of a population moves to a new location.
In a departure from previous studies, Stigall used phylogenetic
analysis, which draws on an understanding of the tree of evolutionary
relationships to examine how individual speciation events occurred.
She focused on one bivalve, Leptodesma (Leiopteria), and two
brachiopods, Floweria and Schizophoria (Schizophoria), as well as a predatory crustacean,
Archaeostraca.
These small, shelled marine animals were some of the most common
inhabitants of the Late Devonian oceans, which had the most extensive reef
system in Earth's history.
The seas teemed with huge predatory fish such as Dunkleosteus, and
smaller 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 land
masses, however, some species gained access to environments they hadn't
inhabited before.
The hardiest of these invasive species that could thrive on a variety
of food sources and in new climates became dominant, wiping out more locally
adapted species.
The invasive species were so prolific at this time that it became
difficult for many new species to arise.
"The main mode of speciation that occurs in the geological record
is shut down during the Devonian," said Stigall. "It just stops in
its tracks."
Of the species Stigall studied, most lost substantial diversity during
the Late Devonian, and one, Floweria, became extinct.
The entire marine ecosystem suffered a major collapse. Reef-forming
corals were decimated and reefs did not appear on Earth again for 100 million years.
The giant fishes, trilobites, sponges and brachiopods also declined
dramatically, while organisms on land had much higher survival rates.
The study is relevant for the current biodiversity crisis, Stigall
said, as human activity has introduced a high number of invasive species into
new ecosystems.
In addition, the modern extinction rate exceeds the rate of ancient
extinction events, including the event that wiped out the dinosaurs 65 million
years ago.
"Even if you can stop habitat loss, the fact that we've moved all
these invasive species around the planet will take a long time to recover from
because the high level of invasions has suppressed the speciation rate
substantially," Stigall said.
Maintaining Earth's ecosystems, she suggests, would be helped by
focusing 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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